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==Chapter Six - Asymmetry of Cerebral Hemispheric Function During Development==
{{DickersonMcGurk1982 header}}
=Chapter Six - Asymmetry of Cerebral Hemispheric Function During Development=


Andrew W. Young  
Andrew W. Young  


==Introduction==


Introduction
The cerebral cortex of the human brain is divided into two cerebral hemispheres. The hemispheres are connected to the body by nerve tracts mediating sensation and movement, whose principal organization is contralateral. In other words, the left hemisphere is primarily responsible for sensation and movement of the right side of the body, whilst the right hemisphere is primarily responsible for sensation and movement of the left side of the body. It should be noted that in both cases there are ipsilateral nerve connections between the left hemisphere and the left side of the body, and between the right hemisphere and the right side of the body. The contralateral nerve fibres predominate, however, and the precise role of the ipsilateral fibres is not fully understood.


The cerebral cortex of the human brain is divided into two cerebral hemispheres.
This ‘crossed’ arrangement of the nervous system is found in many species (Dimond, 1972), though why it evolved is not known. In addition to the ipsilateral and contralateral connections to the body, the cerebral hemispheres are connected to each other by bundles of nerve fibres. In man, the principal interhemispheric connections are mediated through the corpus callosum and the anterior commissure (Seines, 1974; Gazzaniga and Le Doux, 1978).
The hemispheres are connected to the body by nerve tracts mediating sensation
and movement, whose principal organization is contralateral. In other words,
the left hemisphere is primarily responsible for sensation and movement of the
right side of the body, whilst the right hemisphere is primarily responsible for
sensation and movement of the left side of the body. It should be noted that in
both cases there are ipsilateral nerve connections between the left hemisphere
and the left side of the body, and between the right hemisphere and the right side
of the body. The contralateral nerve fibres predominate, however, and the
precise role of the ipsilateral fibres is not fully understood.  


This ‘crossed’ arrangement of the nervous system is found in many species  
The fact that most people show a preference for the use of the right hand for a number of activities was noted in ancient times, and has been much discussed ever since. Although individual members of other species may also exhibit lateral preferences, they tend to be less marked than those observed in most humans, and when preferences are found they average out across individual animals at about 50 % left preference and 50 % right preference. In contrast, no more than 10% of humans are left-handed (Hardyck and Petrinovich, 1977), though the precise figure obtained depends on the strictness of the criteria used.
(Dimond, 1972), though why it evolved is not known. In addition to the
ipsilateral and contralateral connections to the body, the cerebral hemispheres
are connected to each other by bundles of nerve fibres. In man, the principal
interhemispheric connections are mediated through the corpus callosum and the
anterior commissure (Seines, 1974; Gazzaniga and Le Doux, 1978).  


The fact that most people show a preference for the use of the right hand for a
During the nineteenth century it was discovered that the majority of adults who suffer serious speech disturbances after unilateral (one-sided) brain injury do so following damage to the left cerebral hemisphere. As well as expressive language (speech and writing), language comprehension was also found to be more likely to be disturbed following left rather than right hemisphere injury. The possibility of a connection between the involvement of the left cerebral hemisphere in both language and right hand preference was quickly seen, and led to the conception of the left hemisphere as being typically the dominant hemisphere and the right hemisphere as minor or non-dominant. This idea held sway in some quarters until quite recently, though not without opposition. Over the last thirty years, however, a convincing body of evidence for right hemisphere superiorities has accumulated (Joynt and Goldstein, 1975), and it would now seem that the cerebral hemispheres each have their own different functions.
number of activities was noted in ancient times, and has been much discussed
ever since. Although individual members of other species may also exhibit lateral
preferences, they tend to be less marked than those observed in most humans,  


Asymmetric organization, then, is typical of certain cerebral hemispheric functions in the adult human brain. The left cerebral hemisphere is specialized for functions of language and speech, and also controls movement of what is for most people the preferred hand, whilst the right hemisphere is superior for a collection of functions that are often rather loosely characterized as non- linguistic and visuo-spatial. These include the perception and memory of nonlinguistic auditory and visual patterns (such as environmental sounds and people’s faces), and spatial ‘reasoning’ (such as when working from an engineering plan). It is not, at present, clear whether functional asymmetries are also typically found in the brains of non-human animals. They have been found in some cases (e.g. Nottebohm, 1970; Dewson, 1976; Trevarthen, 1978), which suggests that the phenomenon may he more widespread than was thought on the basis of studies of lateral motor preferences.


168
The existence of functional asymmetries between the cerebral hemispheres of the adult human brain raises interesting ontogenetic questions as to how functions are organized in infancy and childhood. For instance, it can be asked whether asymmetry of cerebral hemispheric function is present in infancy, which will be regarded here as the period from birth until two years of age, or whether it develops gradually from an initial bilaterally symmetric organization.


Although such questions are of considerable theoretical and practical importance, they have proved very difficult to answer satisfactorily. The functions being investigated are obviously very complex, and the available methods of investigation are rather indirect. In consequence, conclusions need to be drawn carefully and cautiously. This has not always been done.




ASYMMETRY OF CEREBRAL HEMISPHERIC FUNCTION
The present chapter is intended to examine our knowledge of asymmetry of cerebral hemispheric function during development. In doing this, no attempt will be made to select only those results that fit a preconceived pattern, or to hide where the gaps in our knowledge lie. In some cases, however, criticisms will be- made of studies that exhibit obvious or characteristic deficiencies. This can create a rather negative impression, but it is necessary in order that the results of unsound studies may be discounted and, it is hoped, in order that such pitfalls are avoided in future investigations.


An excellent review of the development of hemispheric function has been published by Witelson (1977a). The present chapter differs from Witelson not only by including more recent material but also in emphasizing more strongly the importance of studies of the development of normal children and the importance of using methods that are themselves properly researched and understood. The potential value of the application of techniques deriving from experimental psychology to enable a degree of precision in pinpointing the sources of obtained laterality effects will also be stressed.


169


==Organization of function in the adult brain==


and when preferences are found they average out across individual animals at
Before examining the available evidence concerning asymmetry of cerebral hemispheric function during development, it is necessary to clarify certain important features of the organization of cerebral hemispheric functions in the adult brain. It needs to be made clear that some functions are more asymmetrically organized than others and, although this chapter will concentrate on the asymmetrically organized functions, it must not be forgotten that there are many functions that are quite symmetrically arranged (Trevarthen, 1978).
about 50 % left preference and 50 % right preference. In contrast, no more than
10% of humans are left-handed (Hardyck and Petrinovich, 1977), though the
precise figure obtained depends on the strictness of the criteria used.  


During the nineteenth century it was discovered that the majority of adults
The most marked asymmetry seems to occur for the production of speech, which is almost exclusively under the control of the left hemisphere (Searleman, 1977). The right hemisphere is usually mute or only capable of highly stereotyped utterances. The motor asymmetry involved in the production of speech is much more marked than other motor asymmetries, and left hemisphere control of speech production is found in nearly all right-handed adults, and many left-handers (Goodglass and Quadfasel, 1954; Branch et al , 1964). Hence, left cerebral control of speech production is more common than right- handedness. This point has important implications for developmental theories, since it renders untenable the view that the ontogeny of hemispheric specialization for speech production arises from an increasing and generalized dominance of the left hemisphere consequent on the development of right hand preference.
who suffer serious speech disturbances after unilateral (one-sided) brain injury
do so following damage to the left cerebral hemisphere. As well as expressive
language (speech and writing), language comprehension was also found to be
more likely to be disturbed following left rather than right hemisphere injury.  
The possibility of a connection between the involvement of the left cerebral
hemisphere in both language and right hand preference was quickly seen, and  
led to the conception of the left hemisphere as being typically the dominant
hemisphere and the right hemisphere as minor or non-dominant. This idea held
sway in some quarters until quite recently, though not without opposition. Over
the last thirty years, however, a convincing body of evidence for right
hemisphere superiorities has accumulated (Joynt and Goldstein, 1975), and it
would now seem that the cerebral hemispheres each have their own different
functions.  


Asymmetric organization, then, is typical of certain cerebral hemispheric
For the purposes of the present review, the interesting questions raised by the existence of interindividual differences in organization of cerebral function will be ignored, since they have not been studied developmentally, and the pattern of organization of function found in the majority of right-handed adults will be regarded as typical. Neither will detailed consideration usually be given to differences between studies in the criteria used for sampling from possible subject populations, since most studies have used subject groups of reasonable size drawn from populations in which the ‘typical’ pattern of organization could be expected to predominate in adulthood.
functions in the adult human brain. The left cerebral hemisphere is specialized
for functions of language and speech, and also controls movement of what is for
most people the preferred hand, whilst the right hemisphere is superior for a
collection of functions that are often rather loosely characterized as non-
linguistic and visuo-spatial. These include the perception and memory of  
nonlinguistic auditory and visual patterns (such as environmental sounds and
people’s faces), and spatial ‘reasoning’ (such as when working from an
engineering plan). It is not, at present, clear whether functional asymmetries are
also typically found in the brains of non-human animals. They have been found
in some cases (e.g. Nottebohm, 1970; Dewson, 1976; Trevarthen, 1978), which  
suggests that the phenomenon may he more widespread than was thought on
the basis of studies of lateral motor preferences.  


The existence of functional asymmetries between the cerebral hemispheres of
Despite its being almost completely lacking in the ability to express itself through speech, the right hemisphere does seem to have some capacity to understand language (Searleman, 1977). Zaidel’s (1976, 1978, 1979) studies, especially, have revealed an extensive auditory and a rather more restricted visual comprehension vocabulary, and some syntactic competence as well. It does not, however, appear to be the case that the right hemisphere’s vocabulary is merely an impoverished version of that of the left hemisphere. Instead, the right hemisphere is relatively capable of understanding concrete, imageable words (Searleman, 1977; Marcel and Patterson, 1979) and poor at understanding abstract words.
the adult human brain raises interesting ontogenetic questions as to how
functions are organized in infancy and childhood. For instance, it can be asked
whether asymmetry of cerebral hemispheric function is present in infancy, which
will be regarded here as the period from birth until two years of age, or whether it
develops gradually from an initial bilaterally symmetric organization.  


Although such questions are of considerable theoretical and practical
There is evidence, then, indicating that there are qualitative differences between the language abilities of the left and right hemispheres of the adult brain. The position is much less clear with regard to those abilities for which the right hemisphere shows superiority. These have been comprehensively reviewed by Joynt and Goldstein (1975). For the sake of simplicity, they will be loosely grouped here into ‘perceptual’ and ‘spatial’ abilities.
importance, they have proved very difficult to answer satisfactorily. The
functions being investigated are obviously very complex, and the available
methods of investigation are rather indirect. In consequence, conclusions need
to be drawn carefully and cautiously. This has not always been done.  


Although real, these right hemisphere superiorities are often not large, and in many cases would seem to represent quantitative rather than qualitative differences to left hemisphere abilities. In the case of nonlinguistic visual and auditory perceptual superiorities, for instance, both the left and right hemispheres are able to carry out the processes concerned, but the right hemisphere is in some way more efficient. This is one reason why the term ‘superiorities’ is used here with reference to the right hemisphere, rather than ‘specializations’. There is no sense in which the left hemisphere might be regarded as blind or deaf. This point is emphasized by Gazzaniga and Le Doux (1978), who regard the existence of right hemisphere superiorities as a side-effect of the left hemisphere’s language specializations. The only known case in which a claim for a qualitative perceptual superiority of the right hemisphere might be made is that of face recognition, but the evidence indicating that this may be a qualitative rather than a quantitative right hemisphere superiority is far from convincing (Ellis, 1975).


Certain complex spatial tasks, such as finding one’s way about and dressing, are more adversely affected by right than by left hemisphere brain injuries (Joynt and Goldstein, 1975). Similarly, in normal people, although there does not seem to be any difference in basic tactual perceptual abilities between the left and right hands, left hand (and hence presumably right hemisphere) superiorities can be shown for tasks with a degree of ‘spatial’ complexity (Corkin, 1978) such as identifying the direction of raised lines felt by touch (Varney and Benton, 1975). Present knowledge of what is involved in such spatial abilities is, however, so rudimentary that it cannot be stated with certainty whether qualitative or quantitative superiorities are involved. Le Doux, Wilson and Gazzaniga (1977) and Gazzaniga and Le Doux (1978) maintain that to the extent that such tasks demand active manipulation of materials (which most do) qualitative interhemisphere differences do arise. They attribute such differences to an involvement of the inferior parietal lobule of the left hemisphere in linguistic at the expense of manipulospatial functions. On this view the right hemisphere is superior for manipulospatial functions only to the extent that the left hemisphere’s language specializations have led to its being deficient in manipulospatial functions.


170
This brief summary of our knowledge of interhemisphere differences in the adult brain gives some idea of the complexity of the phenomenon of cerebral asymmetry, and how little is understood as to its true nature. Many people have found it convenient to adopt summary dichotomies to describe the functions of each hemisphere, such as left-dominant right-minor, left-verbal right-visuo- spatial, or left-analytic right-holistic. Such descriptions should be treated cautiously. In many cases they distort what is known by ignoring the extent to which duplication and symmetry of function actually does take place, and the extent to which the cerebral hemispheres work together as an integrated system.




==The concept of lateralization==


Although the investigation of asymmetry of cerebral hemispheric function during development is seriously hampered both by our lack of knowledge of hemisphere function in the adult brain and by the indirect nature of the methods suitable for work with children, quite comprehensive theoretical statements have been attempted. The most well known of these is that of Lenneberg (1967).


Lenneberg’s principal concern was with language functions, but he also discussed the development of hand preference. He did not really offer a new theory of the ontogeny of cerebral asymmetry, but he did give what was already a widely accepted view its most thoroughly documented and complete expression. Although there are slight changes in emphasis at different points in the book, the main point of Lenneberg’s theory is contained in the view that the extent of lateral asymmetry of organization of particular functions in the left and right cerebral hemispheres is not a fixed characteristic of the human brain, but increases during development in a quite regular manner. In other words, some hemispheric functions are claimed to be progressively lateralized. During the first years of life the cerebral hemispheres are seen as perfectly equipotential for language acquisition, in the sense that either could acquire language with equal facility if the other were injured, and there is no asymmetry of function.


The present chapter is intended to examine our knowledge of asymmetry of
cerebral hemispheric function during development. In doing this, no attempt will
be made to select only those results that fit a preconceived pattern, or to hide
where the gaps in our knowledge lie. In some cases, however, criticisms will be-
made of studies that exhibit obvious or characteristic deficiencies. This can
create a rather negative impression, but it is necessary in order that the results of
unsound studies may be discounted and, it is hoped, in order that such pitfalls
are avoided in future investigations.


An excellent review of the development of hemispheric function has been
Functional asymmetry begins to emerge toward the end of the second year, but it is not marked, and the right hemisphere is involved as well as the left in language acquisition. The degree of asymmetry increases throughout childhood, reaching the adult level at puberty. As the extent of lateralization of function increases and the right hemisphere’s involvement in language functions falls behind that of the left hemisphere equipotentiality declines, so that the final organization is relatively fixed.
published by Witelson (1977a). The present chapter differs from Witelson not
only by including more recent material but also in emphasizing more strongly
the importance of studies of the development of normal children and the  
importance of using methods that are themselves properly researched and
understood. The potential value of the application of techniques deriving from
experimental psychology to enable a degree of precision in pinpointing the  
sources of obtained laterality effects will also be stressed.  


Although directed toward asymmetries of language and hand preference, this type of theory can easily be extended to include the ontogeny of right as well as left hemispheric functional superiorities, though there have been disagreements as to whether the functions of the two hemispheres lateralize concurrently or with one leading the other (e.g. Corballis and Morgan, 1978). There have also been disagreements about the precise age at which lateralization is completed. Krashen (1973) has suggested completion by age five instead of by puberty, whereas Brown and Jaffe (1975) suggest that the process continues into old age. As none of these theories disagrees over the usefulness or the validity of the concept of lateralization they are all regarded here as fundamentally similar to Lenneberg’s position.


Organization of function in the adult brain
Lenneberg’s theory has many attractive features. Many parents feel that it is difficult to tell at first whether a child will be left- or right-handed. The theory brings together a very wide range of observations, and people always seem to have liked theories that postulate general ways in which children and adults differ. None of these, however, is a very good reason for accepting Lenneberg’s position, and during the last ten years it has become clear that his theory is quite wrong. In order to understand why this is the case, it is necessary to look in detail at the available evidence from the developmental studies that have been carried out. These will be grouped into three general types; studies of neuroanatomical asymmetries, studies using noninvasive methods with normal children, and studies of the consequences of cerebral injuries sustained at different ages.


Before examining the available evidence concerning asymmetry of cerebral
==Developmental studies==
hemispheric function during development, it is necessary to clarify certain
important features of the organization of cerebral hemispheric functions in the
adult brain. It needs to be made clear that some functions are more asymmetrically organized than others and, although this chapter will concentrate on
the asymmetrically organized functions, it must not be forgotten that there are
many functions that are quite symmetrically arranged (Trevarthen, 1978).


The most marked asymmetry seems to occur for the production of speech,
Neuroanatomical asymmetries
which is almost exclusively under the control of the left hemisphere (Searleman,
1977). The right hemisphere is usually mute or only capable of highly
stereotyped utterances. The motor asymmetry involved in the production of
speech is much more marked than other motor asymmetries, and left hemisphere
control of speech production is found in nearly all right-handed adults, and
many left-handers (Goodglass and Quadfasel, 1954; Branch et al , 1964). Hence,
left cerebral control of speech production is more common than right-
handedness. This point has important implications for developmental theories,
since it renders untenable the view that the ontogeny of hemispheric specialization for speech production arises from an increasing and generalized dominance
of the left hemisphere consequent on the development of right hand preference.


For the purposes of the present review, the interesting questions raised by the  
Our understanding of functional cerebral asymmetries may be at present limited, but knowledge of any corresponding neuroanatomical asymmetries is very scant indeed. None the less, neuroanatomical asymmetries do exist. The most thoroughly researched is the asymmetry of the planum temporale in the posterior region of the superior surface of the temporal lobe (Geschwind and Levitsky, 1968). The planum temporale of the left temporal lobe, which forms part of an area of known importance in language functions, is larger than or equal in size to the planum temporale of the right temporal lobe in approximately 90 % of adults.
existence of interindividual differences in organization of cerebral function will
be ignored, since they have not been studied developmentally, and the pattern of  
organization of function found in the majority of right-handed adults will be




Is such an asymmetry present in the brains of babies? It is quite clear that the answer is yes. Studies by Teszner et al (1972), Witelson and Pallie (1973) and Wada et al (1975) have demonstrated differences in the relative sizes of the left and right planum temporale of the foetal, newborn and infant brain. Opinions differ as to whether the degree of this neuroanatomical asymmetry increases between infancy and adulthood. This is hardly surprising, since it is by no means clear which measurements should be used to effect such a comparison. There is no disagreement, however, that the nature of the neuroanatomical asymmetry does not differ between infants and adults.


ASYMMETRY OF CEREBRAL HEMISPHERIC FUNCTION
It is clear, then, that if functional asymmetries are found in the infant brain, this would not conflict with neuroanatomical knowledge. Similarly, the existence of neuroanatomical hemispheric asymmetries in the newborn makes it difficult (though not impossible) to believe in the complete equipotentiality of the cerebral hemispheres for language functions. On the other hand, as Witelson (1977a) points out, the existence of a neuroanatomical asymmetry is not in itself sufficient to imply that the cerebral hemispheres function asymmetrically in infancy. It may only represent the structural bias underlying later developing functional specializations. Because neuroanatomical findings are ambiguous in this way, it is necessary to look at results deriving from other methods.


==Noninvasive methods==


171
A number of methods have been devised in an attempt to study functional asymmetries in the normal, intact brain. Following Witelson’s (1977a) terminology, these will be referred to as noninvasive methods.


It is possible, for instance, to study asymmetries of motor control of parts of the body, and lateral preferences. Ontogenetic studies of lateral preference have been carried out for a long time. More recently attention has also been given to asymmetries following auditory, visual or tactile stimulus presentations, and these procedures have been adapted for use with children and, in some cases, infants.


regarded as typical. Neither will detailed consideration usually be given to
In examining these noninvasive methods, studies of asymmetries in children for processing auditory, visual and tactile stimuli will each be considered in turn. Studies involving the auditory or visual presentation of stimuli to infants will then be discussed, and finally studies of asymmetries of motor control and lateral preferences.
differences between studies in the criteria used for sampling from possible subject
populations, since most studies have used subject groups of reasonable size
drawn from populations in which the ‘typical’ pattern of organization could be
expected to predominate in adulthood.  


Despite its being almost completely lacking in the ability to express itself
Auditory presentation. The principal auditory nerve connections are contralateral, so that material presented to the right ear is directed to the left cerebral hemisphere and material presented to the left ear is directed to the right cerebral hemisphere. However, substantial ipsilateral auditory nerve connections between the left ear and left hemisphere and between the right ear and right hemisphere also exist.
through speech, the right hemisphere does seem to have some capacity to
understand language (Searleman, 1977). Zaidel’s (1976, 1978, 1979) studies,
especially, have revealed an extensive auditory and a rather more restricted
visual comprehension vocabulary, and some syntactic competence as well. It
does not, however, appear to be the case that the right hemisphere’s vocabulary
is merely an impoverished version of that of the left hemisphere. Instead, the  
right hemisphere is relatively capable of understanding concrete, imageable
words (Searleman, 1977; Marcel and Patterson, 1979) and poor at understanding abstract words.  


There is evidence, then, indicating that there are qualitative differences
When different linguistic stimuli (such as spoken digits) are presented simultaneously, one to each ear, the stimulus presented to the right ear tends to be reported more accurately than that presented to the left ear (Kimura, 1961, 1967). This general method has come to be known as dichotic stimulation, and is readily adapted for use with children. The finding of right ear superiority for linguistic material would seem to reflect its more efficient transmission to the specialized language areas of the left cerebral hemisphere, but it has also been thought that the ascendancy of the contralateral over the ipsilateral auditory nerve connections is particularly marked when both ears are simultaneously stimulated (Kimura, 1967; Cohen, 1977). When material is presented to one ear at a time the differences between ears are small and their demonstration requires the use of sensitive measures (Studdert-Kennedy, 1972; Fry, 1974; Morais and Darwin, 1974) or difficult tasks (Bakker, 1969, 1970; Frankfurter and Honeck, 1973; Van Duyne et al , 1977).
between the language abilities of the left and right hemispheres of the adult
brain. The position is much less clear with regard to those abilities for which the  
right hemisphere shows superiority. These have been comprehensively reviewed
by Joynt and Goldstein (1975). For the sake of simplicity, they will be loosely
grouped here into ‘perceptual’ and ‘spatial’ abilities.  


Although real, these right hemisphere superiorities are often not large, and in
As well as its use in investigating the language specializations of the left hemisphere, the dichotic stimulation technique can also be used to study right hemisphere (and hence left ear) superiorities for the processing of some nonlinguistic auditory stimuli (Gordon, 1970, 1974; Knox and Kimura, 1970). For clarity and convenience the use of dichotic stimulation techniques to study the development of left and right hemispheric abilities will be discussed separately.
many cases would seem to represent quantitative rather than qualitative
differences to left hemisphere abilities. In the case of nonlinguistic visual and
auditory perceptual superiorities, for instance, both the left and right hemispheres are able to carry out the processes concerned, but the right hemisphere is
in some way more efficient. This is one reason why the term ‘superiorities’ is used
here with reference to the right hemisphere, rather than ‘specializations’. There is
no sense in which the left hemisphere might be regarded as blind or deaf. This
point is emphasized by Gazzaniga and Le Doux (1978), who regard the existence
of right hemisphere superiorities as a side-effect of the left hemisphere’s language
specializations. The only known case in which a claim for a qualitative
perceptual superiority of the right hemisphere might be made is that of face
recognition, but the evidence indicating that this may be a qualitative rather
than a quantitative right hemisphere superiority is far from convincing (Ellis,
1975).  


Certain complex spatial tasks, such as finding one’s way about and dressing,
The studies of the ontogeny of right hemisphere superiorities for processing nonlinguistic sounds can be quickly dealt with, as few have been carried out. The principal studies are those of Knox and Kimura (1970) and Piazza (1977). Neither of these studies, nor the two unpublished studies referred to by Witelson (1977a), found any change in the left ear advantage across age in the range three years to adult.
are more adversely affected by right than by left hemisphere brain injuries (Joynt
and Goldstein, 1975). Similarly, in normal people, although there does not seem
to be any difference in basic tactual perceptual abilities between the left and right
hands, left hand (and hence presumably right hemisphere) superiorities can be
shown for tasks with a degree of ‘spatial’ complexity (Corkin, 1978) such as


The overwhelming majority of dichotic stimulation studies involving children have been addressed to the development of left hemisphere specializations. Witelson (1977a) gives a detailed summary of the methods and findings of over 30 published and unpublished studies carried out up to 1976. These studies differ on many points of methodology. Considering only the published studies reviewed by Witelson, the stimuli used included isolated speech sounds and nonsense syllables (Berlin et a/., 1973; Dorman and Geffner, 1974; Geffner and Dorman, 1976), spoken digits (Kimura, 1963, 1967; Inglis and Sykes, 1967; Bryden, 1970; Knox and Kimura, 1970; Geffner and Hochberg, 1971; Satz et al , 1971; Sommers and Taylor, 1972; Satz et al, 1975; Witelson, 1976a, 19766; Kinsbourne and Hiscock, 1977; Bryden and Allard, 1978), words (Knox and Kimura, 1970; Nagafuchi, 1970; Sommers and Taylor, 1972; Goodglass, 1973; Ingram, 1975a) and animal names (Bever, 1971). In some studies a report was required after each pair of stimuli, whilst in others two, three or even four pairs were presented before report of as many stimuli as possible was required. Both vocal and non vocal (such as pointing to a picture of a word’s referent) methods of reporting were used. There were also differences as to whether only right- handed children were used as subjects, and the criteria for establishing handedness when this was done. In addition, a point not taken up by Witelson (1911a) is that various different methods of aligning the left and right ear stimuli for ‘simultaneous’ presentation have been tried (Morton et a/., 1976).


Given that there have been such marked methodological differences between studies, the findings are surprisingly consistent. Almost all of the studies found right ear superiorities for the processing of linguistic stimuli, and almost all found right ear superiorities in the youngest groups of children studied. This has also been true of reports published since Witelson’s review was written (e.g. Mirabile et al, 1975; Borowy and Goebel, 1976; Geffen, 1976; Hynd and Obrzut, 1977; Hiscock and Kinsbourne, 1977; Piazza, 1977; Geffen, 1978; Geffen and Sexton, 1978; Geffen and Wale, 1979; Sextcn and Geffen, 1979). In a number of the published reports (Nagafuchi, 1970; Bever, 1971; Ingram, 1975a; Hiscock and Kinsbourne, 1977; Kinsbourne and Hiscock, 1977; Piazza, 1977) right ear superiorities have been demonstrated in children as young as three years old. Moreover, none of the studies that have investigated such young children has failed to find right ear superiorities.


172
It is clear, then, that insofar as right ear advantages for reporting dichotic linguistic stimuli are dependent on cerebral asymmetry for language functions, such asymmetries are present from at least three years of age. Supporters of the concept of progressive lateralization have, however, tended to see the most important question as being not so much the ages at which ear asymmetries can be demonstrated, but rather whether the degree of right ear superiority increases across age (Satz et al ., 1975). This is based on the contention that as the degree of cerebral hemispheric functional asymmetry increases, the size of ear advantages for dichotic stimulation should also increase. In other words, dichotic stimulation is regarded as a parametric measure of cerebral asymmetry. This view requires more careful consideration.


The first point that must be made is that even when dichotic stimulation scores are analysed by parametric statistical procedures most studies have not found the degree of right ear superiority for linguistic material to vary across age. In a small minority of studies, however, ear asymmetry was found to increase with increasing age (Bryden, 1970; Satz et al, 1975; Bryden and Allard, 1978). This raises the difficult question of the proper interpretation of findings of this type.


If it were the case that it is appropriate to regard dichotic stimulation as a parametric index of cerebral asymmetry, then the findings of Bryden and of Satz might substantiate the idea that the degree of cerebral asymmetry increases with increasing age. However, there are serious difficulties to be overcome before such a conclusion could be reached. No one has been able to demonstrate satisfactorily that the sizes of ear asymmetries are sufficiently closely or uniquely related to the degree of asymmetry of cerebral hemispheric function to serve as an index (Berlin and Cullen, 1977; Witelson, 1977a; Colbourn, 1978). Although it is probable that some modest relationship exists, there are many factors besides cerebral asymmetry which may influence the magnitude of ear advantages. A list of these factors would include the difficulty level of the task to particular subjects (and hence ‘ceiling’ or ‘floor’ effects), individual differences in the relative functional predominance of contralateral and ipsilateral auditory nerve fibres, different strategies for organizing reports of left and right ear stimuli, and attentional biases toward a particular ear. Moreover, the level or levels of stimulus processing at which ear asymmetries due to functional cerebral asymmetries can arise are not properly understood, and few investigations have explicitly controlled for the possibility that the contribution to observed asymmetries arising from different levels of information processing may vary between subjects. All of these potential influences on the size of obtained ear asymmetries are, of course, particularly likely to influence the outcomes of developmental studies which must necessarily sample across wide ranges of ages.


When these several factors are considered it is even more remarkable that the results of the majority of dichotic stimulation studies have been so consistent. The consistency is probably caused by most of the results happening to arise from the same general source of asymmetry, namely the left hemisphere’s superiority for speech decoding, and the few results that do not fit the main pattern of absence of developmental trends in ear asymmetry are best discounted until methods that allow more control over the factors involved are available. This conclusion is strengthened by the failure of Bakker, Hoefkens and Van Der Vlugt (1979) to confirm the developmental trend of Satz et al (1975) using a longitudinal instead of a cross-sectional research design.


From this discussion it is apparent that studies of ear asymmetry to dichotic linguistic stimulation in children must develop better methods for controlling unwanted sources of variance and for identifying the levels of stimulus processing at which cerebral asymmetries arise. Some researchers are beginning to do this. Most notably, an elegant series of studies by Geffen and her colleagues (Geffen, 1976, 1978; Geffen and Sexton, 1978; Geffen and Wale, 1979; Sexton and Geffen, 1979) has demonstrated that when attentional strategies are properly controlled there is no variation across age in the degree of right ear advantage for speech perception. Conversely, Geffen also found that the ability to deploy attentional strategies did vary across age, and that the use of attentional strategies can affect the size of obtained ear asymmetries, so that this factor does need to be controlled.


identifying the direction of raised lines felt by touch (Varney and Benton, 1975).  
The identification of the levels of stimulus processing at which cerebral asymmetries arise is more difficult to achieve than the control of attentional strategies, but some progress is also being made. Consider, for instance, what aspects of cerebral asymmetry might contribute to the right ear advantage for linguistic stimuli. Two broad classes of effect can be readily distinguished. These are effects attributable to the left hemisphere’s superior abilities for the analysis and temporary storage of speech sounds, which will be called speech decoding asymmetries, and effects attributable to the different types of word that can be recognized by the left and right hemispheres, which will be called lexical asymmetries . Within the class of speech decoding asymmetries a further distinction might be drawn as to whether the asymmetries arise at the level of immediate perceptual analysis, or whether some short-term memory component is involved (as when multiple pairs of stimuli are presented before a report is required).
Present knowledge of what is involved in such spatial abilities is, however, so
rudimentary that it cannot be stated with certainty whether qualitative or
quantitative superiorities are involved. Le Doux, Wilson and Gazzaniga (1977)
and Gazzaniga and Le Doux (1978) maintain that to the extent that such tasks
demand active manipulation of materials (which most do) qualitative interhemisphere differences do arise. They attribute such differences to an involvement of the inferior parietal lobule of the left hemisphere in linguistic at the  
expense of manipulospatial functions. On this view the right hemisphere is  
superior for manipulospatial functions only to the extent that the left hemisphere’s language specializations have led to its being deficient in manipulospatial functions.  


This brief summary of our knowledge of interhemisphere differences in the  
It is quite clear that a major contribution to obtained ear advantages is made by the general class of speech decoding asymmetries, which are sufficient to account for most of the observed results. This is evident from the fact that many of the ear asymmetries in the studies cited did not depend on the presentation of complete words, but could be obtained when isolated speech sounds or nonsense syllables were used as stimuli. It seems, too, that these speech decoding asymmetries can arise at the level of immediate perceptual analysis, but are heightened when a short-term memory component is introduced into the experimental task (Oscar-Berman et al , 1974; Yeni-Komshian and Gordon, 1974). This has important implications for developmental studies, which have been very free in varying the short-term memory requirements of the tasks used, as Porter and Berlin (1975) point out. It is likely that tasks with a large shortterm memory component will produce developmental trends in ear asymmetry not because cerebral asymmetry changes across age but because of age differences in short-term memory abilities and hence task sensitivity.
adult brain gives some idea of the complexity of the phenomenon of cerebral
asymmetry, and how little is understood as to its true nature. Many people have
found it convenient to adopt summary dichotomies to describe the functions of  
each hemisphere, such as left-dominant right-minor, left-verbal right-visuo-
spatial, or left-analytic right-holistic. Such descriptions should be treated
cautiously. In many cases they distort what is known by ignoring the extent to
which duplication and symmetry of function actually does take place, and the
extent to which the cerebral hemispheres work together as an integrated system.  


Ear asymmetries belonging to the lexical class obviously cannot arise when isolated speech sounds or nonsense syllables are used as stimuli. Although findings of lexical class ear asymmetries have been made in studies of adults using words as stimuli (McFarland et al, 1978; Kelly and Orton, 1979) they are by no means always found (e.g. McFarland et al, 1978; Kelly and Orton, 1979; Young and Ellis, 1980) and probably only arise under conditions that are not typical of most dichotic stimulation studies. The only study to date that has separately considered the possible implications for developmental findings of the distinction between the classes of auditory asymmetries described here as due to speech decoding and lexical factors has been that of Eling et al (1979).


The concept of lateralization  
In summary, then, studies of ear asymmetries to dichotic stimulation in children indicate that left hemisphere specializations for speech decoding and right hemisphere superiorities for the analysis of some nonlinguistic sounds are present down to at least three years of age. In most of the studies carried out the magnitude of ear asymmetries did not vary across age. In the few cases where the degree of ear asymmetry did increase with increasing age there is no reason to believe that this was a consequence of any process of increasing lateralization of cerebral hemispheric function.


Although the investigation of asymmetry of cerebral hemispheric function
Visual presentation. The optic nerve pathways are organized in such a way that information about visual stimuli falling to the left of the point at which a person is looking (in the left visual hemifield) is projected initially to the right cerebral hemisphere, whilst information about stimuli falling to the right of the point at which a person is looking (in the right visual hemifield) is projected initially to the left cerebral hemisphere. It is not established with certainty whether or not there is some degree of ipsilateral optic projection for stimuli falling close to the visual midline in the foveal and parafoveal regions of the retina, but outside this disputed area the projections are known to be exclusively contralateral (Cohen, 1977; Haun, 1978). This should not be taken as meaning that the left eye sends projections only to the right hemisphere. The fields of vision of each of the eyes overlap to a considerable extent, so that most left or right visual hemifield stimuli are seen by both eyes, and the contralateral optic projections consequently arise from a grouping together at the optic chiasm of the nerve fibres from the corresponding side of the retina of each eye. Because of this anatomical arrangement, the phenomena of eye dominance bear no clear relation to cerebral asymmetry (Porac and Coren, 1976), and will not be discussed.
during development is seriously hampered both by our lack of knowledge of
hemisphere function in the adult brain and by the indirect nature of the methods
suitable for work with children, quite comprehensive theoretical statements have
been attempted. The most well known of these is that of Lenneberg (1967).  


Lenneberg’s principal concern was with language functions, but he also
If we know where a person is looking, then, it is possible to present visual stimuli in such a way that information is initially projected to whichever cerebral hemisphere we choose. Unfortunately, the presentation of a visual stimulus usually leads to an involuntary movement of the eyes to bring it into central vision. It is thus necessary to restrict the presentation time of stimuli to a time less than that needed to make such an eye movement. Estimates of this time vary, but it is usual to regard presentation times of 200 milliseconds (one-fifth of a second) or less as acceptable (Cohen, 1977).
discussed the development of hand preference. He did not really offer a new
theory of the ontogeny of cerebral asymmetry, but he did give what was already
a widely accepted view its most thoroughly documented and complete
expression. Although there are slight changes in emphasis at different points in
the book, the main point of Lenneberg’s theory is contained in the view that the  
extent of lateral asymmetry of organization of particular functions in the left and
right cerebral hemispheres is not a fixed characteristic of the human brain, but  
increases during development in a quite regular manner. In other words, some
hemispheric functions are claimed to be progressively lateralized. During the
first years of life the cerebral hemispheres are seen as perfectly equipotential for
language acquisition, in the sense that either could acquire language with equal
facility if the other were injured, and there is no asymmetry of function.  


The need to use briefly presented stimuli falling outside central vision obviously places a serious constraint on what can be studied using this technique, but a surprising amount has been achieved despite the limitations. It must be made clear, however, that the method can only permit the initial projection of stimulus information to one cerebral hemisphere or the other. What happens after that is not well understood, though it is probable that information is coordinated by means of the neocortical commissures, and the anterior commissure in particular (Risse et al ., 1978). Most investigators have been sufficiently reassured by the contralateral nature of the optic pathways to use unilateral stimulus presentations (in which stimuli appear only in one visual hemifield), but a case that bilateral presentation (in which different stimuli appear simultaneously in each of the visual hemifields) is a rather better procedure can be made out (McKeever and Huling, 1971; Hines, 1975). Although methods that can allow continuous lateralized input have been developed (e.g. Zaidel, 1975) these have not been adapted for use with children.


When words are presented briefly in the left or the right visual hemifield and right-handed adults are asked to name them it is usual to find a right visual hemifield (RYF) superiority (Mishkin and Forgays, 1952; McKeever and Huling, 1971). This RVF superiority is principally due to information about words falling in the RVF being directly projected to the left cerebral hemisphere. However, it has also been claimed to relate to the fact that English is read from left to right. The argument in this case is that the memory trace of the stimulus is initially ‘examined’ by the subject with a left to right scan starting from the point of fixation (Heron, 1957; White, 1969, 1972, 1973). Hence, the RVF superiority would arise from a ‘post-exposural trace-scanning’ mechanism deriving from experience in reading.


ASYMMETRY OF CEREBRAL HEMISPHERIC FUNCTION
This trace-scanning notion no longer needs to be taken very seriously. It can be varied so freely as to become almost unfalsifiable, and even if true it could only be making a minor contribution to the patterns of results found in studies that have used words as stimuli rather than arrays of unrelated letters (McKeever, 1974; Pirozzolo, 1977). It is known, for instance, that the RVF advantage holds for vertically as well as horizontally arranged words and for words in the Hebrew language, which is read from right to left (Barton et al , 1965; Carmon et al , 1976). Furthermore, the size of the RVF superiority is not constant for all types of word, but has been shown to be larger for abstract than concrete words (Ellis and Shepherd, 1974; Hines, 1976, 1977). This pattern of results is most readily interpreted by postulating that both cerebral hemispheres of the adult brain possess at least rudimentary abilities to decode print stimuli, so that the word-class effect derives from the different types of word that can be recognized by the left and right hemispheres.


Another convincing reason for interpreting the results of studies using brief lateral presentations of visual stimuli in terms of functional cerebral asymmetry is that in several studies using nonlinguistic visual stimuli left visual hemifield (LVF), and hence presumably right hemisphere, superiorities have been demonstrated (Kimura and Durnford, 1974). Face recognition has proved to be a particularly useful task in this respect, with many subsequent reports confirming the findings of LVF superiorities by Rizzolatti et al (1971) and Hilliard (1973).


173
The use of the visual modality of stimulus presentation in studies of asymmetry of cerebral hemispheric function in children is potentially of great interest because of the wide range of skills that can be examined and the considerable range of ages at which the differing skills are learnt. The ability to identify visually represented words, for instance, is achieved at a much older age than is the ability to recognize faces. Unfortunately, a large rumber of theoretical and methodological difficulties are encountered in the case of visual presentation, and progress has been slow in comparison with that made by studies using dichotic stimulation. There have not been nearly as many studies carried out, and several of those that have been attempted are seriously flawed.


The most pressing methodological requirement in studies of visual hemifield asymmetries in children is to control fixation. Studies of adults usually rely on a central fixation spot, which subjects are asked to fixate before each stimulus is presented. This procedure is obviously of dubious validity in a developmental investigation. Children may fail to fixate when instructed to do so for a number of reasons. The consequence of a failure by children to fixate when instructed is that stimuli will not fall in the positions in the visual field intended by the experimenter, and will probably be distributed randomly, thus reducing or eliminating ‘visual hemifield’ differences. Since younger children will be more likely to fail to fixate than older children, a bias will be introduced making it probable that findings of differences in asymmetries across age will arise as an artifact of lack of fixation. Moreover, the temptation not to fixate when instructed may be itself related in a complex manner to the difficulty of particular experimental tasks to particular subjects. For these reasons, some form of fixation control is necessary in developmental studies of visual hemifield asymmetries, and all developmental trends found in studies without adequate fixation control (such as Jeeves, 1972; Miller and Turner, 1973; Barosso, 1976; Reynolds and Jeeves, 1978a, 1978b; Tomlinson-Keasey et al ., 1978) must be discounted as irrelevant to any considerations of asymmetry of cerebral hemispheric function.


Functional asymmetry begins to emerge toward the end of the second year, but
A difficulty which is partly methodological and partly theoretical is that of ensuring that subjects of different ages are relying on the same cognitive processes or strategies when faced with a given task. It is often assumed that the use of linguistic stimuli will automatically engage specialized left hemisphere mechanisms and lead to a RVF advantage, whilst the use of nonlinguistic stimuli will produce no visual hemifield difference or a small LVF advantage. Cases are known in the adult literature, however, where this generalization breaks down. Matching tasks provide a simple example. Suppose that a pair of words or a pair of letters is presented in one visual hemifield, and subjects are asked to say whether they are the same or different. This can be determined either by comparing the physical appearances of the stimuli (physical match) or by naming them and comparing the names (name match). Studies by Cohen (1972) and Gibson, Dimond and Gazzaniga (1972) have demonstrated that whereas name matches yield RYF advantages, physical matches may be more effectively carried out for LVF stimuli. The implication for developmental studies is that if matching tasks are used they must be arranged in such a way that subjects are forced to adopt only one of the possible strategies. If this is not done, differences across age may simply be attributable to strategy differences. Witelson (19776) first noticed this potential artifact in one of her own studies, but the criticism applies equally to the differences across age found by Tomlinson-Keasey et al. (1978) and in Broman’s (1978) experiment involving matching pairs of letters. The general point that it is important to know how subjects actually approach experimental tasks applies, of course, to a lot more than just matching tasks.
it is not marked, and the right hemisphere is involved as well as the left in  
language acquisition. The degree of asymmetry increases throughout childhood,  
reaching the adult level at puberty. As the extent of lateralization of function
increases and the right hemisphere’s involvement in language functions falls
behind that of the left hemisphere equipotentiality declines, so that the final
organization is relatively fixed.  


Although directed toward asymmetries of language and hand preference, this
Having made these methodological cautions and eliminated some of the more poorly designed studies, the principal studies of visual hemifield asymmetries in children will now be considered, starting with studies of right hemisphere (LVF) superiorities.
type of theory can easily be extended to include the ontogeny of right as well as
left hemispheric functional superiorities, though there have been disagreements
as to whether the functions of the two hemispheres lateralize concurrently or
with one leading the other (e.g. Corballis and Morgan, 1978). There have also
been disagreements about the precise age at which lateralization is completed.
Krashen (1973) has suggested completion by age five instead of by puberty,
whereas Brown and Jaffe (1975) suggest that the process continues into old age.
As none of these theories disagrees over the usefulness or the validity of the
concept of lateralization they are all regarded here as fundamentally similar to
Lenneberg’s position.  


Lenneberg’s theory has many attractive features. Many parents feel that it is
The most common task used to investigate LVF superiorities, as in the adult literature, has been face recognition. Young and Ellis (1976) found LVF superiorities for face recognition in five-, seven- and eleven-year-old children, with no differences across age in the degree of visual hemifield asymmetry. Broman (1978) found no developmental differences in LVF superiority for face recognition in the age range seven years to adult. Marcel and Rajan’s (1975) study of seven-year-old children also showed a LVF superiority for face recognition. In contrast, failures to find LVF superiority in seven- and eight-year-old children have been reported by Leehey (1976) and Reynolds and Jeeves (1978b). Reynolds and Jeeves’ study, however, lacks adequate fixation control. Leehey (1976) reports three developmental experiments on visual hemifield asymmetries for face recognition by subjects aged eight to adult that are in most respects carefully designed. When she used bilaterally presented photographs of the faces of people known to her subjects a LVF superiority was found at all ages, but with bilaterally presented unfamiliar faces the eight-year-old children gave no visual hemifield difference in two experiments. Unfortunately, Young and Bion (1980a) were unable to replicate this result, and have suggested that it was probably due to an age difference in directional reporting strategies arising from Leehey’s use of bilateral stimuli without controlled order of report. Studies of visual hemifield asymmetries thus give no grounds at present for claiming any developmental change in the extent of the right hemisphere’s superiority for face recognition.
difficult to tell at first whether a child will be left- or right-handed. The theory
brings together a very wide range of observations, and people always seem to  
have liked theories that postulate general ways in which children and adults
differ. None of these, however, is a very good reason for accepting Lenneberg’s
position, and during the last ten years it has become clear that his theory is quite
wrong. In order to understand why this is the case, it is necessary to look in detail
at the available evidence from the developmental studies that have been carried
out. These will be grouped into three general types; studies of neuroanatomical
asymmetries, studies using noninvasive methods with normal children, and
studies of the consequences of cerebral injuries sustained at different ages.  


Developmental studies
Studies of right hemisphere superiorities using visual presentation and tasks other than face recognition have also failed to reveal developmental trends. Witelson (1977b) found a LVF superiority for matching pictures of human figures (a task which can only be done by means of a physical match) in boys aged six to thirteen years. Witelson (1977a) described an unpublished experiment finding a tendency to greater LVF accuracy for dot enumeration (p < 0.1) in six- to thirteen-year-old boys. Young and Bion (1979) found greater LVF accuracy for dot enumeration in boys aged five, seven and eleven years, but no visual hemifield accuracy difference in girls. A similar sex difference was found in adult subjects by McGlone and Davidson (1973). The absence of any developmental trend in LVF superiority for dot enumeration is interesting in view of the fact that it is a skill that is present in only a rudimentary form before age three, and even after three years is learnt quite gradually (Klahr and Wallace, 1973; Young and McPherson, 1976), whereas recognition of many faces is possible in the first year of life (Schaffer, 1971; Ellis, 1975). It is thus clear that the absence of reliable developmental trends in asymmetry of face recognition by children aged five and above is not simply due to the early age at which the skill is acquired.


Neuroanatomical asymmetries
Two findings of LVF superiority in children allegedly induced by means of a spatial mental set must also be noted (Kershner et al , 1977; Carter and Kinsbourne, 1979). Only Carter and Kinsbourne tested more than one age group of children, and found no developmental differences in the tendency of spatial priming to produce a LVF superiority for digit naming.


Our understanding of functional cerebral asymmetries may be at present limited,
Most studies of visual hemifield asymmetries for linguistic stimuli in children have used printed words. In several cases the principal focus of interest was not so much whether there were differences across age as the possibility of differences between normal and poor readers (Beaumont and Rugg, 1978). These studies will only be referred to when they provide data relating to normal readers under ten years of age.
but knowledge of any corresponding neuroanatomical asymmetries is very scant
indeed. None the less, neuroanatomical asymmetries do exist. The most
thoroughly researched is the asymmetry of the planum temporale in the
posterior region of the superior surface of the temporal lobe (Geschwind and  
Levitsky, 1968). The planum temporale of the left temporal lobe, which forms
part of an area of known importance in language functions, is larger than or
equal in size to the planum temporale of the right temporal lobe in approximately 90 % of adults.  


The general finding has been one of RVF superiorities in normal readers down to as young as six years of age (Olson, 1973; Marcel, Katz and Smith, 1974; Marcel and Rajan, 1975; Carmon, Nachshon and Starinsky, 1976; and one of the experiments of Turner and Miller, 1975). Forgays (1953), however, did find an increase in visual hemifield asymmetry with increasing age. Turner and Miller (1975) and Butler and Miller (1979) reported larger asymmetries when using five- rather than three-letter words. Turner and Miller (1975) also found changes across age when using five-letter words, but not when using three-letter words, though Butler and Miller’s (1979) results did not confirm this observation. It is probable that these somewhat confusing results derive from a failure properly to control the characteristics of the words used. Longer words are more likely than short words to be abstract, and hence to produce larger visual hemifield asymmetries for reasons already mentioned. Conversely, the words recognized by young children under conditions of brief lateral presentation are likely to be mainly concrete, with older children recognizing a more even mixture of concrete and abstract words. Since smaller visual hemifield asymmetries derive from concrete than from abstract words a change in the size of the obtained visual hemifield asymmetry across age will ensue if scores from abstract and concrete words are pooled, but it has nothing to do with any possible difference across age in the organization of cerebral hemispheric functions. Studies which exercise proper control over the characteristics of stimulus words used are clearly needed.


Some developmental studies of left hemisphere specialization have tried using letters instead of words as stimuli. Of these, only the one reported by Carmon et al (1976) meets the minimal methodological requirements specified here. Carmon et al found traces of a developmental trend in visual hemifield asymmetry when using letters as stimuli, but not when using words. The absence of a developmental trend in asymmetry with words clearly implies that left hemisphere specialization for at least some of the skills involved in the recognition of visually presented linguistic stimuli was present at all ages. Beyond this, all that can be said is that letter recognition is not a very meaningful task for developmental comparisons, since Bryden and Allard (1976) have shown that even with adults the results obtained are easily affected by the difficulty experienced by subjects in reading the typeface employed. The more difficult typefaces tend to give LVF superiorities, and it is obviously the case that the difficulty of particular typefaces will vary across age.


174
Taken together, then, the findings of studies of children using visual hemifield stimulus presentations do not support the idea that the degree of asymmetry of organization of cerebral hemispheric functions varies across age. It has only proved possible to date to work with children down to age five, but this disadvantage is offset by the fact that some of the skills that can be studied are being learned at these ages, allowing the possibility of the investigation of initial stages of organization. What is now needed is a more precise analysis of the particular skills used at different ages for word recognition and other tasks, so that these skills can be examined separately. This might throw some interesting light on the role (or absence of any role) played by the right hemisphere in the early stages of learning to read. A related question which has not received the attention it deserves concerns the way in which the right hemisphere acquires the ability to recognize those words it can identify in adulthood. The results of studies by Ellis and Young (1977) and Young and Bion (1980b) suggest that the nature of the difference between the ‘visual vocabularies’ of the left and right hemispheres is semantic, and unrelated to the ages at which different words are first learnt.


Tactile presentation. Although both ipsilateral and contralateral somatosensory nerve connections exist, they are organized into discrete systems that probably serve different purposes (Wall, 1975). It is thought that ‘active’ touch and proprioception (Gibson, 1962) are mediated primarily through the contra- laterally organized pathway passing through the dorsal column and medial lemniscus, whilst passive touch, pain and temperature depend on the spinothalamic system, which has both ipsilateral and contralateral projections (Gazzaniga and Le Doux, 1978).


A useful review of the evidence relating to the role of different cerebral structures in tactile perception is given by Corkin (1978), who points out that there is no evidence for any asymmetry in elementary tactual functions. Tactile asymmetries only occur when the task used engages some higher-order function for which one cerebral hemisphere is superior (Corkin, 1978; Young and A. Ellis, 1979). Most of the findings of tactile asymmetries have derived from studies in which active tactile exploration of stimuli was required.




It is often the case that left hand superiorities are found for complex tactile perception, but in some cases right hand superiorities are observed. Cioffi and Kandel (1979) found a right hand superiority for identifying two-letter abstract words by touch, which was present down to age six. A right hand superiority for the report of sequentially touched fingers was found down to age seven by Bakker and Van der Kleij (1978).


Is such an asymmetry present in the brains of babies? It is quite clear that the
Left hand superiorities for the identification of tactually perceived nonsense shapes have been found down to age six by Witelson (1974,1976a) and by Cioffi and Kandel (1979). These left hand superiorities did not increase with increasing age, but inconsistent sex differences were observed. Using an accuracy measure, Flanery and Balling (1979) also found a left hand superiority for this type of task which did not vary across age down to age seven. However, when they computed laterality coefficients’, differences across age were observed by Flanery and Balling. Since the computation of such coefficients involves several unjustified theoretical assumptions (Colbourn, 1978), and many different coefficients are available that may all lead to differing outcomes, it is not possible to satisfactorily interpret this particular result.
answer is yes. Studies by Teszner et al (1972), Witelson and Pallie (1973) and  
Wada et al (1975) have demonstrated differences in the relative sizes of the left  
and right planum temporale of the foetal, newborn and infant brain. Opinions
differ as to whether the degree of this neuroanatomical asymmetry increases
between infancy and adulthood. This is hardly surprising, since it is by no means
clear which measurements should be used to effect such a comparison. There is
no disagreement, however, that the nature of the neuroanatomical asymmetry
does not differ between infants and adults.  


It is clear, then, that if functional asymmetries are found in the infant brain,  
In some studies, Braille patterns of raised dots have been used as stimuli. Hermelin and O’Connor (1971a, 1971b) found that blind adults and children aged eight to ten years were better at reading Braille with the left than right hand. Rudel et al (1974), however, found that sighted children did not learn Braille letters more accurately using the left hand until they were over ten years of age. Witelson (1977a) objected that the use of a naming task with raised dot stimulus patterns confounds the linguistic and spatial components of the task, but Rudel et al (1977) repeated the finding in a study that required that raised dot configurations only be discriminated, not named.
this would not conflict with neuroanatomical knowledge. Similarly, the  
existence of neuroanatomical hemispheric asymmetries in the newborn makes it
difficult (though not impossible) to believe in the complete equipotentiality of  
the cerebral hemispheres for language functions. On the other hand, as Witelson
(1977a) points out, the existence of a neuroanatomical asymmetry is not in itself
sufficient to imply that the cerebral hemispheres function asymmetrically in
infancy. It may only represent the structural bias underlying later developing
functional specializations. Because neuroanatomical findings are ambiguous in
this way, it is necessary to look at results deriving from other methods.  


Noninvasive methods
One curious aspect of Rudel et a/.’s (1977) findings was that not only did children aged over ten years show left hand superiority, but children below ten showed a tendency toward right hand superiority. Attention has been drawn to this because it illustrates the danger inherent in regarding the results of studies of this type as direct measures of asymmetry of cerebral organization. Surely no- one would want to maintain that spatial functions moved from the left to the right hemisphere at age ten? What is evidently happening is that the type of task used by Rudel and her colleagues can be approached using more than one solution strategy, and the younger children rely on a method that involves the left hemisphere to some extent. Their conclusion should thus have been not that cerebral asymmetry varies across age but that more needs to be known about the possible ways in which subjects can approach this type of task. A similar point has been made by Bertelson (1978).


A number of methods have been devised in an attempt to study functional
It should by now be clear that the minimum requirement for demonstrating that the extent to which particular cerebral hemispheric functions are asymmetrically organized changes across age is to show that younger children do not give a lateral superiority when using the same method of dealing with the given task that produces the lateral superiority observed in older children. This requirement applies generally to studies using auditory, visual or tactile presentation, and it has never been met by any of the studies claiming to find developmental differences in asymmetric cerebral organization. Consequently, the only valid conclusion at present with regard to tactile asymmetries is that left or right hand superiorities for tactile perception can be demonstrated in children down to at least age six under appropriate conditions.
asymmetries in the normal, intact brain. Following Witelson’s (1977a)
terminology, these will be referred to as noninvasive methods.  


It is possible, for instance, to study asymmetries of motor control of parts of  
Studies of asymmetries in infants for processing auditorily or visually presented stimuli. The failure of studies of asymmetries during childhood for processing laterally presented stimuli to provide any convincing evidence of changes across age in the asymmetric organization of cerebral hemispheric functions, and the existence of neuroanatomical asymmetries in infants, has led researchers to explore the possibility that functional cerebral asymmetries are present in infancy. A number of techniques have been devised, mostly using electrophysio- logical measures.
the body, and lateral preferences. Ontogenetic studies of lateral preference have
been carried out for a long time. More recently attention has also been given to  
asymmetries following auditory, visual or tactile stimulus presentations, and
these procedures have been adapted for use with children and, in some cases,
infants.  


In examining these noninvasive methods, studies of asymmetries in children
Electrophysiological studies have shown cerebral hemisphere differences in early infancy in terms of auditory and visual evoked potentials (Molfese et al, 1975; Molfese et al , 1976; Davis and Wada, 1977; Molfese, 1977; Molfese and Molfese, 1979), EEG power distributions (Davis and Wada, 1977; Gardiner and Walter, 1977), and photic driving (Crowell et al , 1973).
for processing auditory, visual and tactile stimuli will each be considered in turn.
Studies involving the auditory or visual presentation of stimuli to infants will
then be discussed, and finally studies of asymmetries of motor control and lateral
preferences.  


Auditory presentation. The principal auditory nerve connections are contralateral, so that material presented to the right ear is directed to the left cerebral  
The dichotic stimulation technique has been adapted in order to demonstrate cerebral asymmetries in infants by Glanville et al. (1977), who used a response measure based on heart rate habituation. Entus (1977) also used dichotic stimulation with a sucking response, but a subsequent study (Vargha-Khadem and Corballis, 1979) has not been able to replicate her results.
hemisphere and material presented to the left ear is directed to the right cerebral
hemisphere. However, substantial ipsilateral auditory nerve connections
between the left ear and left hemisphere and between the right ear and right
hemisphere also exist.  


The findings of these several studies of hemisphere function in infancy convincingly demonstrate that asymmetric organization of function is present, which is incompatible with Lenneberg’s (1967) views. Most investigators have, however, been satisfied to establish the basic point that functional asymmetries can be shown in infancy. Whilst the similarity of the asymmetries found in infants to those found in adults is usually obvious, this tactic avoids questions as to the precise mechanisms involved, and leaves open the possibility that some changes across age might occur. However, since studies of asymmetries for processing perceptually presented stimuli in infants and children have so consistently failed to produce any satisfactory supporting evidence for the notion of progressive lateralization of abilities, it is unreasonable to believe that such changes do occur unless strong supporting evidence can be found elsewhere.


Asymmetries of motor control and lateral preferences. It was mentioned in the introduction to this chapter that the principal innervation of movements of the body is mediated through contralateral nerve tracts. Although ipsilateral nerve fibres also exist, their role is not fully understood, but it is thought that their influence is confined to relatively gross movements. An example might be moving a hand by moving the whole arm. Fine motor movements, and especially movements from the level of the wrist of the hands and fingers, are seen as normally involving a relatively high degree of contralateral control (Brinkman and Kuypers, 1972, 1973; Trevarthen, 1974,1978). For this reason, in examining asymmetries of motor control and lateral preferences, particular attention will be paid to the fine control of movements of the hands and fingers.


ASYMMETRY OF CEREBRAL HEMISPHERIC FUNCTION
An important distinction which needs to be made when considering motor asymmetries concerns the difference between lateral preference and relative skill (Annett, 1970; Ingram, 19757?; G. Young, 1977). This is perhaps best illustrated by means of an example. Most right-handed people will always write with their right hand, and nearly always pick up a pen with their right hand. The degree of preference for use of the right hand is similar for both activities. If, however, a right-handed person is asked to carry out these activities using his left hand, he is not likely to experience any difficulty in picking up the pen, but left-handed writing will prove to be much more tricky. The degree of relative skill of the hands for both activities is quite different. It is evident that relative manual skill and hand preference are not the same thing, though they are related (Annett, 1976). Their relation is probably most close for the more difficult and skilled tasks, as Brown (1962) found. Even with difficult tasks, however, the relation of preference and relative skill is not exact, and it is possible to find motor tasks that right-handed people can better execute with the left hand (Kimura and Vanderwolf, 1970). In addition to the contribution of asymmetry of cerebral hemispheric motor functions, hand preference can involve an element of choice, with one hand often being preferred regardless of whether the activities might cause considerable or little difficulty to the other. This means that studies of relative skill of the hands at different ages are of more direct relevance to asymmetry of cerebral hemispheric function than are studies of hand preferences (Denckla, 1974; G. Young, 1977).


The distinction of studies of lateral preference from studies of relative skill makes the results of an otherwise confusing body of studies of motor asymmetries during development fall into a neat pattern. Put simply, studies of relative skill have not found increases in asymmetry across age (one or two have actually found decreases), whereas studies of lateral preference have generated a mixture of results seen as indicating changes in lateral preference and results indicating absence of change in lateral preference across age.


175
A favourite type of task in studies of relative skill has involved comparisons between the hands for the highest speed or greatest accuracy with which repetitive movements can be carried out. Examples would be moving pegs on a pegboard, or tapping rhythms, and studies of this type which have used children down to age five or below include those of Knights and Moule (1967), Annett (1970), Denckla (1973, 1974), Ingram (19755), Finlayson (1976), and Wolff and Hurwitz (1976). The total range of ages covered by these studies is from three to sixteen years. All found right hand superiorities, and none produced evidence of an increase in the degree of right hand superiority with increasing age. In some cases, however, asymmetries were found to decrease in magnitude with increasing age (Denckla, 1974; Wolff and Hurwitz, 1976). These results may be attributable to a decrease in sensitivity of particular tasks across the considerable ranges of ages used in the studies concerned. They do, however, also raise the interesting possibility that there may be changes across age in the extent of asymmetric organization of some skills which do not take the form specified by the concept of lateralization.


Other tests of relative skill which have led to right hand superiorities include hand strength (Ingram, 19755; Finlayson, 1976), speed of writing (Reitan, 1971), and duration of grasp of a rattle (Caplan and Kinsbourne, 1976). Caplan and Kinsbourne’s finding, from a study of two- to four-month-old babies, remains the earliest demonstration of a manual asymmetry.


When different linguistic stimuli (such as spoken digits) are presented
In two tasks used in Ingram’s (1975 5) study of three- to five-year-old children, which involved imitating hand postures or finger spacings, left hand superiorities were obtained, presumably reflecting the right hemisphere’s superiority for the complex spatial component of the tasks. This finding can thus be seen as both confirming the presence of superior right hemisphere spatial functions at age three and illustrating the importance of distinguishing questions of relative manual skill for different tasks from those of hand preference.
simultaneously, one to each ear, the stimulus presented to the right ear tends to
be reported more accurately than that presented to the left ear (Kimura, 1961,
1967). This general method has come to be known as dichotic stimulation, and is
readily adapted for use with children. The finding of right ear superiority for
linguistic material would seem to reflect its more efficient transmission to the
specialized language areas of the left cerebral hemisphere, but it has also been
thought that the ascendancy of the contralateral over the ipsilateral auditory
nerve connections is particularly marked when both ears are simultaneously
stimulated (Kimura, 1967; Cohen, 1977). When material is presented to one ear
at a time the differences between ears are small and their demonstration requires
the use of sensitive measures (Studdert-Kennedy, 1972; Fry, 1974; Morais and
Darwin, 1974) or difficult tasks (Bakker, 1969, 1970; Frankfurter and Honeck,
1973; Van Duyne et al , 1977).  


As well as its use in investigating the language specializations of the left  
An interesting variation on the basic studies of relative skill on single tasks involves dual-task performance. Studies of adults have demonstrated that requiring them to talk whilst carrying out an independent manual task interferes more with right than with left hand performance (e.g. Kinsbourne and Cook, 1971; Hicks, 1975). Such interference probably occurs when speech and right hand movements demand the use of common left hemisphere functions (Lomas and Kimura, 1976). Studies of interference in dual-task performance in children down to age three have shown that the same types of effect occur (Kinsbourne and McMurray, 1975; Piazza, 1977; Hiscock and Kinsbourne, 1978). The only hint of any change across age arises in the report of McFarland and Ashton (1975), but since their groups contained as few as four subjects, sampling bias cannot be ruled out.
hemisphere, the dichotic stimulation technique can also be used to study right
hemisphere (and hence left ear) superiorities for the processing of some
nonlinguistic auditory stimuli (Gordon, 1970, 1974; Knox and Kimura, 1970).  
For clarity and convenience the use of dichotic stimulation techniques to study
the development of left and right hemispheric abilities will be discussed
separately.  


The studies of the ontogeny of right hemisphere superiorities for processing
Early studies of motor asymmetries in infants and children were almost exclusively addressed to questions of lateral preference (e.g. Wile, 1934; Giesecke, 1936; Gesell and Ames, 1947; Hildreth, 1949). Although most of these studies would now receive low marks for adequacy of methodology and clarity in reporting what was actually done they were in general agreement that lateral preferences, and especially hand preferences, are established gradually throughout childhood, with periods of absence of preference or preferences opposite to those finally adopted. Some more recent studies have also reported results of this type both for infants (Cohen, 1966; Cernacek and Podivinsky, 1971; Seth, 1973; Ramsay, 1979) and for older children (Belmont and Birch, 1963), though there are also studies that have not found changes in hand preference across age in infancy (Ramsay, Campos and Fenson, 1979) or childhood (Annett, 1970).
nonlinguistic sounds can be quickly dealt with, as few have been carried out. The
principal studies are those of Knox and Kimura (1970) and Piazza (1977).
Neither of these studies, nor the two unpublished studies referred to by Witelson
(1977a), found any change in the left ear advantage across age in the range three
years to adult.  


The overwhelming majority of dichotic stimulation studies involving children
The explanation of these discrepant findings from studies of hand preference lies in the measures used. Annett (1970) and Ramsay et al (1979) both studied hand preference for quite difficult skills. Hand preference for difficult skills is, for reasons explained previously, likely to be relatively closely related to differential skill, and it is studies of differential skill which do not tend to find changes across age. Most of the studies which did find developmental trends in hand preference used measures based on preference for picking up objects. There is no reason to assume that preference for the same actions is being examined at different ages, since there are a number of different ways of manipulating and picking up objects (Elliott and Connolly, 1973; Kopp, 1974; Bresson et al, 1977).
have been addressed to the development of left hemisphere specializations.  
Witelson (1977a) gives a detailed summary of the methods and findings of over
30 published and unpublished studies carried out up to 1976. These studies differ
on many points of methodology. Considering only the published studies
reviewed by Witelson, the stimuli used included isolated speech sounds and  
nonsense syllables (Berlin et a/., 1973; Dorman and Geffner, 1974; Geffner and
Dorman, 1976), spoken digits (Kimura, 1963, 1967; Inglis and Sykes, 1967;
Bryden, 1970; Knox and Kimura, 1970; Geffner and Hochberg, 1971; Satz et al ,
1971; Sommers and Taylor, 1972; Satz et al, 1975; Witelson, 1976a, 19766;
Kinsbourne and Hiscock, 1977; Bryden and Allard, 1978), words (Knox and
Kimura, 1970; Nagafuchi, 1970; Sommers and Taylor, 1972; Goodglass, 1973;
Ingram, 1975a) and animal names (Bever, 1971). In some studies a report was
required after each pair of stimuli, whilst in others two, three or even four pairs
were presented before report of as many stimuli as possible was required. Both


A type of investigation involving motor asymmetries which does not really fit into the scheme of studies of relative skill or studies of lateral preference also deserves mention. In several studies Turkewitz and his colleagues have shown that very young infants turn their heads more often to the right than to the left (e.g. Turkewitz et al ., 1965; Turkewitz et al, 1969; Turkewitz and Creighton, 1975). Although the demonstration of any motor asymmetry at early ages is of interest, no really satisfactory explanation as to the cause of the bias in head turning has been offered.


Studies of the consequences of cerebral injury at different ages


176
The findings of studies of infants and children using noninvasive methods have failed to provide evidence indicating that the extent to which particular cerebral hemispheric functions are symmetrically or asymmetrically organized changes across age in the manner implied by the concept of progressive lateralization of abilities. Moreover, they have shown that asymmetry of hemispheric function is present in some form in infancy. Both outcomes are clearly at variance with Lenneberg’s (1967) theoretical position. In fairness, however, it must be pointed out that most of this evidence was not available to Lenneberg, and that his theory was mainly derived from studies of hand preference and from studies of the consequences of cerebral injury at different ages.


It is notoriously difficult to draw valid inferences concerning the organization of cerebral functions from the effects of cerebral injuries, and this difficulty is compounded when it is necessary to draw conclusions about possible organizational differences across age. Several of the problems of methodology and interpretation that can arise have detailed in the reviews of Kinsbourne (1976) and Witelson (1977a), which seriously criticized many of the interpretations that have been offered. This is not, of course, to deny the great importance of studies of the developmental sequelae of cerebral injuries, but their relevance to understanding asymmetry of cerebral hemispheric function during development has often been overestimated and misunderstood.


The four main aspects of studies of the consequences of cerebral injury that have received attention will be examined in turn. These are the differences across age in the extent of recovery from unilateral cerebral injuries, the claim of the equipotentiality of the cerebral hemispheres for language acquisition, differences across age in the nature of aphasic symptoms, and the possible involvement of the right hemisphere in the early stages of language acquisition. As it will become clear that many of the studies carried out add little or nothing to our understanding of cerebral asymmetry, a systematic review of all the studies will not be attempted.


Age and the extent of recovery from unilateral cerebral injuries. Many studies of language disturbances in children following left hemisphere injury have shown that the younger the child the more rapid and complete is the recovery. Reviews are provided by Basser (1962), Lenneberg (1967) and Witelson (1977a); see also Parker, this volume. This recovery may be in part due to intrahemispheric reorganization of functions within the damaged left hemisphere (Hecaen, 1976), but it is known from cases where the extent of the injury eventually led to left hemispherectomy that considerable acquisition of language functions by the right hemisphere is possible in the first years of life.


These findings have often been taken to indicate that the lateralization of language abilities proceeds gradually throughout childhood, with the right hemisphere being involved in language functions in the early years. In fact, the findings do not indicate this at all. They simply attest to the remarkable ability of the young brain to recover and reorganize functions in response to injury. This is a complex phenomenon in its own right, widespread throughout the animal kingdom, to which there are a number of different contributory processes (Hecaen and Albert, 1978; Lund, 1978). None the less, the finding that recovery can take place tells us nothing about the organization of function before injury. There is no reason to connect loss of‘plasticity’ with an increase in lateralization.


vocal and non vocal (such as pointing to a picture of a word’s referent) methods
The claim of the equipotentiality of the cerebral hemispheres for language acquisition. The extent of the recovery of language abilities following early left hemisphere injury is so marked that Lenneberg (1967) was led to the conclusion that the cerebral hemispheres are initially perfectly equipotential for language acquisition. This conclusion was supported by reports of the observations of clinicians, but more systematic and quantitative studies have shown that perfect equipotentiality does not obtain (Dennis and Kohn, 1975; Dennis and Whitaker, 1976; Dennis and Whitaker, 1977). Although extensive language functioning can be achieved by the right hemisphere following early injury to the left hemisphere, the left hemisphere is better able than the right to subserve language acquisition even in infancy.
of reporting were used. There were also differences as to whether only right-
handed children were used as subjects, and the criteria for establishing
handedness when this was done. In addition, a point not taken up by Witelson
(1911a) is that various different methods of aligning the left and right ear stimuli
for ‘simultaneous’ presentation have been tried (Morton et a/., 1976).  


Given that there have been such marked methodological differences between
Differences across age in the nature of aphasic symptoms. It has long been known that cerebral lesions causing disturbances of language in children (acquired aphasias) do not produce the same pattern of symptoms as found in adults (Guttman, 1942; Alajouanine and Lhermitte, 1965; Hecaen, 1976). The most common form of acquired aphasia in children involves difficulty with or absence of spontaneous expression (mutism), whilst jargonaphasia and logorrhea occur only in adults. Brown and Jaffe (1975) and Brown (1977) have extended these observations, arguing that the different types of aphasia are systematically related to age not only in childhood but throughout the human lifespan.
studies, the findings are surprisingly consistent. Almost all of the studies found
right ear superiorities for the processing of linguistic stimuli, and almost all
found right ear superiorities in the youngest groups of children studied. This has
also been true of reports published since Witelson’s review was written (e.g.
Mirabile et al, 1975; Borowy and Goebel, 1976; Geffen, 1976; Hynd and
Obrzut, 1977; Hiscock and Kinsbourne, 1977; Piazza, 1977; Geffen, 1978;
Geffen and Sexton, 1978; Geffen and Wale, 1979; Sextcn and Geffen, 1979). In a
number of the published reports (Nagafuchi, 1970; Bever, 1971; Ingram, 1975a;
Hiscock and Kinsbourne, 1977; Kinsbourne and Hiscock, 1977; Piazza, 1977)  
right ear superiorities have been demonstrated in children as young as three
years old. Moreover, none of the studies that have investigated such young
children has failed to find right ear superiorities.  


It is clear, then, that insofar as right ear advantages for reporting dichotic
Such differences across age in the nature of acquired aphasias are of undoubted intrinsic importance and interest, but what do they tell us about asymmetry of cerebral hemispheric function? They might indicate that, at the 'psychological’ level, the organization of language functions and the relative contribution made by different linguistic skills changes during the lifespan, with some skills being developed to the level of practised fluency at which jargon- aphasia and logorrhea can occur. These changes could, however, be associated with intrahemispheric development and organization of processes principally located in the left hemisphere, and the concept of lateralization is not needed.
linguistic stimuli are dependent on cerebral asymmetry for language functions,  
such asymmetries are present from at least three years of age. Supporters of the  
concept of progressive lateralization have, however, tended to see the most
important question as being not so much the ages at which ear asymmetries can  
be demonstrated, but rather whether the degree of right ear superiority increases
across age (Satz et al ., 1975). This is based on the contention that as the degree of
cerebral hemispheric functional asymmetry increases, the size of ear advantages
for dichotic stimulation should also increase. In other words, dichotic stimulation is regarded as a parametric measure of cerebral asymmetry. This view
requires more careful consideration.  


The first point that must be made is that even when dichotic stimulation
The possible involvement of the right hemisphere in the early stages of language acquisition. Although it has been customary to include them in discussions of asymmetry of cerebral hemispheric function during development, it is apparent that the lines of evidence concerning the consequences of cerebral injury at different ages described thus far are not really of central importance to the topic. There is one claim, however, which is potentially crucial, and which has been held apart from the others to show its special role in making the other lines of evidence appear to contribute more to our understanding of the problem than they actually do. The claim falls into two parts, which require separate consideration. Firstly, it is held that childhood aphasias are more likely than adult aphasias to occur as a consequence of injury to the right cerebral hemisphere, and secondly it is held that this implies that the right hemisphere is involved as well as the left hemisphere in the early stages of the acquisition of language functions.
scores are analysed by parametric statistical procedures most studies have not  
found the degree of right ear superiority for linguistic material to vary across age.  
In a small minority of studies, however, ear asymmetry was found to increase
with increasing age (Bryden, 1970; Satz et al, 1975; Bryden and Allard, 1978).
This raises the difficult question of the proper interpretation of findings of this
type.  


If it were the case that it is appropriate to regard dichotic stimulation as a
The evidence concerning the first part of the claim is not completely convincing. It is clear that the proportion of children over five years of age experiencing aphasic difficulties following left as opposed to right hemisphere injury is comparable to the proportion found for adults (Krashen, 1973; Hecaen, 1976). For children aged two to five years, however, aphasia following right hemisphere injury would seem to be relatively frequent from the cases reported in the literature. Witelson (1977 a) gives a rough figure of 30%, but there are several difficulties in taking such a figure at its face value, as Kinsbourne (1976) and Witelson (1977a) have stressed. These difficulties include the possibility that many of the right hemisphere injuries were so extensive as to also involve parts of the left hemisphere, the danger of bias toward referral to specialists and reporting of the more unusual cases (i.e. those where aphasia apparently followed right hemisphere injury) and the poverty of the assessments typically given as to the nature, severity and duration of the aphasic symptoms. These methodological problems are not caused by any lack of competence of investigators, and it is difficult to see how they could all be fully overcome. Kinsbourne (1976) concluded that the existence of such difficulties is sufficient to invalidate the reports indicating greater frequency of aphasias following right hemisphere injuries in young children than in adults; Witelson also advocated that such reports should be treated with caution.
parametric index of cerebral asymmetry, then the findings of Bryden and of Satz
might substantiate the idea that the degree of cerebral asymmetry increases with
increasing age. However, there are serious difficulties to be overcome before such  
a conclusion could be reached. No one has been able to demonstrate satis-


The attention paid to the methodological problems inherent in attempts to calculate the relative frequency of aphasias following right hemisphere injury in young children and adults has tended to draw attention away from the question of what the finding of a greater frequency in young children, if valid, should be taken to mean (a notable exception is the discussion by Moscovitch, 1977). Witelson (1977a) felt that it means that the right hemisphere may participate in the execution of language functions in the early stages of language acquisition, but that its contribution is always less than that of the left hemisphere. What needs to be clarified, though, is whether the right hemisphere’s contribution is of the same type as that made by the left hemisphere, as the concept of progressive lateralization of language abilities would imply, or whether it is important because of functions it can execute which would not normally be viewed as linguistic yet are integral to the early stages of language acquisition. Evidence from psychological studies of language acquisition, for instance, indicates that much of the initial organization involved is closely related to understanding of and interactions with the world of objects, events and other people (R. Brown, 1973; Lock, 1978). It is unfortunate that the level of analytic sophistication attained by psychologists has not been applied to neuropsychological studies of childhood aphasia. If this were done, differences between the types of aphasia following left and right hemisphere injuries sustained in childhood might be found. With mutism being the most common symptom this would obviously be difficult, but detectable differences could arise in the patterns of recovery.


At present, then, firm answers to the important questions that have arisen concerning the possible involvement of the right hemisphere in the early stages of language acquisition have not been provided by studies of childhood aphasias following right hemisphere injuries.


ASYMMETRY OF CEREBRAL HEMISPHERIC FUNCTION
==Overview and conclusions==


Having examined the available evidence concerning asymmetry of cerebral hemispheric function during development, it is now possible to consider what general conclusions can be drawn. This will necessarily involve discussion of what type of conceptual and theoretical framework is most useful in describing the existing findings and generating new lines of investigation.


177
The results of the numerous studies that have been carried out show that asymmetric organization of at least some cerebral hemispheric functions is characteristic of the human brain at all ages during postnatal development. Although considerable recovery and reorganization of function can take place following unilateral cerebral injury sustained early in life, the cerebral hemispheres are not equipotential for language acquisition. Thus the claims of absence of functional asymmetry in infancy and perfect hemispheric equi- potentiality for language put forward by Lenneberg (1967) are simply incorrect.


The question as to whether the degree to which functions are asymmetrically organized increases across age cannot be given such a straightforward answer, and requires some clarification. The total number of asymmetrically organized functions may well increase during the first years of life for the simple reason that many are acquired during this period. In this trivial sense, ‘laterality’ quite probably does increase across age. The concept of lateralization, however, is only of real interest as applied to particular functions, for which it implies that unilateral organization develops progressively from an initial organization that is at least to some extent bilateral. It is this sense that was clearly intended by Lenneberg (1967), Krashen (1973) and Brown and Jaffe (1975).


factorily that the sizes of ear asymmetries are sufficiently closely or uniquely
This hypothesis of progressive lateralization of abilities has not found adequate support in the studies that have been carried out, irrespective of whether it is regarded as valid or as invalid to use parametric statistical analyses. When findings have been claimed to demonstrate progressive lateralization of abilities, it has been shown that enthusiasm for the concept of lateralization has led to lack of attention to more prosaic alternative explanations. Of course, as has been pointed out, the available methods of investigation have not always been adapted for work with all ages of children, so that all of the conceivable lines of enquiry have by no means been exhausted. It thus remains possible for people to believe that substantial positive evidence of genuine progressive changes in lateralization will one day be found. However, this is more a statement of faith than a scientific inference, and a more realistic theoretical framework for research findings needs to be built up.
related to the degree of asymmetry of cerebral hemispheric function to serve as
an index (Berlin and Cullen, 1977; Witelson, 1977a; Colbourn, 1978). Although
it is probable that some modest relationship exists, there are many factors
besides cerebral asymmetry which may influence the magnitude of ear
advantages. A list of these factors would include the difficulty level of the task to  
particular subjects (and hence ‘ceiling’ or ‘floor’ effects), individual differences in
the relative functional predominance of contralateral and ipsilateral auditory
nerve fibres, different strategies for organizing reports of left and right ear
stimuli, and attentional biases toward a particular ear. Moreover, the level or
levels of stimulus processing at which ear asymmetries due to functional cerebral
asymmetries can arise are not properly understood, and few investigations have  
explicitly controlled for the possibility that the contribution to observed
asymmetries arising from different levels of information processing may vary
between subjects. All of these potential influences on the size of obtained ear
asymmetries are, of course, particularly likely to influence the outcomes of
developmental studies which must necessarily sample across wide ranges of ages.  


When these several factors are considered it is even more remarkable that the  
The research approach dictated by the concept of lateralization has been to look for progressive changes in childhood in the extent of the asymmetric organization of certain functions. This means that studies have often been directed toward the possibility of change in functions that are already adequately established. The typical investigative tactic has involved the use of one or two tasks and a wide range of ages of subjects. Such studies have been worthwhile insofar as they have led to the conclusion that progressive lateralization of already acquired functions does not take place. Further studies of this type can still be of value in filling in the many missing details. It may now be more interesting, however, to look for changes in organization whilst functions are actually being acquired. For this purpose, the concept of lateralization should be abandoned, since it arbitrarily predetermines what form such changes would be conceptualized as taking, and they may turn out to be more varied. There is, for instance, no reason to discount the possibility that for some skills the extent of asymmetric organization may actually decrease as they become firmly established and integrated into a child’s repertoire.
results of the majority of dichotic stimulation studies have been so consistent.  
The consistency is probably caused by most of the results happening to arise
from the same general source of asymmetry, namely the left hemisphere’s
superiority for speech decoding, and the few results that do not fit the main
pattern of absence of developmental trends in ear asymmetry are best discounted
until methods that allow more control over the factors involved are available.  
This conclusion is strengthened by the failure of Bakker, Hoefkens and Van Der
Vlugt (1979) to confirm the developmental trend of Satz et al (1975) using a  
longitudinal instead of a cross-sectional research design.  


From this discussion it is apparent that studies of ear asymmetry to dichotic
A useful approach, then, may be to define the basic problem as one of understanding how newly learned skills are integrated with existing functions that are already symmetrically or asymmetrically organized. This shifts emphasis on to the possibility of relatively rapid changes occurring whilst functions are being acquired rather than long-term changes in already acquired functions, and does not prescribe the form such changes might take. It would require careful studies directed toward quite specific skills at the ages at which they are learned. A few studies of this type have been achieved, and suggestions have already been offered where others are obviously necessary, but they demand precise methods of investigation which have only recently begun to be available. As such methods are developed the studies of isolated tasks across wide ranges of ages deriving from the conceptual framework dictated by the concept of lateralization will probably become of less interest than very detailed studies carried out whilst functions such as prehension, enumeration or reading are being acquired.
linguistic stimulation in children must develop better methods for controlling
unwanted sources of variance and for identifying the levels of stimulus
processing at which cerebral asymmetries arise. Some researchers are beginning
to do this. Most notably, an elegant series of studies by Geffen and her colleagues
(Geffen, 1976, 1978; Geffen and Sexton, 1978; Geffen and Wale, 1979; Sexton
and Geffen, 1979) has demonstrated that when attentional strategies are  
properly controlled there is no variation across age in the degree of right ear
advantage for speech perception. Conversely, Geffen also found that the ability
to deploy attentional strategies did vary across age, and that the use of  
attentional strategies can affect the size of obtained ear asymmetries, so that this
factor does need to be controlled.  


The identification of the levels of stimulus processing at which cerebral
asymmetries arise is more difficult to achieve than the control of attentional
strategies, but some progress is also being made. Consider, for instance, what


Acknowledgements


The assistance provided by SSRC grants HR 5078, HR 6398, and HR 6876 is gratefully acknowledged. I am very grateful to Andrew Ellis for helpful discussion of several points of interpretation.


178


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linguistic stimuli. Two broad classes of effect can be readily distinguished. These
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It is quite clear that a major contribution to obtained ear advantages is made
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account for most of the observed results. This is evident from the fact that many
of the ear asymmetries in the studies cited did not depend on the presentation of
complete words, but could be obtained when isolated speech sounds or nonsense
syllables were used as stimuli. It seems, too, that these speech decoding
asymmetries can arise at the level of immediate perceptual analysis, but are
heightened when a short-term memory component is introduced into the
experimental task (Oscar-Berman et al , 1974; Yeni-Komshian and Gordon,
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as Porter and Berlin (1975) point out. It is likely that tasks with a large shortterm memory component will produce developmental trends in ear asymmetry  
not because cerebral asymmetry changes across age but because of age
differences in short-term memory abilities and hence task sensitivity.  


Ear asymmetries belonging to the lexical class obviously cannot arise when
isolated speech sounds or nonsense syllables are used as stimuli. Although
findings of lexical class ear asymmetries have been made in studies of adults
using words as stimuli (McFarland et al, 1978; Kelly and Orton, 1979) they are
by no means always found (e.g. McFarland et al, 1978; Kelly and Orton, 1979;
Young and Ellis, 1980) and probably only arise under conditions that are not
typical of most dichotic stimulation studies. The only study to date that has
separately considered the possible implications for developmental findings of the
distinction between the classes of auditory asymmetries described here as due to
speech decoding and lexical factors has been that of Eling et al (1979).


In summary, then, studies of ear asymmetries to dichotic stimulation in  
Bakker, D. J. and Van Der Kleij, P. C. M. (1978) Development of lateral asymmetry in the perception of sequentially touched fingers. Acta Psychologica, 42, 357-365.
children indicate that left hemisphere specializations for speech decoding and
right hemisphere superiorities for the analysis of some nonlinguistic sounds are
present down to at least three years of age. In most of the studies carried out the
magnitude of ear asymmetries did not vary across age. In the few cases where the
degree of ear asymmetry did increase with increasing age there is no reason to


Barosso, F. (1976) ‘Hemispheric asymmetry of function in children’, in The Neuropsychology of Language (ed. R. W. Rieber), Plenum, New York, 157-180.


Barton, M., Goodglass, H. and Shai, A. (1965) Differential recognition of tachistoscopically presented English and Hebrew words in right and left visual fields. Perceptual and Motor Skills, 21, 431-437.


ASYMMETRY OF CEREBRAL HEMISPHERIC FUNCTION
Basser, L. S. (1962) Hemiplegia of early onset and the faculty of speech with special reference to the effects of hemispherectomy. Brain, 85, 427-460.


Beaumont, J. G. and Rugg, M. D. (1978) Neuropsychological laterality of function and dyslexia: a new hypothesis. Dyslexia Rev., 1,18-21.


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Belmont, L. and Birch, H. G. (1963) Lateral dominance and right-left awareness in normal children. Child Devel, 34, 257-270.


Berlin, C. I. and Cullen, J. K. (1977) ‘Acoustic problems in dichotic listening tasks’, in Language Development and Neurological Theory (eds. S. J. Segalowitz and F. A. Gruber), Academic Press, New York, 75-88.


believe that this was a consequence of any process of increasing lateralization of
Berlin, C. I., Hughes, L. F., Lowe-Bell, S. S. and Berlin, H. L. (1973) Dichotic right ear advantage in children 5 to 13. Cortex, 9, 394-402.
cerebral hemispheric function.  


Visual presentation. The optic nerve pathways are organized in such a way that
Bertelson, P. (1978) Interpreting developmental studies of human hemispheric specialization. Behavioral and Brain Sci., 1, (2), 281-282.
information about visual stimuli falling to the left of the point at which a person
is looking (in the left visual hemifield) is projected initially to the right cerebral
hemisphere, whilst information about stimuli falling to the right of the point at
which a person is looking (in the right visual hemifield) is projected initially to
the left cerebral hemisphere. It is not established with certainty whether or not
there is some degree of ipsilateral optic projection for stimuli falling close to the
visual midline in the foveal and parafoveal regions of the retina, but outside this
disputed area the projections are known to be exclusively contralateral (Cohen,
1977; Haun, 1978). This should not be taken as meaning that the left eye sends
projections only to the right hemisphere. The fields of vision of each of the eyes
overlap to a considerable extent, so that most left or right visual hemifield
stimuli are seen by both eyes, and the contralateral optic projections consequently arise from a grouping together at the optic chiasm of the nerve fibres
from the corresponding side of the retina of each eye. Because of this anatomical
arrangement, the phenomena of eye dominance bear no clear relation to
cerebral asymmetry (Porac and Coren, 1976), and will not be discussed.  


If we know where a person is looking, then, it is possible to present visual
Bever, T. G. (1971) ‘The nature of cerebral dominance in speech behaviour of the child and adult’, in Language Acquisition: Models and Methods (eds. R. Huxley and E. Ingram), Academic Press, New York, 231-261.
stimuli in such a way that information is initially projected to whichever cerebral
hemisphere we choose. Unfortunately, the presentation of a visual stimulus
usually leads to an involuntary movement of the eyes to bring it into central
vision. It is thus necessary to restrict the presentation time of stimuli to a time
less than that needed to make such an eye movement. Estimates of this time vary,  
but it is usual to regard presentation times of 200 milliseconds (one-fifth of a
second) or less as acceptable (Cohen, 1977).  


The need to use briefly presented stimuli falling outside central vision
Branch, C., Milner, B. and Rasmussen, T. (1964) Intracarotid sodium amytal for the lateralization of cerebral speech dominance. J. Neurosurgery, 21, 399-405.
obviously places a serious constraint on what can be studied using this
technique, but a surprising amount has been achieved despite the limitations. It
must be made clear, however, that the method can only permit the initial
projection of stimulus information to one cerebral hemisphere or the other.  
What happens after that is not well understood, though it is probable that
information is coordinated by means of the neocortical commissures, and the
anterior commissure in particular (Risse et al ., 1978). Most investigators have
been sufficiently reassured by the contralateral nature of the optic pathways to
use unilateral stimulus presentations (in which stimuli appear only in one visual
hemifield), but a case that bilateral presentation (in which different stimuli
appear simultaneously in each of the visual hemifields) is a rather better
procedure can be made out (McKeever and Huling, 1971; Hines, 1975).  
Although methods that can allow continuous lateralized input have been


Bresson, F., Maury, L., Pieraut-Le Bonniec, G. and De Schonen, S. (1977) Organization and lateralization of reaching in infants: an instance of asymmetric functions in hands collaboration. Neuropsychologia, 15, 311-320.


Brinkman, J. and Kuypers, H. G. J. M. (1972) Splitbrain monkeys: cerebral control of ipsilateral and contralateral arm, hand and finger movements. Science, 176, 536-539.


180
Brinkman, J. and Kuypers, H. G. J. M. (1973) Cerebral control of contralateral and ipsilateral arm, hand and finger movements in the splitbrain rhesus monkey. Brain, 96, 653-674.


Borowy, T. and Goebel, R. (1976) Cerebral lateralization of speech: the effects of age, sex, race and socioeconomic class. Neuropsychologia, 14, 363-370.


Broman, M. (1978) Reaction-time differences between the left and right hemispheres for face and letter discrimination in children and adults. Cortex, 14, 578-591.


Brown, J. L. (1962) Differential hand usage in three-year-old children. J. Genet. Psychol., 100, 167-175.


Brown, J. W. (1977) Mind, Brain and Consciousness: The Neuropsychology of Cognition. Academic Press, New York.


developed (e.g. Zaidel, 1975) these have not been adapted for use with children.  
Brown, J. W. and Jaffe, J. (1975) Hypothesis on cerebral dominance. Neuropsychologia, 13, 107-H0.


When words are presented briefly in the left or the right visual hemifield and
Brown, R. (1973) A First Language: the Early Stages. Allen and Unwin.
right-handed adults are asked to name them it is usual to find a right visual
hemifield (RYF) superiority (Mishkin and Forgays, 1952; McKeever and
Huling, 1971). This RVF superiority is principally due to information about
words falling in the RVF being directly projected to the left cerebral hemisphere.
However, it has also been claimed to relate to the fact that English is read from
left to right. The argument in this case is that the memory trace of the stimulus is
initially ‘examined’ by the subject with a left to right scan starting from the point
of fixation (Heron, 1957; White, 1969, 1972, 1973). Hence, the RVF superiority
would arise from a ‘post-exposural trace-scanning’ mechanism deriving from
experience in reading.  


This trace-scanning notion no longer needs to be taken very seriously. It can
Bryden, M. P. (1970) Laterality effects in dichotic listening: relations with handedness and reading ability in children. Neuropsychologia, 8, 443-450.
be varied so freely as to become almost unfalsifiable, and even if true it could
only be making a minor contribution to the patterns of results found in studies
that have used words as stimuli rather than arrays of unrelated letters
(McKeever, 1974; Pirozzolo, 1977). It is known, for instance, that the RVF
advantage holds for vertically as well as horizontally arranged words and for
words in the Hebrew language, which is read from right to left (Barton et al ,
1965; Carmon et al , 1976). Furthermore, the size of the RVF superiority is not
constant for all types of word, but has been shown to be larger for abstract than
concrete words (Ellis and Shepherd, 1974; Hines, 1976, 1977). This pattern of
results is most readily interpreted by postulating that both cerebral hemispheres
of the adult brain possess at least rudimentary abilities to decode print stimuli,
so that the word-class effect derives from the different types of word that can be
recognized by the left and right hemispheres.  


Another convincing reason for interpreting the results of studies using brief
Bryden, M. P. and Allard, F. (1976) Visual hemifield differences depend on typeface. Brain and Language, 3, 191-200.
lateral presentations of visual stimuli in terms of functional cerebral asymmetry
is that in several studies using nonlinguistic visual stimuli left visual hemifield
(LVF), and hence presumably right hemisphere, superiorities have been demonstrated (Kimura and Durnford, 1974). Face recognition has proved to be a
particularly useful task in this respect, with many subsequent reports confirming
the findings of LVF superiorities by Rizzolatti et al (1971) and Hilliard (1973).  


The use of the visual modality of stimulus presentation in studies of
Bryden, M. P. and Allard, F. (1978) ‘Dichotic listening and the development of linguistic processes’, in Asymmetrical Function of the Brain (ed. M. Kinsbourne). University Press, Cambridge, 392-404.
asymmetry of cerebral hemispheric function in children is potentially of great
interest because of the wide range of skills that can be examined and the
considerable range of ages at which the differing skills are learnt. The ability to
identify visually represented words, for instance, is achieved at a much older age
than is the ability to recognize faces. Unfortunately, a large rumber of
theoretical and methodological difficulties are encountered in the case of visual
presentation, and progress has been slow in comparison with that made by


Butler, D. C. and Miller, L. K. (1979) Role of order of approximation to English and letter array length in the development of visual laterality. Devel. Psychol, 15, 522-529.


Caplan, P. J. and Kinsbourne, M. (1976) Baby drops the rattle: asymmetry of duration of grasp by infants. Child Devel., 47, 532-534.


ASYMMETRY OF CEREBRAL HEMISPHERIC FUNCTION
Carmon, A., Nachshon, I. and Starinsky, R. (1976) Developmental aspects of visual hemifield differences in perception of verbal material. Brain and Language, 3, 463-469.


Carter, G. L. and Kinsbourne, M. (1979) The ontogeny of right cerebral lateralization of spatial mental set. Devel. Psychol, 15, 241-245.


181
Cernacek, J. and Podivinsky, F. (1971) Ontogenesis of handedness and somatosensory cortical response. Neuropsychologia, 9 , 219-232.


Cioffi, J. and Kandel, G. L. (1979) Laterality of stereognostic accuracy of children for words, shapes, and bigrams: a sex difference for bigrams. Science. 204 , 1432-1434.


studies using dichotic stimulation. There have not been nearly as many studies
Cohen, A. I. (1966) Hand preference and developmental status of infants. J. Genetic Psychol, 108 , 337-345.
carried out, and several of those that have been attempted are seriously flawed.  


The most pressing methodological requirement in studies of visual hemifield
Cohen, G. (1972) Hemisphere differences in a letter classification task. Perception and Psychophysics, 11,139-142.
asymmetries in children is to control fixation. Studies of adults usually rely on a
central fixation spot, which subjects are asked to fixate before each stimulus is
presented. This procedure is obviously of dubious validity in a developmental
investigation. Children may fail to fixate when instructed to do so for a number
of reasons. The consequence of a failure by children to fixate when instructed is
that stimuli will not fall in the positions in the visual field intended by the
experimenter, and will probably be distributed randomly, thus reducing or
eliminating ‘visual hemifield’ differences. Since younger children will be more
likely to fail to fixate than older children, a bias will be introduced making it
probable that findings of differences in asymmetries across age will arise as an
artifact of lack of fixation. Moreover, the temptation not to fixate when
instructed may be itself related in a complex manner to the difficulty of
particular experimental tasks to particular subjects. For these reasons, some
form of fixation control is necessary in developmental studies of visual hemifield
asymmetries, and all developmental trends found in studies without adequate
fixation control (such as Jeeves, 1972; Miller and Turner, 1973; Barosso, 1976;
Reynolds and Jeeves, 1978a, 1978b; Tomlinson-Keasey et al ., 1978) must be
discounted as irrelevant to any considerations of asymmetry of cerebral
hemispheric function.  


A difficulty which is partly methodological and partly theoretical is that of
Cohen, G. (1977) The Psychology of Cognition. Academic Press.
ensuring that subjects of different ages are relying on the same cognitive
processes or strategies when faced with a given task. It is often assumed that the
use of linguistic stimuli will automatically engage specialized left hemisphere
mechanisms and lead to a RVF advantage, whilst the use of nonlinguistic stimuli
will produce no visual hemifield difference or a small LVF advantage. Cases are
known in the adult literature, however, where this generalization breaks down.
Matching tasks provide a simple example. Suppose that a pair of words or a pair
of letters is presented in one visual hemifield, and subjects are asked to say
whether they are the same or different. This can be determined either by
comparing the physical appearances of the stimuli (physical match) or by
naming them and comparing the names (name match). Studies by Cohen (1972)
and Gibson, Dimond and Gazzaniga (1972) have demonstrated that whereas
name matches yield RYF advantages, physical matches may be more effectively
carried out for LVF stimuli. The implication for developmental studies is that if
matching tasks are used they must be arranged in such a way that subjects are
forced to adopt only one of the possible strategies. If this is not done, differences
across age may simply be attributable to strategy differences. Witelson (19776)
first noticed this potential artifact in one of her own studies, but the criticism
applies equally to the differences across age found by Tomlinson-Keasey et al.  


Colbourn, C. J. (1978) Can laterality be measured? Neuropsychologia, 16 , 283-289.


Corballis, M. C. and Morgan, M. J. (1978) On the biological basis of human laterality: I. Evidence for a maturational left-right gradient. Behavioral and Brain Sciences, 1, (2), 261-269.


182
Corkin, S. (1978) The role of different cerebral structures in somesthetic perception’, in Handbook of Perception, VIB: Feeling and Hurting (eds. E. C. Carterette and M. P. Friedman), Academic Press, New York, 105-155.


Crowell, D. H., Jones, R. H., Kapuniai, L. E. and Nakagawa, J.. K. (1973) Unilateral cortical activity in newborn humans: an early index of cerebral dominance? Science, 180 , 205-208.


Davis, A. E. and Wada, J. A. (1977) Hemispheric asymmetries in human infants: spectral analysis of flash and click evoked potentials. Brain and Language, 4, 23-31.


Denckla, M. B. (1973) Development of speed in repetitive and successive finger movements in normal children. Devel. Med. Child Neurol., 15 , 635-645.


Denckla, M. B. (1974) Development of motor coordination in normal children. Devel. Med. Child Neurol, 16 , 729-741.


(1978) and in Broman’s (1978) experiment involving matching pairs of letters.  
Dennis, M. and Kohn, B. (1975) Comprehension of syntax in infantile hemiplegics after cerebral hemidecortication: left-hemisphere superiority. Brain and Language, 2 , 472-482.
The general point that it is important to know how subjects actually approach
experimental tasks applies, of course, to a lot more than just matching tasks.  


Having made these methodological cautions and eliminated some of the more
Dennis, M. and Whitaker, H. A. (1976) Language acquisition following hemidecortication: linguistic superiority of the left over the right hemisphere. Brain and Language, 3,404-433.
poorly designed studies, the principal studies of visual hemifield asymmetries in
children will now be considered, starting with studies of right hemisphere (LVF)
superiorities.  


The most common task used to investigate LVF superiorities, as in the adult
Dennis, M. and Whitaker, H. A. (1977) ‘Hemispheric equipotentiality and language acquisition’, in Language Development and Neurological Theory (eds. S. J. Segalowitz and F. A. Gruber), Academic Press, New York, 93-106.
literature, has been face recognition. Young and Ellis (1976) found LVF superiorities for face recognition in five-, seven- and eleven-year-old children, with
no differences across age in the degree of visual hemifield asymmetry. Broman
(1978) found no developmental differences in LVF superiority for face recognition in the age range seven years to adult. Marcel and Rajan’s (1975) study of
seven-year-old children also showed a LVF superiority for face recognition. In
contrast, failures to find LVF superiority in seven- and eight-year-old children
have been reported by Leehey (1976) and Reynolds and Jeeves (1978b).  
Reynolds and Jeeves’ study, however, lacks adequate fixation control. Leehey
(1976) reports three developmental experiments on visual hemifield asymmetries
for face recognition by subjects aged eight to adult that are in most respects
carefully designed. When she used bilaterally presented photographs of the faces
of people known to her subjects a LVF superiority was found at all ages, but
with bilaterally presented unfamiliar faces the eight-year-old children gave no
visual hemifield difference in two experiments. Unfortunately, Young and Bion
(1980a) were unable to replicate this result, and have suggested that it was
probably due to an age difference in directional reporting strategies arising from
Leehey’s use of bilateral stimuli without controlled order of report. Studies of
visual hemifield asymmetries thus give no grounds at present for claiming any
developmental change in the extent of the right hemisphere’s superiority for face
recognition.  


Studies of right hemisphere superiorities using visual presentation and tasks
Dewson, J. H. (1976) ‘Preliminary evidence of hemispheric asymmetry of auditory function in monkeys’, in Lateralization in the Nervous System (eds. S. Harnad, R. W. Doty, L. Goldstein, J. Jaynes and G. Krauthamer). Academic Press, New York, 63-74.
other than face recognition have also failed to reveal developmental trends.  
Witelson (1977b) found a LVF superiority for matching pictures of human
figures (a task which can only be done by means of a physical match) in boys
aged six to thirteen years. Witelson (1977a) described an unpublished experiment finding a tendency to greater LVF accuracy for dot enumeration (p < 0.1)
in six- to thirteen-year-old boys. Young and Bion (1979) found greater LVF
accuracy for dot enumeration in boys aged five, seven and eleven years, but no
visual hemifield accuracy difference in girls. A similar sex difference was found in
adult subjects by McGlone and Davidson (1973). The absence of any developmental trend in LVF superiority for dot enumeration is interesting in view of the
fact that it is a skill that is present in only a rudimentary form before age three,  
and even after three years is learnt quite gradually (Klahr and Wallace, 1973;


Dimond, S. J. (1972) The Double Brain. Churchill-Livingstone, London.


Dorman, M. F. and Geffner, D. S. (1974) Hemispheric specialization for speech perception in six- year-old black and white children from low and middle socioeconomic classes. Cortex, 10 , 171-176.


ASYMMETRY OF CEREBRAL HEMISPHERIC FUNCTION
Eling, P., Marshall, J. and Van Galen, G. (1979) Language processing levels and right ear advantages in dichotic listening. Int. Neuropsychol. Soc. Bull, June, 17.


Elliott, J. M. and Connolly, K. (1973) ‘Hierarchical structure in skill development’, in The Growth of Competence (eds. K. Connolly and J. Bruner), Academic Press, New York, 135-168.


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Ellis, H. D. (1975) Recognizing faces. Brit. J. Psychol, 66, 409-426.


Ellis, H. D. and Shepherd, J. W. {1914) Recognition of abstract and concrete words presented in left and right visual fields. J. Exp. Psychol, 103 , 1035-1036.


Young and McPherson, 1976), whereas recognition of many faces is possible in  
Ellis, H. D. and Young, A. W. (1977) Age-of-acquisition and recognition of nouns presented in the left and right visual fields: a failed hypothesis. Neuropsychologia, 15 , 825-828.
the first year of life (Schaffer, 1971; Ellis, 1975). It is thus clear that the absence of
reliable developmental trends in asymmetry of face recognition by children aged
five and above is not simply due to the early age at which the skill is acquired.  


Two findings of LVF superiority in children allegedly induced by means of a
Entus, A. K. (1977) ‘Hemispheric asymmetry in processing of dichotically presented speech and nonspeech stimuli by infants’, in Language Development and Neurological Theory (eds. S. J. Segalowitz and F. A. Gruber), Academic Press, New York, 63-73.
spatial mental set must also be noted (Kershner et al , 1977; Carter and  
Kinsbourne, 1979). Only Carter and Kinsbourne tested more than one age
group of children, and found no developmental differences in the tendency of
spatial priming to produce a LVF superiority for digit naming.  


Most studies of visual hemifield asymmetries for linguistic stimuli in children  
Finlayson, M. A. J. (1976) A behavioral manifestation of the development of interhemispheric transfer of learning in children. Cortex, 12, 290-295.
have used printed words. In several cases the principal focus of interest was not
so much whether there were differences across age as the possibility of differences
between normal and poor readers (Beaumont and Rugg, 1978). These studies
will only be referred to when they provide data relating to normal readers under
ten years of age.  


The general finding has been one of RVF superiorities in normal readers down
Flanery, R. C. and Balling, J. D. (1979) Developmental changes in hemispheric specialization for tactile spatial ability. Devel. Psychol, 15 , 364-372.
to as young as six years of age (Olson, 1973; Marcel, Katz and Smith, 1974;
Marcel and Rajan, 1975; Carmon, Nachshon and Starinsky, 1976; and one of
the experiments of Turner and Miller, 1975). Forgays (1953), however, did find
an increase in visual hemifield asymmetry with increasing age. Turner and Miller
(1975) and Butler and Miller (1979) reported larger asymmetries when using five-
rather than three-letter words. Turner and Miller (1975) also found changes  
across age when using five-letter words, but not when using three-letter words,
though Butler and Miller’s (1979) results did not confirm this observation. It is
probable that these somewhat confusing results derive from a failure properly to
control the characteristics of the words used. Longer words are more likely than
short words to be abstract, and hence to produce larger visual hemifield
asymmetries for reasons already mentioned. Conversely, the words recognized
by young children under conditions of brief lateral presentation are likely to be
mainly concrete, with older children recognizing a more even mixture of
concrete and abstract words. Since smaller visual hemifield asymmetries derive
from concrete than from abstract words a change in the size of the obtained
visual hemifield asymmetry across age will ensue if scores from abstract and
concrete words are pooled, but it has nothing to do with any possible difference
across age in the organization of cerebral hemispheric functions. Studies which
exercise proper control over the characteristics of stimulus words used are
clearly needed.  


Some developmental studies of left hemisphere specialization have tried using
Forgays, D. G. (1953) The development of differential word recognition. J. Exp. Psychol, 45 , 165-168.
letters instead of words as stimuli. Of these, only the one reported by Carmon et
al (1976) meets the minimal methodological requirements specified here.  
Carmon et al found traces of a developmental trend in visual hemifield
asymmetry when using letters as stimuli, but not when using words. The absence


Frankfurter, A. and Honeck, R. P. (1973) Ear differences in the recall of monaurally presented sentences. Quart. J. Exp. Psychol, 25,138-146.




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Fry, D. B. (1974) Right ear advantage for speech presented monaurally. Language and Speech 17, 142-151.


Gardiner, M. F. and Walter, D. O. (1977) ‘Evidence of hemispheric specialization from infant EEG’, in Lateralization in the Nervous System (eds. S. Harnad, R. W. Doty, L. Goldstein, J. Jaynes and G. Krauthamer), Academic Press, New York, 481-502.


Gazzaniga, M. S. and Le Doux, J. E. (1978) The Integrated Mind. Plenum, New York.


Geffen, G. (1976) Development of hemispheric specialization for speech perception Cortex, 12, 337-346.


Geffen, G. (1978) The development of the right ear advantage in dichotic listening with focused attention. Cortex, 14, 169-177.


of a developmental trend in asymmetry with words clearly implies that left
Geffen, G. and Sexton, M. A. (1978) The development of auditory strategies of attention. Devel Psychol., 14, 11-17.
hemisphere specialization for at least some of the skills involved in the
recognition of visually presented linguistic stimuli was present at all ages.  
Beyond this, all that can be said is that letter recognition is not a very meaningful
task for developmental comparisons, since Bryden and Allard (1976) have
shown that even with adults the results obtained are easily affected by the
difficulty experienced by subjects in reading the typeface employed. The more
difficult typefaces tend to give LVF superiorities, and it is obviously the case that
the difficulty of particular typefaces will vary across age.  


Taken together, then, the findings of studies of children using visual hemifield
Geffen, G. and Wale, J. (1979) The development of selective listening and hemispheric asymmetry. Devel. Psychol., 15, 138-146.
stimulus presentations do not support the idea that the degree of asymmetry of
organization of cerebral hemispheric functions varies across age. It has only
proved possible to date to work with children down to age five, but this
disadvantage is offset by the fact that some of the skills that can be studied are
being learned at these ages, allowing the possibility of the investigation of initial
stages of organization. What is now needed is a more precise analysis of the
particular skills used at different ages for word recognition and other tasks, so
that these skills can be examined separately. This might throw some interesting
light on the role (or absence of any role) played by the right hemisphere in the
early stages of learning to read. A related question which has not received the
attention it deserves concerns the way in which the right hemisphere acquires the
ability to recognize those words it can identify in adulthood. The results of
studies by Ellis and Young (1977) and Young and Bion (1980b) suggest that the
nature of the difference between the ‘visual vocabularies’ of the left and right
hemispheres is semantic, and unrelated to the ages at which different words are
first learnt.  


Tactile presentation. Although both ipsilateral and contralateral somatosensory
Geffner, D. S. and Dorman, M. F. (1976) Hemispheric specialization for speech perception in four- year-old children from low and middle socio-economic classes. Cortex, 12, 71-73.
nerve connections exist, they are organized into discrete systems that probably
serve different purposes (Wall, 1975). It is thought that ‘active’ touch and
proprioception (Gibson, 1962) are mediated primarily through the contra-  
laterally organized pathway passing through the dorsal column and medial
lemniscus, whilst passive touch, pain and temperature depend on the spinothalamic system, which has both ipsilateral and contralateral projections
(Gazzaniga and Le Doux, 1978).  


A useful review of the evidence relating to the role of different cerebral
Geffner, D. S. and Hochberg, I. (1971) Ear laterality performance of children from low and middle socioeconomic levels on a verbal dichotic listening task. Cortex, 7, 193-203.
structures in tactile perception is given by Corkin (1978), who points out that
there is no evidence for any asymmetry in elementary tactual functions. Tactile
asymmetries only occur when the task used engages some higher-order function
for which one cerebral hemisphere is superior (Corkin, 1978; Young and A. Ellis,
1979). Most of the findings of tactile asymmetries have derived from studies in
which active tactile exploration of stimuli was required.  


Geschwind, N. and Levitsky, W. (1968) Human brain: left-right asymmetries in temporal speech region. Science, 161, 186-187.


Gesell, A. and Ames, L. B. (1947) The development of handedness. J. Genet. Psychol., 70,155-175.


ASYMMETRY OF CEREBRAL HEMISPHERIC FUNCTION
Gibson, A. R., Dimond, S. J. and Gazzaniga, M. S. (1972) Left field superiority for word matching. N europyschologia, 10,463-466.


Gibson, J. J. (1962) Observations on active touch. Psychol. Rev., 69, 477-491.


185
Giesecke, M. (1936) The genesis of hand preference. Monographs of the Society for Research in Child Development, 1, No. 5.


Glanville, B., Best, C. and Levenson, R. (1977) A cardiac measure of cerebral asymmetries in infant auditory perception. Devel. Psychol., 13, 54-59.


It is often the case that left hand superiorities are found for complex tactile
Goodglass, H. (1973) Developmental comparison of vowels and consonants in dichotic listening. J. Speech and Hearing Res., 16, 744-752.
perception, but in some cases right hand superiorities are observed. Cioffi and
Kandel (1979) found a right hand superiority for identifying two-letter abstract
words by touch, which was present down to age six. A right hand superiority for
the report of sequentially touched fingers was found down to age seven by
Bakker and Van der Kleij (1978).  


Left hand superiorities for the identification of tactually perceived nonsense
Goodglass, H. and Quadfasel, F. A. (1954) Language laterality in left-handed aphasics. Brain, 77, 521-548.
shapes have been found down to age six by Witelson (1974,1976a) and by Cioffi
and Kandel (1979). These left hand superiorities did not increase with increasing
age, but inconsistent sex differences were observed. Using an accuracy measure,
Flanery and Balling (1979) also found a left hand superiority for this type of task
which did not vary across age down to age seven. However, when they computed
laterality coefficients’, differences across age were observed by Flanery and
Balling. Since the computation of such coefficients involves several unjustified
theoretical assumptions (Colbourn, 1978), and many different coefficients are
available that may all lead to differing outcomes, it is not possible to
satisfactorily interpret this particular result.  


In some studies, Braille patterns of raised dots have been used as stimuli.  
Gordon, H. W. (1970) Hemispheric asymmetries in the perception of musical chords. Cortex, 6, 387-398.
Hermelin and O’Connor (1971a, 1971b) found that blind adults and children
aged eight to ten years were better at reading Braille with the left than right
hand. Rudel et al (1974), however, found that sighted children did not learn
Braille letters more accurately using the left hand until they were over ten years
of age. Witelson (1977a) objected that the use of a naming task with raised dot
stimulus patterns confounds the linguistic and spatial components of the task,  
but Rudel et al (1977) repeated the finding in a study that required that raised
dot configurations only be discriminated, not named.  


One curious aspect of Rudel et a/.’s (1977) findings was that not only did
Gordon, H. W. (1974) ‘Auditory specialization of the right and left hemispheres’, in Hemispheric Disconnection and Cerebral Function (eds. M. Kinsbourne and W. L. Smith), Thomas, Springfield, 126-136.
children aged over ten years show left hand superiority, but children below ten
showed a tendency toward right hand superiority. Attention has been drawn to
this because it illustrates the danger inherent in regarding the results of studies of
this type as direct measures of asymmetry of cerebral organization. Surely no-
one would want to maintain that spatial functions moved from the left to the  
right hemisphere at age ten? What is evidently happening is that the type of task
used by Rudel and her colleagues can be approached using more than one
solution strategy, and the younger children rely on a method that involves the
left hemisphere to some extent. Their conclusion should thus have been not that
cerebral asymmetry varies across age but that more needs to be known about the
possible ways in which subjects can approach this type of task. A similar point
has been made by Bertelson (1978).  


It should by now be clear that the minimum requirement for demonstrating
Guttman, E. (1942) Aphasia in children. Brain, 65, 205-219.
that the extent to which particular cerebral hemispheric functions are asymmetrically organized changes across age is to show that younger children do not


Hardyck, C. and Petrinovich, L. F. (1977) Left-handedness. Psychol. Bull., 84, 385-404.


Haun, F. (1978) Functional dissociation of the hemispheres using foveal visual input. Neuro- psychologia, 16, 725-733.


186
Hecaen, H. (1976) Acquired aphasia in children and the ontogenesis of hemispheric functional specialization. Brain and Language, 3, 114-134.


Hecaen, H. and Albert, M. L. (1978) Human Neuropsychology. Wiley, New York.


Hermelin, B. and O’Connor, N. (1971a) Right and left handed reading of Braille. Nature, 231, 470.


Hermelin, B. and O’Connor, N. (19717?) Functional asymmetry in the reading of Braille. Neuropsychologia, 9, 431-435.


Heron, W. (1957) Perception as a function of retinal locus and attention. Amer. J. Psychol, 70, 38-48.


give a lateral superiority when using the same method of dealing with the given
Hicks, R. E. (1975) Intrahemispheric response competition between vocal and unimanual performance in normal adult human males. J. Comp. Physiol. Psychol., 89, 50-60.
task that produces the lateral superiority observed in older children. This
requirement applies generally to studies using auditory, visual or tactile
presentation, and it has never been met by any of the studies claiming to find
developmental differences in asymmetric cerebral organization. Consequently,  
the only valid conclusion at present with regard to tactile asymmetries is that left
or right hand superiorities for tactile perception can be demonstrated in children
down to at least age six under appropriate conditions.  


Studies of asymmetries in infants for processing auditorily or visually presented
Hildreth, G. (1949) The development and training of hand dominance: II. Developmental tendencies in handedness. J. Genet. Psychol., 75, 221-254.
stimuli. The failure of studies of asymmetries during childhood for processing
laterally presented stimuli to provide any convincing evidence of changes across
age in the asymmetric organization of cerebral hemispheric functions, and the
existence of neuroanatomical asymmetries in infants, has led researchers to
explore the possibility that functional cerebral asymmetries are present in
infancy. A number of techniques have been devised, mostly using electrophysio-  
logical measures.  


Electrophysiological studies have shown cerebral hemisphere differences in
Hilliard, R. D. (1973) Hemispheric laterality effects on a facial recognition task in normal subjects. Cortex, 9, 246-258.
early infancy in terms of auditory and visual evoked potentials (Molfese et al,
1975; Molfese et al , 1976; Davis and Wada, 1977; Molfese, 1977; Molfese and
Molfese, 1979), EEG power distributions (Davis and Wada, 1977; Gardiner and
Walter, 1977), and photic driving (Crowell et al , 1973).  


The dichotic stimulation technique has been adapted in order to demonstrate
Hines, D. (1975) Independent functioning of the two cerebral hemispheres for recognizing bilaterally presented visual-half-field stimuli. Cortex, 11, 132-143.
cerebral asymmetries in infants by Glanville et al. (1977), who used a response
measure based on heart rate habituation. Entus (1977) also used dichotic
stimulation with a sucking response, but a subsequent study (Vargha-Khadem
and Corballis, 1979) has not been able to replicate her results.  


The findings of these several studies of hemisphere function in infancy
Hines, D. (1976) Recognition of verbs, abstract nouns and concrete nouns from the left and right visual-half-fields. Neuropsychologia, 14, 211-216.
convincingly demonstrate that asymmetric organization of function is present,  
which is incompatible with Lenneberg’s (1967) views. Most investigators have,
however, been satisfied to establish the basic point that functional asymmetries
can be shown in infancy. Whilst the similarity of the asymmetries found in
infants to those found in adults is usually obvious, this tactic avoids questions as
to the precise mechanisms involved, and leaves open the possibility that some
changes across age might occur. However, since studies of asymmetries for
processing perceptually presented stimuli in infants and children have so
consistently failed to produce any satisfactory supporting evidence for the
notion of progressive lateralization of abilities, it is unreasonable to believe that
such changes do occur unless strong supporting evidence can be found
elsewhere.  


Asymmetries of motor control and lateral preferences. It was mentioned in the
Hines, D. (1977) Differences in tachistoscopic recognition between abstract and concrete words as a function of visual half-field and frequency. Cortex, 13, 66-73.
introduction to this chapter that the principal innervation of movements of the


Hiscock, M. and Kinsbourne, M. (1977) Selective listening asymmetry in preschool children. Devel. Psychol., 13, 217-224.


Hiscock, M. and Kinsbourne, M. (1978) Ontogeny of cerebral dominance: evidence from timesharing asymmetry in children. Devel. Psychol, 14, 321-329.


ASYMMETRY OF CEREBRAL HEMISPHERIC FUNCTION
Hynd, G. W. and Obrzut, J. E. (1977) Effects of grade level and sex on the magnitude of the dichotic ear advantage. Neuropsychologia, 15, 689-692.


Inglis, J. and Sykes, D. H. (1967) Some sources of variation in dichotic listening performance in children. J. Exp. Child Psychol, 5, 480-488.


187
Ingram, D. (1975a) Cerebral speech lateralization in young children. Neuropsychologia, 13,103-105.


Ingram, D. (19756) Motor asymmetries in young children. Neuropsychologia, 13, 95-102.


body is mediated through contralateral nerve tracts. Although ipsilateral nerve
Jeeves, M. A. (1972) Hemisphere differences in response rates to visual stimuli in children. Psychonomic Sci., 27, 201-203.
fibres also exist, their role is not fully understood, but it is thought that their
influence is confined to relatively gross movements. An example might be
moving a hand by moving the whole arm. Fine motor movements, and especially
movements from the level of the wrist of the hands and fingers, are seen as
normally involving a relatively high degree of contralateral control (Brinkman
and Kuypers, 1972, 1973; Trevarthen, 1974,1978). For this reason, in examining
asymmetries of motor control and lateral preferences, particular attention will
be paid to the fine control of movements of the hands and fingers.  


An important distinction which needs to be made when considering motor
Joynt, R. J. and Goldstein, M. N. (1975) ‘Minor cerebral hemisphere’, in Advances in Neurology, Vol. 7 (ed. W. J. Friedlander), Raven Press, New York, 147-183.
asymmetries concerns the difference between lateral preference and relative skill
(Annett, 1970; Ingram, 19757?; G. Young, 1977). This is perhaps best illustrated
by means of an example. Most right-handed people will always write with their
right hand, and nearly always pick up a pen with their right hand. The degree of
preference for use of the right hand is similar for both activities. If, however, a
right-handed person is asked to carry out these activities using his left hand, he is
not likely to experience any difficulty in picking up the pen, but left-handed
writing will prove to be much more tricky. The degree of relative skill of the
hands for both activities is quite different. It is evident that relative manual skill
and hand preference are not the same thing, though they are related (Annett,
1976). Their relation is probably most close for the more difficult and skilled
tasks, as Brown (1962) found. Even with difficult tasks, however, the relation of
preference and relative skill is not exact, and it is possible to find motor tasks
that right-handed people can better execute with the left hand (Kimura and
Vanderwolf, 1970). In addition to the contribution of asymmetry of cerebral
hemispheric motor functions, hand preference can involve an element of choice,
with one hand often being preferred regardless of whether the activities might
cause considerable or little difficulty to the other. This means that studies of
relative skill of the hands at different ages are of more direct relevance to
asymmetry of cerebral hemispheric function than are studies of hand preferences
(Denckla, 1974; G. Young, 1977).  


The distinction of studies of lateral preference from studies of relative skill
Kelly, R. R. and Orton, K. D. (1979) Dichotic perception of word-pairs with mixed image values. Neuropsychologia, 17, 363-371.
makes the results of an otherwise confusing body of studies of motor asymmetries during development fall into a neat pattern. Put simply, studies of
relative skill have not found increases in asymmetry across age (one or two have
actually found decreases), whereas studies of lateral preference have generated a
mixture of results seen as indicating changes in lateral preference and results
indicating absence of change in lateral preference across age.  


A favourite type of task in studies of relative skill has involved comparisons
Kershner, J., Thomae, R. and Callaway, R. (1977) Nonverbal fixation control in young children induces a left-field advantage in digit recall. Neuropsychologia, 15, 569-576.
between the hands for the highest speed or greatest accuracy with which
repetitive movements can be carried out. Examples would be moving pegs on a
pegboard, or tapping rhythms, and studies of this type which have used children  


Kimura, D. (1961) Cerebral dominance and the perception of verbal stimuli. Can. J. Psychol, 15, 166-171.


Kimura, D. (1963) Speech lateralization in young children as determined by an auditory test. J. Comp. Physiol. Psychol, 56, 899-902.


188
Kimura, D. (1966) Dual functional asymmetry of the brain in visual perception. Neuropsychologia, 4, 275-285.


Kimura, D. (1967) Functional asymmetry of the brain in dichotic listening. Cortex, 3, 163-178.


Kimura, D. and Durnford, M. (1974) ‘Normal studies on the function of the right hemisphere in vision’, in Hemisphere Function in the Human Brain (eds. S. J. Dimond and J. G. Beaumont), Elek, London, 25-47.


Kimura, D. and Vanderwolf, C. H. (1970) The relation between hand preference and the performance of individual finger movements by left and right hands. Brain, 93, 769-774.


Kinsbourne, M. (1976) ‘The ontogeny of cerebral dominance’, in The Neuropsychology of Language (ed. R. W. Rieber), Plenum, New York, 181-191.


down to age five or below include those of Knights and Moule (1967), Annett
Kinsbourne, M. and Cook, J. (1971) Generalized and lateralized effects of concurrent verbalization on a unimanual skill. Quart. J. Exp. Psychol, 23, 341-345.
(1970), Denckla (1973, 1974), Ingram (19755), Finlayson (1976), and Wolff and
Hurwitz (1976). The total range of ages covered by these studies is from three to
sixteen years. All found right hand superiorities, and none produced evidence of
an increase in the degree of right hand superiority with increasing age. In some
cases, however, asymmetries were found to decrease in magnitude with
increasing age (Denckla, 1974; Wolff and Hurwitz, 1976). These results may be
attributable to a decrease in sensitivity of particular tasks across the considerable ranges of ages used in the studies concerned. They do, however, also raise
the interesting possibility that there may be changes across age in the extent of
asymmetric organization of some skills which do not take the form specified by
the concept of lateralization.  


Other tests of relative skill which have led to right hand superiorities include
Kinsbourne, M. and Hiscock, M. (1977) ‘Does cerebral dominance develop?’, in Language Development and Neurological Theory (eds. S. J. Segalowitz and F. A. Gruber), Academic Press, New York, 171-191.
hand strength (Ingram, 19755; Finlayson, 1976), speed of writing (Reitan, 1971),  
and duration of grasp of a rattle (Caplan and Kinsbourne, 1976). Caplan and
Kinsbourne’s finding, from a study of two- to four-month-old babies, remains
the earliest demonstration of a manual asymmetry.  


In two tasks used in Ingram’s (1975 5) study of three- to five-year-old children,  
Kinsbourne, M. and McMurray, J. (1975) The effect of cerebral dominance on time sharing between speaking and tapping by preschool children. Child Devel, 46, 240-242.
which involved imitating hand postures or finger spacings, left hand superiorities
were obtained, presumably reflecting the right hemisphere’s superiority for the
complex spatial component of the tasks. This finding can thus be seen as both
confirming the presence of superior right hemisphere spatial functions at age
three and illustrating the importance of distinguishing questions of relative
manual skill for different tasks from those of hand preference.  


An interesting variation on the basic studies of relative skill on single tasks
Klahr, D. and Wallace, J. G. (1973) The role of quantification operators in the development of conservation of quantity. Cog. Psychol, 4, 301-327.
involves dual-task performance. Studies of adults have demonstrated that
requiring them to talk whilst carrying out an independent manual task interferes
more with right than with left hand performance (e.g. Kinsbourne and Cook,
1971; Hicks, 1975). Such interference probably occurs when speech and right
hand movements demand the use of common left hemisphere functions (Lomas
and Kimura, 1976). Studies of interference in dual-task performance in children
down to age three have shown that the same types of effect occur (Kinsbourne
and McMurray, 1975; Piazza, 1977; Hiscock and Kinsbourne, 1978). The only
hint of any change across age arises in the report of McFarland and Ashton
(1975), but since their groups contained as few as four subjects, sampling bias
cannot be ruled out.  


Early studies of motor asymmetries in infants and children were almost
Knights, R. M. and Moule, A. D. (1967) Normative and reliability data on finger and foot tapping in children. Perceptual and Motor Skills, 25, 717-720.
exclusively addressed to questions of lateral preference (e.g. Wile, 1934;
Giesecke, 1936; Gesell and Ames, 1947; Hildreth, 1949). Although most of these
studies would now receive low marks for adequacy of methodology and clarity
in reporting what was actually done they were in general agreement that lateral
preferences, and especially hand preferences, are established gradually


Knox, C. and Kimura, D. (1970) Cerebral processing of nonverbal sounds in boys and girls. Neuropsychologia, 8, 227-237.


Kopp, C. B. (1974) Fine motor abilities of infants. Devel Med. Child Neurol, 16, 629-636.


ASYMMETRY OF CEREBRAL HEMISPHERIC FUNCTION
Krashen, S. (1973) Lateralization, language learning and the critical period: some new evidence. Language Learning, 23, 63-74.


Le Doux, J. E., Wilson, D. H. and Gazzaniga, M. S. (1977) Manipulo-spatial aspects of cerebral lateralization: clues to the origin of lateralization. N europsychologia, 15, 743-750.


189
Leehey, S. C. (1976) Face recognition in children. Evidence for the development of right hemisphere specialization. Unpublished Ph.D. thesis, Massachusetts Institute of Technology.


Lenneberg, E. H. (1967) Biological Foundations of Language. Wiley, New York.


throughout childhood, with periods of absence of preference or preferences
Lock, A. J. (ed.) (1978) Action, Gesture and Symbol: the Emergence of Language. Academic Press, New York.
opposite to those finally adopted. Some more recent studies have also reported
results of this type both for infants (Cohen, 1966; Cernacek and Podivinsky,
1971; Seth, 1973; Ramsay, 1979) and for older children (Belmont and Birch,
1963), though there are also studies that have not found changes in hand
preference across age in infancy (Ramsay, Campos and Fenson, 1979) or
childhood (Annett, 1970).  


The explanation of these discrepant findings from studies of hand preference
Lomas, J. and Kimura, D. (1976) Intrahemispheric interaction between speaking and sequential manual activity. Neuropsychologia, 14, 23-33.
lies in the measures used. Annett (1970) and Ramsay et al (1979) both studied
hand preference for quite difficult skills. Hand preference for difficult skills is, for
reasons explained previously, likely to be relatively closely related to differential
skill, and it is studies of differential skill which do not tend to find changes across
age. Most of the studies which did find developmental trends in hand preference
used measures based on preference for picking up objects. There is no reason to
assume that preference for the same actions is being examined at different ages,
since there are a number of different ways of manipulating and picking up
objects (Elliott and Connolly, 1973; Kopp, 1974; Bresson et al, 1977).  


A type of investigation involving motor asymmetries which does not really fit
Lund, R. D. (1978) Development and Plasticity of the Brain: An Introduction. Oxford University Press, New York.
into the scheme of studies of relative skill or studies of lateral preference also
deserves mention. In several studies Turkewitz and his colleagues have shown
that very young infants turn their heads more often to the right than to the left
(e.g. Turkewitz et al ., 1965; Turkewitz et al, 1969; Turkewitz and Creighton,
1975). Although the demonstration of any motor asymmetry at early ages is of
interest, no really satisfactory explanation as to the cause of the bias in head
turning has been offered.  


Studies of the consequences of cerebral injury at different ages
Marcel, T., Katz, L. and Smith, M. (1974) Laterality and reading proficiency. Neuropsychologia, 12, 131-139.


The findings of studies of infants and children using noninvasive methods have
Marcel, T. and Patterson, K. (1979) ‘Word recognition and production: reciprocity in clinical and normal studies’, in Attention and Performance, VII (ed. J. Requin), Erlbaum, New Jersey.
failed to provide evidence indicating that the extent to which particular cerebral
hemispheric functions are symmetrically or asymmetrically organized changes
across age in the manner implied by the concept of progressive lateralization of
abilities. Moreover, they have shown that asymmetry of hemispheric function is
present in some form in infancy. Both outcomes are clearly at variance with
Lenneberg’s (1967) theoretical position. In fairness, however, it must be pointed
out that most of this evidence was not available to Lenneberg, and that his
theory was mainly derived from studies of hand preference and from studies of
the consequences of cerebral injury at different ages.  


It is notoriously difficult to draw valid inferences concerning the organization
Marcel, T. and Rajan, P. (1975) Lateral specialisation for recognition of words and faces in good and poor readers. Neuropsychologia, 13, 489-497.
of cerebral functions from the effects of cerebral injuries, and this difficulty is
compounded when it is necessary to draw conclusions about possible organizational differences across age. Several of the problems of methodology and  
interpretation that can arise have detailed in the reviews of Kinsbourne (1976)


McFarland, K. and Ashton, R. (1975) A developmental study of the influence of cognitive activity on an ongoing manual task. Acta Psychologica, 39, 447-456.


McFarland, K., McFarland, M. L., Bain, J. D. and Ashton, R. (1978) Ear differences of abstract and concrete word recognition. Neuropsychologia, 16, 555-561.


190
McGlone, J. and Davidson, W. (1973) The relation between cerebral speech laterality and spatial ability with special reference to sex and hand preference. Neuropsychologia, 11,105-113.


McKeever, W. F. (1974) Does post-exposural directional scanning offer a sufficient explanation for lateral differences in tachistoscopic recognition? Perceptual and Motor Skills, 38, 43- 50.


McKeever, W. F. and Huling, M. D. (1971) Lateral dominance in tachistoscopic word recognition performances obtained with simultaneous bilateral input. Neuropsychologia, 9, 15-20.


Miller, L. K. and Turner, S. (1973) Development of hemifield differences in word recognition. J. Educ. Psychol, 65, 172-176.


Mirabile, P. J., Porter, R. J., Hughes, L. F. and Berlin, C. I. (1975) Dichotic lag effect in children 7 to 15. Devel. Psychol, 14, 277-285.


and Witelson (1977a), which seriously criticized many of the interpretations that
Mishkin, M. and Forgays, D. G. (1952) Word recognition as a function of retinal locus. J. Exp. Psychol, 43, 43-48.
have been offered. This is not, of course, to deny the great importance of studies
of the developmental sequelae of cerebral injuries, but their relevance to
understanding asymmetry of cerebral hemispheric function during development
has often been overestimated and misunderstood.  


The four main aspects of studies of the consequences of cerebral injury that
Molfese, D. L. (1977) ‘Infant cerebral asymmetry’, in Language Development and Neurological Theory (eds. S. J. Segalowitz and F. A. Gruber), Academic Press, New York, 21-35.
have received attention will be examined in turn. These are the differences across
age in the extent of recovery from unilateral cerebral injuries, the claim of the
equipotentiality of the cerebral hemispheres for language acquisition, differences
across age in the nature of aphasic symptoms, and the possible involvement of
the right hemisphere in the early stages of language acquisition. As it will become
clear that many of the studies carried out add little or nothing to our
understanding of cerebral asymmetry, a systematic review of all the studies will
not be attempted.  


Age and the extent of recovery from unilateral cerebral injuries. Many studies of
Molfese, D. L., Freeman, R. B. and Palermo, D. S. (1975) The ontogeny of brain lateralization for speech and nonspeech stimuli. Brain and Language, 2, 356-368.
language disturbances in children following left hemisphere injury have shown
that the younger the child the more rapid and complete is the recovery. Reviews
are provided by Basser (1962), Lenneberg (1967) and Witelson (1977a); see also
Parker, this volume. This recovery may be in part due to intrahemispheric
reorganization of functions within the damaged left hemisphere (Hecaen, 1976),  
but it is known from cases where the extent of the injury eventually led to left
hemispherectomy that considerable acquisition of language functions by the
right hemisphere is possible in the first years of life.  


These findings have often been taken to indicate that the lateralization of
Molfese, D. L. and Molfese, V. J. (1979) Hemisphere and stimulus differences as reflected in the cortical responses of newborn infants to speech stimuli. Devel. Psychol, 15, 505-511.
language abilities proceeds gradually throughout childhood, with the right
hemisphere being involved in language functions in the early years. In fact, the
findings do not indicate this at all. They simply attest to the remarkable ability of  
the young brain to recover and reorganize functions in response to injury. This is
a complex phenomenon in its own right, widespread throughout the animal
kingdom, to which there are a number of different contributory processes
(Hecaen and Albert, 1978; Lund, 1978). None the less, the finding that recovery
can take place tells us nothing about the organization of function before injury.
There is no reason to connect loss of‘plasticity’ with an increase in lateralization.  


The claim of the equipotentiality of the cerebral hemispheres for language
Molfese, D. L., Nunez, V., Seibert, S. M. and Ramanaiah, N. V. (1976) ‘Cerebral asymmetry: changes in factors affecting its development’, in Origins and Evolution of Language and Speech (eds. S. Harnad, H. Steklis and J. Lancaster), Ann. N.Y. Acad. Scl, 280, 821-833.
acquisition. The extent of the recovery of language abilities following early left
hemisphere injury is so marked that Lenneberg (1967) was led to the conclusion
that the cerebral hemispheres are initially perfectly equipotential for language
acquisition. This conclusion was supported by reports of the observations of
clinicians, but more systematic and quantitative studies have shown that perfect
equipotentiality does not obtain (Dennis and Kohn, 1975; Dennis and


Morais, J, and Darwin, C. J. (1974) Ear differences for same-different reactions to monaurally presented speech. Brain and Language, 1, 383-390.


Morton, J., Marcus, S. and Frankish, C. (1976) Perceptual centers (P-centers). Psychol Rev., 83, 405-408.


ASYMMETRY OF CEREBRAL HEMISPHERIC FUNCTION
Moscovitch, M. (1977) ‘The development of lateralization of language functions and its relation to cognitive and linguistic development: a review and some theoretical speculations’, in Language Development and Neurological Theory (eds. S. J. Segalowitz and F. A. Gruber), Academic Press, New York, 193-211.


Nagafuchi, M. (1970) Development of dichotic and monaural hearing abilities in young children. Acta Otolaryngol, 69, 409-414.


191
Nottebohm, F. (1970) Ontogeny of bird song. Science, 167, 950-956.


Olson, M. E. (1973) Laterality differences in tachistoscopic word recognition in normal and delayed readers in elementary school. Neuropsychologia, 11, 343-350.


Whitaker, 1976; Dennis and Whitaker, 1977). Although extensive language
Oscar-Berman, M., Goodglass, H. and Donnenfeld, H. (1974) Dichotic ear-order effects with nonverbal stimuli. Cortex, 10, 270-277.
functioning can be achieved by the right hemisphere following early injury to the
left hemisphere, the left hemisphere is better able than the right to subserve
language acquisition even in infancy.  


Differences across age in the nature of aphasic symptoms. It has long been known
Piazza, D. M. (1977) Cerebral lateralization in young children as measured by dichotic listening and finger tapping tasks. Neuropsychologia, 15, 417-425.
that cerebral lesions causing disturbances of language in children (acquired
aphasias) do not produce the same pattern of symptoms as found in adults
(Guttman, 1942; Alajouanine and Lhermitte, 1965; Hecaen, 1976). The most
common form of acquired aphasia in children involves difficulty with or absence
of spontaneous expression (mutism), whilst jargonaphasia and logorrhea occur
only in adults. Brown and Jaffe (1975) and Brown (1977) have extended these
observations, arguing that the different types of aphasia are systematically
related to age not only in childhood but throughout the human lifespan.  


Such differences across age in the nature of acquired aphasias are of
Pirozzolo, F. J. (1977) Lateral asymmetries in visual perception: a review of tachistoscopic visual half-field studies. Perceptual and Motor Skills, 45, 695-701.
undoubted intrinsic importance and interest, but what do they tell us about
asymmetry of cerebral hemispheric function? They might indicate that, at the
'psychological’ level, the organization of language functions and the relative
contribution made by different linguistic skills changes during the lifespan, with
some skills being developed to the level of practised fluency at which jargon-  
aphasia and logorrhea can occur. These changes could, however, be associated
with intrahemispheric development and organization of processes principally
located in the left hemisphere, and the concept of lateralization is not needed.  


The possible involvement of the right hemisphere in the early stages of language
Porac, C. and Coren, S. (1976) The dominant eye. Psychol. Bull, 83, 880-897.
acquisition. Although it has been customary to include them in discussions of
asymmetry of cerebral hemispheric function during development, it is apparent
that the lines of evidence concerning the consequences of cerebral injury at
different ages described thus far are not really of central importance to the topic.  
There is one claim, however, which is potentially crucial, and which has been
held apart from the others to show its special role in making the other lines of
evidence appear to contribute more to our understanding of the problem than
they actually do. The claim falls into two parts, which require separate
consideration. Firstly, it is held that childhood aphasias are more likely than
adult aphasias to occur as a consequence of injury to the right cerebral
hemisphere, and secondly it is held that this implies that the right hemisphere is
involved as well as the left hemisphere in the early stages of the acquisition of
language functions.  


The evidence concerning the first part of the claim is not completely
Porter, R. J. and Berlin, C. I. (1975) On interpreting developmental changes in the dichotic right-ear advantage. Brain and Language, 2,186-200.
convincing. It is clear that the proportion of children over five years of age
experiencing aphasic difficulties following left as opposed to right hemisphere
injury is comparable to the proportion found for adults (Krashen, 1973; Hecaen,  


Ramsay, D. S. (1979) Manual preference for tapping in infants. Devel Psychol., 15, 437-442.


Ramsay, D. S., Campos, J. J. and Fenson, L. (1979) Onset of bimanual handedness in infants. Infant Behav. Devel, 2, 69-76.


192
Reitan, R. M. (1971) Sensorimotor functions in brain-damaged and normal children of early school age. Perceptual and Motor Skills, 33, 655-664.


Reynolds, D. McQ. and Jeeves, M. A. (1978a). A developmental study of hemisphere specialization for alphabetical stimuli. Cortex, 14, 259-267.


Reynolds, D. McQ. and Jeeves, M. A. (19785) A developmental study of hemisphere specialization for recognition of faces in normal subjects. Cortex, 14, 511-520.


Risse, G. L., Le Doux, J., Springer, S. P., Wilson, D. H. and Gazzaniga, M. S. (1978) The anterior commissure in man: functional variation in a multisensory system. Neuropsychologia, 16, 23-32.


Rizzolatti, G., Umilta, C. and Berlucchi, G. (1971) Opposite superiorities of the right and left cerebral hemispheres in discriminative reaction time to physiognomic and alphabetical material. Brain, 94, 431-442.


1976). For children aged two to five years, however, aphasia following right
Rudel, R. G., Denckla, M. B. and Hirsch, S. (1977) The development of left-hand superiority for discriminating Braille configurations. Neurology, 27, 160-164.
hemisphere injury would seem to be relatively frequent from the cases reported
in the literature. Witelson (1977 a) gives a rough figure of 30%, but there are
several difficulties in taking such a figure at its face value, as Kinsbourne (1976)
and Witelson (1977a) have stressed. These difficulties include the possibility that
many of the right hemisphere injuries were so extensive as to also involve parts
of the left hemisphere, the danger of bias toward referral to specialists and
reporting of the more unusual cases (i.e. those where aphasia apparently
followed right hemisphere injury) and the poverty of the assessments typically
given as to the nature, severity and duration of the aphasic symptoms. These
methodological problems are not caused by any lack of competence of
investigators, and it is difficult to see how they could all be fully overcome.
Kinsbourne (1976) concluded that the existence of such difficulties is sufficient to
invalidate the reports indicating greater frequency of aphasias following right
hemisphere injuries in young children than in adults; Witelson also advocated
that such reports should be treated with caution.  


The attention paid to the methodological problems inherent in attempts to
Rudel, R. G., Denckla, M. B. and Spalten, E. (1974) The functional asymmetry of Braille letter learning in normal, sighted children. Neurology, 24, 733-738.
calculate the relative frequency of aphasias following right hemisphere injury in
young children and adults has tended to draw attention away from the question
of what the finding of a greater frequency in young children, if valid, should be
taken to mean (a notable exception is the discussion by Moscovitch, 1977).  
Witelson (1977a) felt that it means that the right hemisphere may participate in
the execution of language functions in the early stages of language acquisition,
but that its contribution is always less than that of the left hemisphere. What
needs to be clarified, though, is whether the right hemisphere’s contribution is of
the same type as that made by the left hemisphere, as the concept of progressive
lateralization of language abilities would imply, or whether it is important
because of functions it can execute which would not normally be viewed as
linguistic yet are integral to the early stages of language acquisition. Evidence
from psychological studies of language acquisition, for instance, indicates that
much of the initial organization involved is closely related to understanding of
and interactions with the world of objects, events and other people (R. Brown,
1973; Lock, 1978). It is unfortunate that the level of analytic sophistication
attained by psychologists has not been applied to neuropsychological studies of
childhood aphasia. If this were done, differences between the types of aphasia
following left and right hemisphere injuries sustained in childhood might be
found. With mutism being the most common symptom this would obviously be
difficult, but detectable differences could arise in the patterns of recovery.  


At present, then, firm answers to the important questions that have arisen
Satz, P., Rardin, D. and Ross, J. (1971) An evaluation of a theory of specific developmental dyslexia. Child Devel, 42, 2009-2021.
concerning the possible involvement of the right hemisphere in the early stages
of language acquisition have not been provided by studies of childhood aphasias
following right hemisphere injuries.  


Satz, P., Bakker, D. J., Teunissen, J., Goebel, R. and Van Der Vlugt, H. (1975) Developmental parameters of the ear asymmetry: a multivariate approach. Brain and Language, 2, 171-185.


Schaffer, H. R. (1971) The Growth of Sociability. Penguin, Harmondsworth.


ASYMMETRY OF CEREBRAL HEMISPHERIC FUNCTION
Searleman, A. (1977) A review of right hemisphere linguistic capabilities. Psychol. Bull, 84, 503-528.


Seines, O. A. (1974) The corpus callosum: some anatomical and functional considerations with reference to language. Brain and Language, 1, 111-139.


193
Seth, G. (1973) Eye-hand co-ordination and ‘handedness’: a developmental study of visuo-motor behaviour in infancy. Brit. J. Educ. Psychol, 43, 35-49.


Sexton, M. A. and Geffen, G. (1979) The development of three strategies of attention in dichotic monitoring. Devel Psychol, 15, 299-310.


Overview and conclusions
Sommers, R. K. and Taylor, M. L. (1972) Cerebral speech dominance in language-disordered and normal children. Cortex, 8, 224-232.


Having examined the available evidence concerning asymmetry of cerebral
Studdert-Kennedy, M. (1972) A right-ear advantage in choice reaction time to monaurally presented vowels: a pilot study. Haskins Laboratories Status Report on Speech Research, (31/32), 75-81.
hemispheric function during development, it is now possible to consider what
general conclusions can be drawn. This will necessarily involve discussion of
what type of conceptual and theoretical framework is most useful in describing
the existing findings and generating new lines of investigation.  


The results of the numerous studies that have been carried out show that
Teszner, D., Tzavaras, A., Gruner, J. and Hecaen, H. (1972) L’asymetrie droite-gauche du planum temporale; a propos de l’etude anatomique de 100 cervaux. Revue Neurologique, 126, 444-449.
asymmetric organization of at least some cerebral hemispheric functions is
characteristic of the human brain at all ages during postnatal development.  
Although considerable recovery and reorganization of function can take place
following unilateral cerebral injury sustained early in life, the cerebral hemispheres are not equipotential for language acquisition. Thus the claims of
absence of functional asymmetry in infancy and perfect hemispheric equi-
potentiality for language put forward by Lenneberg (1967) are simply incorrect.  


The question as to whether the degree to which functions are asymmetrically
Tomlinson-Keasey, C., Kelly, R. R. and Burton, J. K. (1978) Hemispheric changes in information processing during development. Devel Psychol, 14, 214-223.
organized increases across age cannot be given such a straightforward answer,  
and requires some clarification. The total number of asymmetrically organized
functions may well increase during the first years of life for the simple reason that
many are acquired during this period. In this trivial sense, ‘laterality’ quite
probably does increase across age. The concept of lateralization, however, is
only of real interest as applied to particular functions, for which it implies that
unilateral organization develops progressively from an initial organization that
is at least to some extent bilateral. It is this sense that was clearly intended by
Lenneberg (1967), Krashen (1973) and Brown and Jaffe (1975).  


This hypothesis of progressive lateralization of abilities has not found
Trevarthen, C. (1974) ‘Functional relations of disconnected hemispheres with the brain stem, and with each other: monkey and man’, in Hemispheric Disconnection and Cerebral Function (eds. M. Kinsbourne and W. L. Smith), Thomas, Springfield, 187-207.
adequate support in the studies that have been carried out, irrespective of
whether it is regarded as valid or as invalid to use parametric statistical analyses.  
When findings have been claimed to demonstrate progressive lateralization of  
abilities, it has been shown that enthusiasm for the concept of lateralization has
led to lack of attention to more prosaic alternative explanations. Of course, as
has been pointed out, the available methods of investigation have not always
been adapted for work with all ages of children, so that all of the conceivable
lines of enquiry have by no means been exhausted. It thus remains possible for
people to believe that substantial positive evidence of genuine progressive
changes in lateralization will one day be found. However, this is more a
statement of faith than a scientific inference, and a more realistic theoretical
framework for research findings needs to be built up.  


The research approach dictated by the concept of lateralization has been to
Trevarthen, C. (1978) ‘Manipulative strategies of baboons and origins of cerebral asymmetry’, in Asymmetrical Function of the Brain (ed. M. Kinsbourne), University Press, Cambridge, 329- 391.
look for progressive changes in childhood in the extent of the asymmetric
organization of certain functions. This means that studies have often been
directed toward the possibility of change in functions that are already


Turkewitz, G. and Creighton, S. (1975) Changes in lateral differentiation of head posture in the human neonate. Devel. Psychobiol, 8, 85-89.


Turkewitz, G., Gordon, E. W. and Birch, H. G. (1965) Head turning in the human neonate: spontaneous patterns. J. Genet. Psychol, 107, 143-158.


194
Turkewitz, G., Moreau, T., Davis, L. and Birch, H. G. (1969) Factors affecting lateral differentiation in the human newborn. J. Exp. Child Psychol, 8, 483-493.


Turner, S. and Miller, L. K. (1975) Some boundary conditions for laterality effects in children. Devel. Psychol, 11, 342-352.


Van Duyne, H. J., Bakker, D. and De Jong, W. (1977) Development of ear-asymmetry related to coding processes in memory in children. Brain and Language, 4, 322-334.


Vargha-Khadem, F. and Corballis, M. C. (1979) Cerebral asymmetry in infants. Brain and Language,


Varney, N. R. and Benton, A. L. (1975) Tactile perception of direction in relation to handedness and familial handedness. Neuropsychologia, 13, 449-454.


adequately established. The typical investigative tactic has involved the use of
Wada, J. A., Clarke, R. and Hamm, A. (1975) Cerebral hemispheric asymmetry in humans: cortical speech zones in 100 adult and 100 infant brains. Arch. Neurol, 32, 239-246.
one or two tasks and a wide range of ages of subjects. Such studies have been
worthwhile insofar as they have led to the conclusion that progressive
lateralization of already acquired functions does not take place. Further studies
of this type can still be of value in filling in the many missing details. It may now
be more interesting, however, to look for changes in organization whilst
functions are actually being acquired. For this purpose, the concept of
lateralization should be abandoned, since it arbitrarily predetermines what form
such changes would be conceptualized as taking, and they may turn out to be
more varied. There is, for instance, no reason to discount the possibility that for
some skills the extent of asymmetric organization may actually decrease as they
become firmly established and integrated into a child’s repertoire.  


A useful approach, then, may be to define the basic problem as one of  
Wall, P. D. (1975) ‘The somatosensory system’, in Handbook of Psychobiology (eds. M. S. Gazzaniga and C. Blakemore), Academic Press, New York, 373-392.
understanding how newly learned skills are integrated with existing functions
that are already symmetrically or asymmetrically organized. This shifts
emphasis on to the possibility of relatively rapid changes occurring whilst
functions are being acquired rather than long-term changes in already acquired
functions, and does not prescribe the form such changes might take. It would
require careful studies directed toward quite specific skills at the ages at which
they are learned. A few studies of this type have been achieved, and suggestions
have already been offered where others are obviously necessary, but they
demand precise methods of investigation which have only recently begun to be
available. As such methods are developed the studies of isolated tasks across
wide ranges of ages deriving from the conceptual framework dictated by the
concept of lateralization will probably become of less interest than very detailed
studies carried out whilst functions such as prehension, enumeration or reading
are being acquired.  


White, M. J. (1969) Laterality differences in perception: a review. Psychol Bull, 72, 387-405.


Acknowledgements
White, M. J. (1972) Hemispheric asymmetries in tachistoscopic information processing Brit J Psychol, 63, 497-508.


The assistance provided by SSRC grants HR 5078, HR 6398, and HR 6876 is gratefully
White, M. J. (1973) Does cerebral dominance offer a sufficient explanation for laterality differences in tachistoscopic recognition ? Perceptual and Motor Skills, 36, 479-485.
acknowledged. I am very grateful to Andrew Ellis for helpful discussion of several points of
interpretation.  


Wile, I. S. (1934) Handedness: Right and Left. Lothrop, Lee and Shepard, Boston.


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Hines, D. (1975) Independent functioning of the two cerebral hemispheres for recognizing bilaterally
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198
 
 
 
 
 
Hines, D. (1976) Recognition of verbs, abstract nouns and concrete nouns from the left and right
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200
 
 
 
 
 
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Witelson, S. F. (1977a) ‘Early hemisphere specialization and interhemisphere plasticity: an empirical
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202
 
 
 
 
 
Zaidel, E. (1975) A technique for presenting lateralized visual input with prolonged exposure. Vision
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CHAPTER SEVEN
 
 
DETERMINATE AND PLASTIC PRINCIPLES IN
NEUROPSYCHOLOGICAL DEVELOPMENT
 
DENIS M. PARKER
 
 
Introduction
 
To those interested in the relationship between brain mechanisms and
behaviour, study of the outcome of damage to the central nervous system
currently provides the most useful information concerning the structural basis of
cognition and action. Observation of the pattern of behavioural loss and the
extent to which recovery is possible following specific brain injury enables
differing models of brain organization to be specifically tested. In fact, this
question of the pattern of loss and the extent of recovery lies at the heart of a
controversy, between the advocates of functional localization and those who
proposed a diffuse physical basis for cognitive functions, which began during the
nineteenth century. Some investigators stressed the return of almost complete
function following a transient period of loss (Flourens, 1824), or stressed the re-
emergence of functions at a reduced level while denying that behavioural effects
contingent on the damage were specifically related to the region destroyed
(Goltz, 1892). These views were amplified during the present century by the
experimental work of Lashley (1929) who argued that, excluding the primary
sensory and motor areas of the cortex, the association areas contributed in a
unified way to the performance of any complex skill—the well-known principle
of Mass Action. The degree of functional loss that could be detected following
brain damage was assumed to be determined by the extent, rather than the
location, of damaged tissue. The results of Lashley’s experiments, together with
his theoretical exposition of them, supported the views of Goldstein (1939) who
 
 
203
 
 
 
204
 
 
 
 
 
regarded cognitive skills as a function of the entire organism. He saw the
symptoms following brain damage as manifestations of a complex adaptive
syndrome in which higher intellectual abilities, in particular the ability to
abstract, were likely to be depressed. This general intellectual loss was likely to
be seen in the patients’ failure on a variety of cognitive tasks, but particularly in
linguistic skills since these demanded, more than other capabilities, the integrity
of the entire brain. Holding views such as these, there was no problem in
accounting for the return of functions after cerebral injury since the intact areas
of the brain would continue to function in their usual way following an initial
period of shock, although there would of course be a loss of efficiency. Factors
such as the age of the subject at the time of injury were seen to play a part, and
indeed one of Lashley’s students (Tsang, 1937) had provided evidence that
proportionately equivalent lesions in adult and juvenile rats produced less
impairment in the younger animals.
 
In contrast to models of the brain based on assumptions of global processing
and diffuse representation, from the middle nineteenth century onwards
evidence began to accumulate rapidly that restricted brain lesions often
produced highly specific disorders in which the patients’ remaining spectrum of
skills were relatively unaffected. Broca (1861) produced two cases in which the
patients’ verbal expression was drastically impaired but whose comprehension
of language appeared to be relatively good. Wernicke (1874) produced evidence
that patients’ ability to comprehend and utter meaningful linguistic statements
could be grossly impaired yet their pronunciation and rate of verbal output
could remain unimpaired. Other descriptions followed of patients with restricted
and often bizarre symptom complexes. Dejerine’s description (1892) of Mr. C is
perhaps one of the classic cases. This patient, who had right visual field
blindness, could nevertheless recognize objects and continue his everyday life.
Despite apparently normal vision in his left visual field and the ability to speak
and write normally, he was unable to read by sight even the sentences he had
written himself a short while before. He was however able to ‘read’ by feeling the
shapes of cut out letters. Other syndromes described included those of
disordered motor planning (Leipmann, 1908), the inability to recognize objects
(Lissauer, 1890), and the inability to recognize melody (Henschen, 1926). The list
could be continued, but the importance of these cases was that they convinced a
substantial number of investigators that the brain could not be organized in a
diffuse equipotential manner but, on the contrary, must contain relatively
specialized processors even for such uniquely human processes as the comprehension and generation of language.
 
Modern neuropsychology has moved away from the exclusive study of such
rare and extraordinary cases to the systematic examination of groups of patients
in which the behavioural effects of differences in known cerebral pathology can
be investigated. The more extreme forms of cerebral localization have been
 
 
 
DETERMINATE AND PLASTIC PRINCIPLES
 
 
205
 
 
rejected, but the acceptance of specialized processing within and between the
two hemispheres of the brain has gained wide acceptance (see Walsh, 1978, and
Hecaen and Albert, 1978). This current position regarding the distribution of
functional systems within and between the cerebral hemispheres in the adult’s
central nervous system is of considerable importance to those interested in
neuropsychological development. The relative paucity of information concerning the specific effects of brain damage during development necessitates the use
of the effects of adult brain damage as a yardstick against which the outcome
of early brain damage may be set. By contrasting the effects of early brain
injury with those which occur following damage to the mature brain, it may
be possible to ascertain whether the organizational pattern typical of the
adult brain is present also in early development or whether it emerges gradually.
 
It may seem odd that this question should be asked at all, given the fact that,
superficially at least, the anatomical structure of the brain is broadly similar in
the neonate and the adult, even to the extent of the presence of the adult pattern
of anatomical asymmetry (Wada et al, 1975). However, evidence has been
presented from time to time which argues that certain crucial functional
differences are present. Animal studies have indicated that effects of motor cortex
damage in infancy are less deleterious than equivalent damage at maturity since
the younger animals escape the flaccid paralysis and spasticity of their elders and
retain their postural and locomotor capabilities (Kennard, 1940). Even in
humans the claim has been made that children may escape some of the effects of
cerebellar injury suffered by adults (Geschwind, 1972). In the area of language it
has been known for some time that children who become aphasic following
massive left hemisphere injury usually recover speech rapidly (Guttman, 1942), a
phenomenon which is not unknown but is certainly rare in the adult (Dejerine
and Andre-Thomas, 1912). In such cases it is possible to resort to concepts like
plasticity in order to explain the greater resilience of the immature brain, but in
some cases the reorganization following injury appears to be so expeditious that
it is difficult to believe that other areas of the brain not previously involved have
acquired behavioural functions so rapidly (Kennard, 1940; Geschwind, 1972).
Even if the idea of greater neural plasticity is accepted, the question may still be
asked as to how great this capacity for compensation is? When children become
aphasic following unilateral brain damage and language eventually resides in the
contralateral hemisphere, does it cope as adequately as the damaged hemisphere
would have done? The existence of greater plasticity in the developing brain
does not necessarily mean that disparate structures are functionally equivalent.
This inference would be valid only if different structures were shown to attain the
same degree of functional sophistication.
 
These two issues concerning the establishment of the adult pattern of neural
organization and the extent to which it can be modified following early brain
damage will be explored in the following pages.
 
 
 
206
 
 
 
 
 
The context within which damage occurs during development
 
It is clear, that, as a child grows from birth, the various indices of maturation
(behavioural, neurophysiological and morphological) tend to move together
towards levels accepted as indicating greater maturity. The range and complexity of spatial, linguistic and social skills increases. In association with them
electrophysiological measures of brain activity move from showing a slow and
irregular pattern at birth to faster rhythms and widening of the bandwidth
characteristics with increasing age. Neuroanatomical changes are also evident
both at a gross level, where changes in surface area and the fissural pattern of the
brain are seen (Turner, 1948, 1950), and at the microscopic level, where the size
of neurones and the complexity of the dendritic pattern increases (Marshall,
1968). When investigators are concerned exclusively with behavioural development, electrophysiological and anatomical factors may provide an interesting
but non-essential background. However, where one is concerned with the
outcome of brain injury, which occurs early rather than late in the developmental sequence, then the broad neurobiological context in which the injury
occurs assumes increasing prominence. A child who suffers brain injury has not
just lived for a shorter time, and consequently experienced less, than an adult.
The injury has occurred within a system which has a complex and uncompleted
developmental plan which is rapidly unfolding, rather than as in the adult where
injury occurs within a system where the developmental sequence may have
reached a plateau of several decades’ duration. With this in mind some relevant
data which outline the changing status of cerebral structures during development will be examined.
 
 
Electrophysiological and neuroanatomical maturation
 
Electrophysiological measures of cerebral development in man indicate a
progressive increase in mean frequency content of the EEG from birth to
maturity. Whilst in the neonate this is characterized by a labile pattern with
periods of almost total electrical silence and an unclearly differentiated sleep
pattern, the adult shows a pattern of continuous activity with a frequency
spectrum that is at least superficially related to the subject’s state of alertness and
with a well-differentiated pattern during the various stages of sleep (see
Marshall, 1968, and Milner, 1976, for reviews, and Thompson, this volume). The
adult pattern, with the occipital alpha frequency centred in the region of 10 Hz,
and the presence of clear beta activity (14-30 Hz) during arousal, is attained
only gradually (Henry, 1944). The most typical frequency band shifts from the
delta region (1-3 Hz) at age 1 year and below, through the theta band (4-7 Hz)
between 2 and 5 years, to the alpha band (8-13 Hz) between 6 years and
adulthood. Beta frequencies become increasingly common in late childhood, and
 
 
 
DETERMINATE AND PLASTIC PRINCIPLES
 
 
207
 
 
are an increasingly common feature of the adult EEG. Measurement of evoked
potentials reveals that peak latencies are long in infancy and decrease as
maturation proceeds, whilst in general, amplitude tends to decline after an initial
rise. This amplitude trend is also characteristic of the development of the gross
EEG (Shagass, 1972). Whilst the behavioural significance of the EEG in man is
currently an extremely active research area, the status of the research findings is
not always unequivocal. The existence during infancy and childhood of a
differential density of waveforms in the left and right hemispheres (Walter, 1950)
and the presence of lateral differences in auditory evoked potentials (Molfese,
1977) cannot be taken as evidence of functional asymmetry within the cortex at
these ages, since they may indicate changes in the thalamus and corpus striatum,
which are simply mirrored in an as yet incompletely differentiated and immature
cerebral mantle. The EEG data do, however, allow us to see the gradual
emergence of intracerebral rhythms whose mean cycle time becomes shorter
with age, and whose responsiveness to external stimulation becomes crisper.
Furthermore, this developmental trend continues throughout the period from
birth to sexual maturity and may actually show reversal in old age (Shagass,
1972).
 
The impression of a long-term developmental cycle which emerges from
examination of human electrophysiological development perhaps finds even
stronger support when neuroanatomical development is examined. It is
apparent that although the full complement of neurones is probably present in
the central nervous system at birth, the brain continues to grow in overall size
and the characteristic sulcal and gyral pattern of the adult emerges gradually
over the first 6 years, and perhaps even later in the case of the frontal lobe
(Turner, 1948, 1950). When the internal differentiation of the brain is considered
a complex and protracted pattern of development unfolds (Yakovlev and
Lecours, 1967; Lecours, 1975). Using the density of myelin, the lipid sheath
surrounding the axons of neurones, as a criterion of maturation these workers
have mapped the development of CNS pathways and structures and used the
termination of the myelogenetic cycle as an index of when the system reaches
final functional maturity.
 
It is apparent from this research that brain development does not proceed
uniformly in all central nervous subsystems. Sensory fibre tracts and associated
nuclei myelinate before the cortical intra- and interhemispheric communication
systems. Different sensory systems however may show a diversity of time
courses. The optic radiations subserving the visual cortex show a short cycle of
myelination closely following the myelination of the optic tract, and is virtually
complete by 4 months of age. The acoustic radiation subserving the auditory
cortex does not show complete development until the 4th year, in marked
contrast to the prethalamic auditory system, which is mature by the 4th
postnatal month. The intracortical and interhemispheric association fibres,
 
 
 
208
 
 
 
 
 
which allow communication between different regions and whose disruption
leads to many of the bizarre neuropsychological disconnection syndromes
(Geschwind, 1965), have prolonged maturational cycles extending into the
second decade of life. Examination of the cortex itself indicates that some regions
implicated in linguistic competence (inferior parietal lobule) show a slow onset
of myelination and that this process may also continue beyond the second
decade of life. It should be emphasized that although the study of the
myelogenetic cycles of maturation shows a complex extended pattern of
development from birth to maturity, a caveat is necessary. Neural conduction
can occur in fibres before they become myelinated (Ulett et al , 1944) and indeed
myelination may be aided by neural activity (Langworthy, 1933). However,
myelin contributes greatly to the efficiency and speed of neural conduction,
which underlies the complex analysis and planning characteristics of human
cognition and action. The importance of myelin may be appreciated by
considering the devastating behavioural effects of demyelination disease in man
such as occurs in multiple sclerosis. The myelogenetic analysis emphasizes the
manner in which the brain becomes structurally mature. Phylogenetically older
structures, in general, mature earlier than those of recent phyletic origin. There
are exceptions to this, for example the reticular formation, whose developmental
cycle again extends into the second decade of life, partly no doubt because its
final operational capability is not required until cortical systems are completely
mature.
 
Both the electrophysiological and neuroanatomical data indicate that the
human central nervous system develops over an extended period. Particularly
fascinating are the observations that the higher auditory systems (geniculo-
temporal pathways) exhibit a developmental cycle that is more than ten times
longer than visual structures at the same level of the CNS. It is probable that this
reflects the necessity of incorporating into the growing brain the species specific
demand of learning the complexities of language via the acoustic mode and
allows for the building of the culture-specific phoneme system which is
eventually used. However, it is apparent that a considerable part of the
developing child’s life is characterized by the presence of cortical structures
which are functionally (by electrophysiological and anatomical criteria)
immature. The status that these immature structures have in subserving the
growing child’s cognitive repertoire is not straightforward. It is possible to
envisage the mapping of cognitive growth within the cerebral structures which
subserve cognition in the adult in at least two ways. The first view would regard
all those structures which are involved in a given functional system in the adult
as also being involved in infancy, although the immaturity of the system
considered as a whole would limit its capabilities. The limitations seen for
instance in perceptual and motor systems in the neonate would be ascribed to
the immaturity of the cortical system, which was nevertheless functional, rather
 
 
 
DETERMINATE AND PLASTIC PRINCIPLES
 
 
209
 
 
than to the fact that lower but morphologically more mature structures are
controlling the behaviour. The alternative view would regard neurobehavioural
development as a process beginning with relatively primitive analysis and
response control systems residing in phylogenetically older structures which
mature earlier in the developmental sequence. As development proceeds,
phylogenetically newer structures, which attain maturity later in the developmental sequence, will capture and modulate the activity of the older systems as
well as adding more complex control processes. This process will not have a
uniform time course across all systems. In the case of the visual system this
transition may occur relatively early in development at two to three months of
age since the cortical network in this case has a short developmental cycle. In the
case of audition the whole process may be more protracted with cortical control
becoming ascendant in the period of 2-4 years, whilst in the complex action
systems controlled by the frontal lobe, the processes may take even longer, given
the differential growth of the region beyond 6 years of age (Turner, 1948, 1950).
Evidence that it is this latter view which is the more reasonable description of
neurobehavioural development comes from a number of observations that will
be subsequently outlined, but it must be said in advance that the case is by no
means proven.
 
 
Neuropsychological evidence concerning functional maturation
 
When damage occurs in the mature brain the effects are usually immediate; the
degree of functional loss, whether it be sensory, motor or cognitive, is greatest
immediately after the lesion and usually shows some remission over time. In the
immature brain in a number of instances the pattern is almost the reverse of this.
In the case of hemiplegia, sustained as a consequence of prenatal or perinatal
injury, the symptoms of the disorder frequently emerge gradually over a
prolonged period. Abnormalities in the use of the hand and arm may emerge at
between 4 and 6 months, whilst differences in the behaviour of the legs may not
become apparent until 10 months of age or more. Deficits in the use of the lower
limbs may only become apparent when the child begins to walk, a milestone that
is frequently attained in the normal age range (Lyon, 1961). Finally, athetoid
movements do not make their appearance until much later, typically between
three and four years of age (Lenneberg, 1968). Thus the developmental pattern is
seen to move through a series of stages: for example, the grasp reflex is present
initially in both the affected and the normal hand, but it persists on the affected
side and usually the hand becomes clenched into a tight fist. Individual finger
movements do not appear on the hemiplegic side, although movement of the
thumbs may be possible. As the child grows, then, the deficit becomes more
severe. However, it is noticeable that these children escape some of the more
drastic consequences of hemiplegia resulting from cerebral injury at maturity.
 
 
 
210
 
 
 
 
 
The severity of paralysis in the upper limb is less and the lower limb is capable of
greater use than after comparable damage in adults. That the residual function
on the affected side is not attributable to surviving undamaged tissue in the
diseased hemisphere is shown by the fact that removing the diseased hemisphere
later in development does not cause further impairment and may frequently
result in improvement (Cairns and Davidson, 1951).
 
Two features then are apparent in these cases. The first is the greater capacity
of the young brain to compensate and escape some of the consequences of
equivalent injury in adulthood—its plastic propensity. The second feature is
cogently described by Lenneberg (1968): ‘one may say that the child with a
perinatal cerebral injury only gradually grows into his symptoms’. This
emergence of deficit with age is compatible with the view that certain systems are
pre-programmed to appear at certain stages in development and injury to them
will only become apparent when they fail to appear. Similar affects in the motor
sphere were observed by Kennard (1940) in macaque monkeys where ablation of
the motor cortex in infancy resulted in surprisingly little immediate effect, but
precluded the development of fine manipulatory skills, and this, together with
signs of dyskenesia, became more apparent as the animals grew older. It appears
that the motor cortex only begins to exert its influence gradually and its loss or
malfunction may not become apparent until a later stage of development.
 
Further support for the notion that behaviour early in development may be
mediated exclusively by subcortical structures, with cortical processors
becoming involved later, has been provided by Goldman (1976). It has been
apparent for some time that tasks which require a monkey to remember the
location of a stimulus over a brief interval of time are drastically affected by
damage to the dorsolateral prefrontal cortex (Chow and Hutt, 1953). These are
known as the delayed-response and delayed-alternation tasks. When the
dorsolateral frontal cortex is removed within the first 2 months of life however,
the operated animals perform as well on this task as unoperated controls as long
as testing is carried out within the first year of life (Harlow et al , 1970). If these
monkeys are followed into the second year of life they show evidence of
increasing impairment of delayed response tasks. Lesions in subcortical
structures which are functionally connected with the dorsolateral prefrontal
cortex (dorsomedial nucleus of the thalamus and the head of the caudate
nucleus), indicate that the monkeys operated on as juveniles show the same
pattern of deficit as adults, i.e. failure on delayed alternation tasks (Goldman,
1974). These results emphasize that not only may the effects of brain damage fail
to appear early in development and only reveal themselves as the animal grows,
they also show that structures which are required for the adequate performance
of a task in adulthood (the dorsolateral prefrontal cortex), are not necessary for
the performance of the same task when the animal is an infant or a juvenile.
 
These experiments contribute a salutary warning to those who would assume
 
 
 
DETERMINATE AND PLASTIC PRINCIPLES
 
 
211
 
 
that, because a cortical structure is implicated in a functional system in
adulthood it must of necessity be involved in performance of those same
functions at an earlier developmental stage. In infant and juvenile macaques
subcortical systems are capable of mediating responses which in adults require
the participation of the cortex. Furthermore, evidence is available which
indicates that the time course over which the effects of cortical lesions become
apparent during development differs for different regions. While monkeys aged
2\ months with orbital frontal cortex lesions are equivalent to unoperated
controls in the performance of object reversal learning, deficits in performance
became apparent by the time the animals are one year old (Goldman, 1974).
Thus, while the effects of dorsolateral frontal cortex damage sustained in infancy
on delayed alternation tasks do not become apparent until after one year of age,
the effects of orbitofrontal damage in infancy on object reversal learning
become apparent before one year of age. In the case of both delayed alternation
and object reversal learning early in development subcortical structures are
capable of mediating the behaviour successfully. With respect to these studies it
should be pointed out that macaque monkeys attain sexual maturity between 24
and 30 months of age, so the effects of early brain damage may take a
considerable proportion of the developmental cycle before they become
apparent.
 
The evidence cited so far has argued for a model of development in which the
complete maturational cycle is prolonged, but within which different systems
may attain functional maturity at widely divergent times. Initially, behaviour
may be controlled by subcortical systems, and depending on the time-course of
the maturational cycle in the higher reaches (cortical) of each system so control
will pass to the phylogenetically newer and more adaptive system. Bronson
(1974) has argued, on the basis of changes in the pattern of visual behaviour in
infants, that in the case of the visual system a transfer of control passes from the
superior colliculus to the striate cortex, from the ‘ambient’ to the ‘focal’ system
(Trevarthen, 1968), during the 2nd and 3rd postnatal month. Such a view is
certainly compatible with the rapid postnatal development of the geniculo-
striate system (Yakovlev and Lecours, 1967). In the case of motor function the
longer maturational cycle of the medullary pyramids (up to 12 months), which
carry the axons of the Betz cells of the motor cortex to the spinal cord and are
required for control of individual fingers and skilled sequences (Lawrence and
Hopkins, 1972), leads one to expect that transition of control in this case may
take place over a more protracted period. The emergence of comparative motor
deficit between affected and normal side, in cases in infantile hemiplegia, between
4 and 12 months of age (Lyon, 1961) appears to reflect this longer cycle of
development. In man, however, the long-term emergence of a deficit in
previously established skills after early brain injury akin to that reported by
Goldman (1974, 1976) for the macaque monkey, has not as yet been reported.
 
 
 
212
 
 
 
 
 
Nevertheless, evidence is available which indicates that, on some perceptual
tasks, brain-damaged children may show either a consistent difference from
controls over a wide age range (Cobrinik, 1959), or a progressive change, which
either diverges from or converges to control values between the ages of 5 and 15
years depending on the particular measure being considered (Teuber and Rudel,
1962). Thus, slowly emerging effects of early brain damage in man are not
unknown, but long-term effects, with an intervening ‘silent’ interval, have not, to
the present author’s knowledge, been described.
 
 
Interactive effects and their interpretation
 
Generalized effects on intelligence that result from presumed structural
incapacity (Hebb, 1942) or following localized injury (Thompson, 1978) have
indicated that while early injury may produce generalized depression of full-
scale IQ, late injury produces a more specific pattern of deficit, with less
depression of full-scale IQ. Research findings of this nature can be seen as
support either for a model of brain development in which neural structures only
gradually attain their mature state, or alternatively, as support for the view that
reorganizational capacities are at work during development which allow savings
on specific skills but at the cost of overall depression in intellectual attainment.
However, there is no doubt that both views are valid and have independent
evidence to support them. The developing brain is more vulnerable than the
mature brain and this can be seen in the long term generalized deleterious
consequences of nutritional deprivation during the brain-growth spurt, as
measured by both neurochemical and anatomical criteria (Dobbing, 1968), and
by behavioural criteria (Chase, 1973). However, it is also clear that substantial
left hemisphere damage, which usually leads to long-term severe dysphasia in
adulthood, results in only transitory dysphasic symptoms when the damage
occurs early in childhood (Hecaen, 1976), and there is evidence of savings in
visuo-spatial skills following early right hemisphere dysfunction, which is not
apparent with equivalent damage at maturity (Kohn and Dennis, 1974).
 
These findings provide unequivocal evidence of the reorganizational and
plastic capacities of the developing brain. However, it is also evident that the
usual predetermined pattern of left hemisphere specialization for language and
right hemisphere specialization for visuo-spatial skills set limits to this plastic
capacity. Where one hemisphere has sustained early injury, the language skills
exhibited by a remaining and intact right hemisphere are less proficient than
those exhibited by a remaining intact left hemisphere (Dennis and Kohn, 1975).
It also appears that visuospatial abilities are better developed in an intact right
hemisphere than when the left hemisphere alone remains fully functional (Kohn
and Dennis, 1974). Thus, while reorganization is possible following early brain
injury, this may be more limited than has been previously thought. Indeed, given
 
 
 
DETERMINATE AND PLASTIC PRINCIPLES
 
 
213
 
 
the anatomical asymmetry present in the neonatal brain (Wada et al, 1975) it
would be surprising if this structural specialization were not detectable at some
stage in the behavioural effects of unilateral brain injury. The results of unilateral
brain injury in early development nicely juxtapose two major tendencies which
emerge from neurobehavioural research: on the one hand, the trend towards
carrying through the construction of a preprogrammed system over a protracted
time period and, on the other, an adaptive plastic capacity which allows the
partial redirection of functional systems should damage occur.
 
The complexity of the processes involved in the analysis of sensory information (Hubei and Wiesel, 1963; Werner and Whitsell, 1973) that has emerged
from single neurone recording makes it clear why a great deal of preprogramming must be involved in the construction of complex adaptive neural
systems. It is less easy to describe the mechanisms which allow complex brain
systems to exhibit the degree of plasticity that they evidently do in early
development. The explanation may reside in the fact that younger individuals
are less susceptible to transneural degeneration effects and also show higher
levels of biosynthetic activity in brain tissue, which may allow damaged
networks greater restitutional capacity (see Goldman and Lewis, 1978, for
review). It is also evident that behavioural recovery, seen after early injury in
some cases, depends on the ability of young animals to profit from experience in
a way not available to the adult. Monkeys sustaining brain injury in infancy
show greater recovery the earlier training experience is given in development,
although the ability to profit from this experience depends on the nature of the
task and the site of the lesion in the brain (Goldman and Lewis, 1978). The
results of this study gave some indication that it was the nonspecific stimulation
effects of the training which were important in recovery, rather than specific
carry-over from common features of the task.
 
The fact that experience per se may be an important factor in promoting
recovery is of considerable importance, since there has been suspicion about the
value of training programmes in the recovery from brain damage (Byers and
McLean, 1962). Given the intricacy of the neurobiological factors involved in
early brain injury it may be too easy to forget about the cognitive dimension.
During development a child moves from a relatively primitive analysis and
interaction with the external world to a stage where his conceptual structures are
complex and enable sophisticated analysis and prediction of the environment.
The cognitive capacities evident at maturity have been built on simpler ones
gathered progressively during childhood. Thus, impairment of systems which
gather and utilize information as a consequence of cerebral damage could result
in diminished intellectual achievement, not through injury to critical higher-level
systems, but because essential ‘feeder’ mechanisms have malfunctioned. An adult
with a subcortical lesion which destroys the left auditory radiation and the
callosal input from the contralateral hemisphere may show a normal audiogram
 
 
 
214
 
 
 
 
 
but be incapable of interpreting spoken language (Gloning et al ., 1963).
Nevertheless, speech, reading and writing may be normal since their associated
cortical circuitry is undamaged and language has been previously established. In
a child, bilateral disruption of the auditory radiations by a lesion damaging the
lips of the sylvian fissures and the insulae can preclude the normal development
of language, despite evidence of hearing in the normal range (Landau et al ,
1960). Even children who become peripherally deaf after acquiring speech may
not only show arrested language development but lose previously acquired
linguistic skills (Bay, 1975). This ‘cognitive starvation’ effect, which is easy to
comprehend in the instances cited, may also operate within association areas of
the cortex and may underlie the tendency of early brain pathology to produce
more global intellectual depression effects than are seen with lesions in
adolescence and adulthood.
 
The data briefly reviewed in this section emphasize that the effects of brain
injury in the developing nervous system are particularly complex. When injury
occurs, it is within a system whose functional capacities are still unfolding, and
the effects of injury are sometimes not immediately apparent. The plastic
capacity of the system may also mask the extent of any physical injury and lead
to a false assumption that effects have been transitory. There is also the added
complication that, whilst injury may not have damaged structures critical for the
attainment of certain cognitive skills, because critical ‘feeder’ systems have been
impaired, these cognitive mechanisms may not have the experiental basis upon
which to build.
 
The consequences of early brain damage
 
Global and specific processing
 
In 1942 Hebb drew attention to the different patterns seen following brain
damage in both mature and immature nervous systems. Adults, he found, could
be considered to be either aphasic or non-aphasic types (Hebb, 1942). The
aphasic type showed obvious evidence of deterioration on verbal tasks and some
also showed evidence of impairment on such non-verbal tasks as detecting
absurd errors in pictures and block manipulation performance. However, some
of the aphasic patients showed evidence of almost normal performance on nonverbal tasks, and Hebb remarked on the wide disparity of abilities seen on
particular tests in individual aphasic patients. This conclusion of Hebb’s (that
non-verbal skills may be retained to a remarkable extent in some cases of
aphasia) is reinforced by more recent research on this topic (Zangwill, 1964;
Kertesz and McCabe, 1975). In the non-aphasic type of brain injury on the other
hand many verbal tasks could be adequately completed, but there was usually
severe impairment on maze learning, block manipulation and picture absurdity
tasks, as well as impairment on some verbal tasks, e.g. the defining of abstract
 
 
 
DETERMINATE AND PLASTIC PRINCIPLES
 
 
215
 
 
words, or naming opposites. The two types were, however, sufficiently different
for Hebb to state that following adult brain injury a reasonably specific pattern
of deficit often emerges—some skills showing deterioration and others being
relatively intact. In contrast, a group of children with what he termed
‘exogenous’ brain injury showed no evidence of a dual pattern, that is the
‘aphasic’ and ‘non-aphasic’ types did not occur as a consequence of early brain
injury. The group as a whole showed depression of verbal IQ, but since he
thought it unlikely that every case of brain injury in infancy involved damage to
the language areas it must be that ‘low verbal test scores are produced by early
lesions outside the speech areas’ (Hebb, 1942, p. 286). He went on to argue that
the more global pattern of intellectual depression seen after early brain injury
occurs as a result of the differing demands being made on the adult and the child
after cerebral damage. The adult has merely to make use of skills which have
already been acquired, whereas the child has still to assimilate a range of skills.
Since a greater cognitive demand is made during the acquisition of a skill than
by the performance of one already acquired, the growing child is at a greater loss
than the adult when an equivalent amount of brain tissue has been lost in both.
Hebb went further and argued—following Lashley (1929)—that some degree of
equipotentiality must exist in the cortex and that areas outside the classical
language areas must be involved in the development, but not the maintenance, of
linguistic skills once they have been mastered.
 
Two major hypotheses then emerge from Hebb’s work (1942). The first is that
early, rather than late, brain damage has a more global depressive effect upon
intellectual development. The second hypothesis is that the developing nervous
system is characterized by a greater degree of equipotentiality than that of the
adult, since the attainment of normal adult performance on a range of specific
skills seems to depend on the integrity of whole cerebrum. The first hypothesis
has, in general, received support from subsequent research. Bryan and Brown
(1957) found that there is a strong relation between the age of injury and mean
IQ, so that those with an injury present at birth averaged a score of 62, those
injured in infancy averaged 66 while those with injuries occurring between 3 and
10 years and between 10 and 20 years averaged 71 and 85 respectively.
Thompson (1978) reported that in 282 subjects who sustained localized cerebral
injury in childhood, there was a linear relationship between age of injury and
full-scale IQ with those injured before 5 scoring 97 and those injured above 15
years scoring 106.5. It should be noted, however, that whilst McFie (1961a)
found a rise in mean IQ between those injured in the age bands 1-4 and 5-9
years from 88.8 to 106.0, he found a fall in IQ with those sustaining injury
between 10 and 15 years (82.7). However, on balance the findings would seem to
support Hebb’s initial contention.
 
The second hypothesis emerging from Hebb’s (1942) study, that the
developing nervous system is characterized by a greater degree of equi-
 
 
 
216
 
 
 
 
 
potentiality than that of the adult, is rather more contentious since it is more
difficult to test than it might appear at first sight. It has already been pointed out
that general depression of IQ cannot be used as evidence for a type of mass
action operating during development, since it may also argue for an interdependence of separate capabilities being required for the construction of more
complex schemata. It is also apparent that brain-damaged children show widely
differing patterns of impairment, which would be difficult to comprehend if there
were a tendency for the brain to act uniformly in the acquisition of cognitive
skills (Strauss and Lehtinen, 1968). There is also the added difficulty that IQ
tests may be rather insensitive to specific patterns of disability produced by brain
injury, both in children (Boll and Reitan, 1972) and adults (Walsh, 1978), a
factor which has resulted in the construction of specialized test batteries.
 
However, instead of asking whether the general depressive effect of early brain
damage on IQ is due to a greater degree of global processing in the immature
CNS, it might be more fruitful to consider whether a similar pattern of
impairment emerges on specific skills after similar damage in the child and the
adult. McFie (1961a), in an investigation of the effects of localized post-infantile
cerebral lesions in children, found that there was a tendency for Wechsler verbal
scores to be lower following left hemisphere injury and performance scores to be
lower following damage to the right hemisphere. He also noted a similarity in the
pattern of impairment shown on the Memory for Designs component of the
Terman-Merrill scale (1937) between children and adults when comparing the
effects of frontal, temporal and parietal injury. He reported that the greatest
deficit is to be found in both groups following right parietal damage. Fedio and
Mirsky (1969) examined the pattern of impairment exhibited by children with
either unilateral temporal lobe or with centrencephalic epilepsy on a test battery
designed to measure performance on both verbal and non-verbal tasks, and a
task of sustained attention. The children, who had a history of illness dating from
early school years, showed similar impairment profiles to those of adults with
similar pathology. Those with left temporal epileptiform foci required a greater
number of trials to learn lists of ten words and showed greater loss after a 5-
minute interval than those with right temporal or centrencephalic pathology.
Those with right temporal pathology showed greatest impairment on the recall
of the order of random shapes and on production of the Rey-Osterrieth figure.
The centrencephalic group showed the greatest deficit on a task requiring
sustained attention. Annet et al (1961) also found a similar pattern of verbal
and spatial difficulties in children classified on the basis of lateralized EEG
abnormalities. These results would suggest that children show impairments of
the same type as those found in adults with similar pathology. It may be objected
however, that in these cases the damage is characteristic of juvenile rather than
infant brain damage, and, if adult cortical specialization appears gradually, then
patterns of specific loss will also begin to appear, producing the observed
 
 
 
DETERMINATE AND PLASTIC PRINCIPLES
 
 
217
 
 
similarity to the adult type by middle childhood. However, evidence does exist
which would suggest that there is hemisphere specialization, even when damage
occurs perinatally or in infancy.
 
Damage to the left, but not the right, hemisphere before the end of the first
year of life results in impairment in the rate at which combinations of words
(elementary syntax), but not single words, are learned (Bishop, 1967). Furthermore, children who have had either a left or right hemisphere removed as a
consequence of damage sustained during the first year of life show differential
effects depending on which hemisphere is involved. Those with left hemisphere
removal show greater difficulty in the comprehension of syntax than right hemi-
decorticates (Dennis and Kohn, 1975). In particular, difficulties as shown by a
greater number of errors are apparent in comprehension of the passive negative,
e.g. ‘the girl is not pushed by the boy’, as opposed to the active affirmative, ‘the
boy pushes the girl’. In these experiments comprehension was assessed by having
the child choose a picture which depicted the sentence. The greater difficulty in
the comprehension of passive sentences was also apparent in longer response
latencies. Children with early right hemisphere damage followed by hemi-
spherectomy show difficulties on spatial tasks (Kohn and Dennis, 1974). The
types of spatial task on which they show relative deficit are those which continue
to show improvements in normal subjects through the teens, e.g. the WISC and
Porteus mazes, and map reading tasks, which require a subject either to state the
direction to be taken or to follow a route through markers placed on the floor.
Early maturing skills, such as tactile form matching and visual closure, were
unaffected in contrast to right hemisphere injury at maturity which severely
depresses these abilities. Further analyses of three cases where hemispherectomy
antedated the beginnings of speech were presented by Dennis and Whitaker
(1976) and two cases where hemispherectomy for unilateral pathology was
carried out at ages 3 and 4 years respectively were reported by Day and
Ulatowska (1979). In these cases the pattern of deficit is similar to those
previously reported and supports the view that the hemispheres are differentially
involved in different aspects of cognition. Specialization would appear to be an
early-established characteristic of the child’s brain although its potential for
reorganization complicates the issue (see p. 225).
 
Given that differences in the patterning of intelligence subtest scores only
really become obvious during adolescence and beyond (Thompson, 1978),
whereas specific deficits are detectable by specially devised tests in younger
children, IQ tests appear to be insensitive instruments on which to base theories
concerning brain development. One obvious factor which will lessen the
sensitivity of IQ tests to brain damage early in development is the capacity to
transfer the development of linguistic and spatial skills to the contralateral
hemisphere should damage occur. This plastic capacity should not however be
confused with ideas about mass action or equipotentiality. These latter ideas
 
 
 
218
 
 
 
 
 
imply the existence of a type of diffuse processing network to which the
association cortex within each hemisphere contributes uniformly (Lashley,
1929). The transfer of functional capability from one hemisphere to the other,
however, is to be understood in a rather different context. Each cerebral
hemisphere contains anatomical structures which are essentially duplicates of
those found in the other, with the difference that certain cytoarchitectonic areas
may be relatively larger or smaller. In some instances, particularly in the
language area, these cytoarchitectonic differences in size may be as great as
700% in favour of the left hemisphere (Galaburda et al ., 1978). However, each
specialized cortical area has a pattern of connections to the rest of the brain
which, in essence, is a lateral reversal of those found in the other hemisphere.
The ability of these ‘duplicates’ to assume some of the functional capacities
previously assumed by their cytoarchitectonic counterparts is perhaps not too
surprising. Indeed, the puzzling feature is that in a substantial majority of the
adult population this ability is lost. The assumption that somehow the areas
within each hemisphere act as a kind of equipotential unit, is not supported by
the available evidence. If such a diffusely organized system were operative in the
left hemisphere during development, then it would be reasonable to assume that
unilateral damage, resulting in suboptimal processing capacity, would be
sufficient reason to transfer linguistic processing to the remaining intact
hemisphere. Milner (1973) has provided evidence, based on language lateralization, tested by the Wada (intracarotid injection of sodium amytal) technique,
that only when early injury invades those areas shown to be important for
linguistic processing in the adult, will language transfer to the right hemisphere.
These results suggest that certain key regions, not the global processing capacity
or total operational mode of one hemisphere, promote the establishment of
language within that hemisphere.
 
In considering the question of global or specific processing in development,
the evidence on balance suggests a specific processing configuration not
dissimilar to that found in the adult brain. It has also been argued that one of the
reasons why early brain damage often has a markedly depressive effect on
intellectual growth may be that the substrate of complex cognitive processes
may require the integrity of fundamental systems in order to attain their full
potential. One line of evidence which supports this is the finding that right
hemisphere damage, which occurs before one year of age, may have more
depressive effects on cognitive growth than damage to the left hemisphere at an
equivalent age, or damage to either hemisphere after one year of age (McFie,
1961 b; Woods, 1980). This greater deleterious effect of early right hemisphere
pathology on both verbal and performance IQ scores argues for the participation of the right hemisphere in certain fundamental processes that may
underlie both linguistic and spatial competence. Given the specialization of the
right hemisphere for the acquisition of spatial skills it appears possible that a
 
 
 
DETERMINATE AND PLASTIC PRINCIPLES 219
 
certain elementary sensori-motor coordinate system may normally be established by the right hemisphere, and in some way aid the differentiation of more
complex skills which do not, at first sight, appear directly connected to it. The
importance of elementary sensori-motor expedience in evolving more complex
cognitive operations has been stressed by many theorists, and by Piaget (1979)
in particular. However, regardless of whether or not the presence of this
correspondence between theories of cognitive development and neuropsychological research finding is accepted, the results obtained by McFie (1971a, b) and
Woods (1980) emphasize the importance of the presence of a particular structure
at a particular stage during development rather than supporting the view that
different structures are functionally equivalent.
 
 
Aphasia in children
 
The study of language disorders in children is important to a number of
problems in developmental neuropsychology. It has provided evidence concerning the extent of functional specialization in the cerebral hemispheres during
maturation; data concerning the reorganizational or plastic capacity of the
developing brain; and a third, equally important, question, evidence as to the
manner in which language becomes established in cortical structures. This
question is, in fact, separate from those of hemispheric specialization and
plasticity. Here we are concerned with the similarity between the aphasic
symptoms of the child and the adult. The greater the similarity of the syndromes
the more likely it is that the adult structural pattern has become established,
even if subsequently the recovery of the child is more complete because
duplication of function in the contralateral hemisphere is still possible. In the
present section each of these three topics will be examined, but the characteristics of childhood aphasia will be examined first.
 
The clinical picture found in childhood aphasia was described by Guttman
(1942), who noted that despite claims that the syndrome was rare, he had found
it not an unusual accompaniment of head injury or intracranial pathology. In
contrast to the adult, where complaints with speech difficulties, failures to name
objects and paraphrasia are common, the aphasic child is usually apathetic and
morose with such extreme poverty of speech that it approaches mutism. Absence
of spontaneous speech, lack of willingness to speak, and a hesitant dysarthric
telegrammatic-style speech are frequently noted, these symptoms being more
common in the younger child. In contrast to the extreme poverty of speech
production, comprehension of simple instructions is evident so that parts of the
body or objects can be appropriately indicated when a request is made. When
prompted to speak difficulties in the manipulation of lips and tongue may be
apparent together with failure to produce sound. As recovery progresses,
initially the child will speak single words when prompted. It will then move to
 
 
 
220
 
 
 
 
 
sparse spontaneous utterances before speech moves into a stage when
impoverished but spontaneous conversation occurs with persistent dysarthria.
This pattern, which bears the imprint of an almost exclusively motor disorder,
occurred in all cases below 10 years of age and occurred regardless of the
location of the lesion within a hemisphere. Where damage occurred after the
tenth year in some cases speech showed a lack of spontaneity but symptoms
more characteristic of the adult pattern with impaired auditory comprehension,
syntactical and paraphrasic errors, together with difficulties of naming were
found. Dysarthria may or may not accompany these symptoms.
 
This picture of aphasia in childhood has been supported by subsequent
research in which the onset of the language loss is sudden following external
injury or internal pathology. In Guttman’s cases the five instances of injury
which produced aphasia in which the symptom was exclusively one of speech
production difficulty were aged 8 years or below, and the two instances in which
speech output was not affected but other aphasic symptoms were present were
over 10 years of age. The series of cases reported by Hecaen (1976) show that in
two instances where the disorder was exclusively one of speech production the
children were aged 6, and in the remaining case 3j years. In children aged 7 years
and more, comprehension, naming and paraphrasic disorders were more likely
to occur. Both Guttman (1942) and Hecaen (1976) stress the fact that recovery
may be extremely rapid, marked improvement sometimes being noted in as little
as 6 weeks. In children aged over 10 years however, the time course of the
disability may sometimes be prolonged. In middle to late childhood the aphasic
symptoms begin to mimic the adult pattern, whereas in early childhood the
disorder appears to be purely expressive. In adults expressive aphasia is often
produced by lesions located in the anterior region of the hemisphere and
difficulties in comprehension occur more frequently with temporal and inferior
parietal damage (Geschwind, 1970). However, in children below 10 years,
disorders of expression appear to occur regardless of the location of the lesion
(Guttman, 1942). A second distinctive feature of the aphasic syndrome in young
children is the absence of cases of jargon aphasia, where the speech output is
rapid with relatively normal articulation but contains many circumlocutions
and paraphrasias (Geschwind, 1972). Woods and Teuber (1978) claim to have
one documented case of jargon aphasia in a 5-year-old child, but the clinical
description is unlike the adult form. The child produced a stream of meaningless
sounds, but when recognizable words were uttered they were most often the
names of objects. The child also showed evidence of a purely apraxic disturbance, e.g. sticking out his tongue when asked to blow out a light. To the
present author, the picture is too dissimilar to the adult form to be classed as an
instance of jargon aphasia, the only feature in common being the high rate of
vocal, as opposed to verbal, output.
 
The cases of language loss in childhood so far described were instances where
 
 
 
DETERMINATE AND PLASTIC PRINCIPLES
 
 
221
 
 
loss was abrupt, following external or internal injury, and where injury was
usually confined to one cerebral hemisphere. A rather different type of childhood
aphasia occurs when the language loss is associated with either the onset or
development of bilateral epileptiform abnormalities (Landau and Kleffner,
1957; Worster-Drought, 1971; Gascon et al , 1973). In these cases, the loss of
language is associated with difficulties in understanding speech, which, in some
cases, may evolve over a matter of days or weeks. The child shows lack of
response to speech, which may be mistaken for peripheral deafness. Audiometric
testing reveals either mild or moderate hearing loss, but this loss is insufficient to
account for the comprehension disorder and, in any case, hearing usually shows
progressive improvement after an initial depression. In some cases auditory
evoked potentials to pure tones may be normal, but evoked potentials to speech
show abnormalities (Gascon et al ., 1973). Loss of language is gradual and
persistent, and while in some cases recovery may occur over a period of years
(Landau and Kleffner, 1957), in other cases it appears to be permanent (Worster-
Drought, 1971). In some cases loss of speech may be almost total and auditory
comprehension limited to less than a dozen words. Despite gross impairment in
the development of language, frequently these children do not show impairment
on non-verbal tasks in intelligence tests. Of the 14 cases described by Worster-
Drought (1971), performance IQ ranged from 96 to 140, with only one case
falling below 100. This remains true despite the fact that, in many cases, the onset
of pathology is at less than 5 years of age. These cases of bilateral abnormality
are in contrast to cases where a unilateral lesion produces aphasia, from which
the child subsequently recovers yet shows a low overall IQ (Hecaen, 1976).
 
When damage to a single hemisphere produces aphasia the child usually
recovers language, and in the young child this recovery is usually better than
when damage occurs above 10 years (Lenneberg, 1967). This has often been seen
as evidence that the two cerebral hemispheres are initially equipotential as far as
the development of language is concerned. Further, it has sometimes been
claimed that both cerebral hemispheres are involved initially in language
development with lateralization increasing with age (see Dennis and Whitaker,
1977 for a review). It has already been noted that as far as attainment on certain
language tests is concerned the two hemispheres are not equivalent. The view
that the right hemisphere is involved in language acquisition in the infant and
young child comes from reports of the high incidence of speech disturbances
following right hemisphere damage. The incidence of language disorders with
lesions of the left and right hemispheres described by different researchers varies
widely. In the case of the left hemisphere, damage has been estimated to produce
language disorder with an incidence varying from 25% (Ingram, 1964) to over
90% (Dunsdon, 1952). In the case of the right hemisphere the estimated
incidence has varied from less than 1 % (Ingram, 1964) to nearly 38 % (Dunsdon,
1952). Only one investigator has claimed an equal frequency of language dis-
 
 
 
222
 
 
 
 
 
order following either left or right cerebral damage (Basser, 1962). The discrepancies seem too large to attribute to statistical sampling fluctuations. One of the
problems encountered in this area is the definition of what constitutes an aphasic
language disturbance. Language difficulties are associated with depressed
general intelligence (Mein, 1960) so that severe brain damage which produces
severe retardation may produce language disturbance indirectly. There is also
the problem of whether speech disturbance should be considered an aphasic
disturbance (Ingram, 1965). It is already apparent that the syndrome of aphasia •
in children may vary from almost total mutism to a clinical picture similar to
that of the adult with comprehension disturbance and naming impairment. That
the type of impairment can vary not just with age of the child but also be related
to the damaged hemisphere can be seen by examining the series of Hecaen
(1976). Of 6 cases of right hemisphere damage, only the two youngest (6 and 3^
years) showed any disturbance and this was articulatory. Bishop (1967) has
reported that in cases of infantile hemiplegia, articulatory disturbances are
equally likely following damage to either hemisphere, but that left hemisphere
damage additionally delays the acquisition of word combinations rather than
single words.
 
The possibility of a different pattern of impairment following left and right
hemisphere injury is not the only factor which complicates the issue. Woods and
Teuber (1978) have pointed out that there is a tendency for investigators since
1940 to report a lower incidence of aphasia following right hemisphere injury
than earlier workers. They attribute this to the fact that in older investigations
aphasias and hemiplegias were frequently complications of systemic infectious
illnesses such as scarlet fever, bacterial pneumonia and diphtheria, which can
produce not only focal lesions but also diffuse bilateral encephalopathy.
Undoubtedly the frequent reliance on hemiplegia alone as the sign indicating
exclusive damage to one hemisphere is likely to result in the inclusion of cases
where a less extensive pathology is also present in the hemisphere that is
assumed to be intact. Bearing these facts in mind, it would obviously be
hazardous to speculate concerning the true incidence of language disturbance
following right hemisphere pathology. For the moment it is sufficient to say that
the incidence of aphasia following right hemisphere damage may be considerably less than previously thought, perhaps as little as 5 % in those who were
previously right-handed (Woods and Teuber, 1978).
 
A finding that has already been mentioned several times is that concerning the
capacity of the right hemisphere to acquire language following early left
hemisphere injury. There is little doubt that the capacity to transfer language to
the right hemisphere is a real factor in the recovery from aphasia in children.
However, it cannot be assumed that in every case of childhood aphasia recovery
of language is due to transfer to the contralateral hemisphere. Milner (1974)
noted, on the basis of the Wada test, that in adults who were left-handed but had
 
 
 
DETERMINATE AND PLASTIC PRINCIPLES
 
 
223
 
 
sustained early left hemisphere damage, language was present in the left
hemisphere in 30 % and bilaterally present in 16 % of cases. Thus, in 46 % of cases
who had left hemisphere injury, the left hemisphere was still involved in language
to some degree. Whether or not language transferred depended on whether
certain critical areas were damaged. In cases where left hemispherectomy is
performed, following widespread unilateral damage, it is clear that the presence
of linguistic competence is dependent on the remaining hemisphere (Dennis and
Whitaker, 1977). When such language transfer does occur, while verbal IQ may
not be significantly depressed relative to performance IQ, it should be remembered that such tests do not directly sample knowledge of language structure.
Where tests are designed to evaluate grammatical comprehension then deficits
appear (Dennis and Kohn, 1975; Teuber, 1975; Dennis and Whitaker, 1976;
Day and Ulatowska, 1979). However, with these reservations in mind, children
exposed to left hemispherectomy do show an adequate degree of language
competence in relation to their overall IQ and it has frequently been remarked
that it would be an incredible improvement if each adult aphasic could recover
the same level of language competence (Geschwind, 1972).
 
The duration of such plasticity in the developing brain has been the subject of
disagreement. Lenneberg (1967) believed that the period of plasticity in regard to
language mechanisms lasted until puberty. Krashen (1973) has challenged this
view mainly on the basis that right hemisphere damage above the age of 5 does
not often produce aphasia whereas below this age it frequently does. However it
should be understood that the issue of the degree to which both hemispheres are
involved in language acquisition early in life (and evidence has already been
cited that right hemisphere aphasia may be quite different in form from left
hemisphere aphasia in young children) is quite a different one from the question
of whether interhemispheric transfer is possible. Children between 5 and 10 years
do show good recovery from aphasia and it would be surprising indeed if
language could have survived in the left hemisphere given the extent and severity
of the damage in some instances, e.g. right hemiplegia and hemianopsia (Hecaen,
1976). On the balance the evidence would appear to favour a period of plasticity
extending to at least 10 years of age. There is even some indication that a period
of reduced plasticity may extend far beyond this age although whether it
involves inter-hemispheric transfer or improved within-hemisphere recovery is
another question. Teuber (1975) noted that an analysis of 167 cases of brain
injury sustained during the Korean campaign showed that the population who
were under 22 at the time of injury showed better recovery of language than
those who were 23 years and over. It may be premature then to try to set rigid
cut-off points for recovery.
 
The evidence presented here suggests that aphasia in children is not one
syndrome but several. In children of 6 years and below mutism and dysarthria
appear as the main symptoms with comprehension being relatively well
 
 
 
224
 
 
 
 
 
preserved. Furthermore this pattern appears to occur regardless of whether the
lesion is in the left or right hemisphere and also appears to be insensitive to the
precise location of the lesion within a hemisphere (whether it is frontal, temporal
or parietal). Above 6 years, symptoms which are regarded by many as truly
aphasic (comprehension and naming disorders) appear. The symptoms appear
to occur largely following left rather than right hemisphere lesions. Between the
ages of 6 and 14, jargon aphasia in its adult form is infrequent although the
extended circumlocutions that are one of the characteristics of aphasia do occur
(Guttman, 1942). The rapidity of the recovery process in some cases and, in very
young children, the preservation of comprehension, makes it extremely unlikely
that language has been totally relearned by the right hemisphere (Geschwind,
1972). This factor has suggested to some investigators that the right hemisphere
must be involved in linguistic processing at an early developmental stage and in
fact retains some capacity for comprehension even in the adult after cerebral
differentiation (Kinsbourne, 1975).
 
It is possible then that during the early stages of language learning both
hemispheres acquire comprehension and share control of the speech mechanism.
This may be necessary in the initial stages, because fine bilateral control of the
speech mechanism is required since suitable motor synergisms for a culture-
specific phoneme system are not yet well established in subcortical structures.
The consequence of this arrangement is that a lesion to either hemisphere can
disrupt speech production but comprehension is relatively unaffected because
the structural basis of language as opposed to speech does not require a bilateral
component. However, the establishment of subcortical synergisms for the
execution of the basic components of speech production together with the
presence of structurally more specialized language mechanisms in the temporoparietal region of the left hemisphere normally leads to left hemisphere capture
of the speech output mechanism. This process is probably a gradual one, but as it
proceeds there is less functional demand for right hemisphere processing of
language and there may even be active inhibition of its linguistic processing by
the left hemisphere. Eventually this isolation of the right hemisphere may lead to
structural changes at the synaptic level so that the re-establishment of control is
no longer possible. To the extent that this isolation process is incomplete transfer
of control is still possible. Thus in young children (under 5 years) the loss of the
left hemisphere will show itself in only transitory speech output disturbances
since this process of capture is just beginning and both hemispheres are still
involved. Even at this age however the linguistic superiority of the left
hemisphere is already apparent (see Young, Chapter 6, this volume, for a review
of the psychophysical literature), and it is in fact this superiority that will allow
the eventual suppression of the right hemisphere. In children between the ages of
six and ten years the speech mechanism is probably under the control of the left
hemisphere but right hemisphere control mechanisms have not yet functionally
 
 
 
DETERMINATE AND PLASTIC PRINCIPLES
 
 
225
 
 
atrophied. Left hemisphere damage at this age can produce speech disturbances
solely and/or truly aphasic disturbances which are transitory, while damage to
the right hemisphere only rarely affects these mechanisms. With the passage of
time however, the capability of the right hemisphere wanes through disuse and
in the majority of cases only a token linguistic capacity remains. Even if in later
life the possibility of direct inhibition by the left hemisphere is removed, by
severing the corpus callosum, the right hemisphere has residual linguistic
comprehension but remains mute (Gazzaniga and Sperry, 1967). This is
probably because the process of gaining control of the speech mechanisms
involves the regulation of neuromuscular synergisms at subcortical levels, and
these remain under left hemisphere control. It should be noted that the control of
the vocal apparatus by the left hemisphere may be specific to its use in the
context of spoken language. Where lesions of the left hemisphere produce
expressive (Broca’s type) aphasia the ability to use the voice in the context of
singing including the fluent production of words may be well preserved
(Yamadori et al ., 1977). Evidence for a motor capture account of left hemisphere
language dominance can also be found in studies of adult aphasics (cf.
Kinsbourne, 1975).
 
 
The plasticity of the developing brain
 
It is usually accepted that the younger the individual when the brain sustains
injury, the greater the resilience and the greater the capacity for functional
restitution. Against this one must set the view that the developing brain is
particularly vulnerable and long-term effects emerge if normal development is
impaired. These two views may be partially reconciled by proposing that
following early brain damage, specific skills may be spared but at a cost that will
be seen in the overall lowered cognitive capacity of the brain (Teuber, 1975).
Thus language or visuo-spatial skills may be spared following left or right
hemisphere injury respectively, but intellectual achievement as measured by IQ
tests or by school performance will show depression. Evidence already cited
concerning the specific effects of early brain damage makes it clear that the
consequences are not just seen in a uniformly lowered total processing capacity
but depend on the site of injury. Language achievement is specifically lowered
following left hemisphere injury and spatial skills depressed specifically
following right hemisphere injury.
 
The evidence in favour of a greater degree of plasticity comes from a number
of sources. Some animal species show spared sensory capacity following cortical
lesions in infancy (Schneider, 1969) while in other species age at time of injury
does not appear to affect the magnitude of the deficit (Doty, 1973). Even where
pattern vision is spared following early lesions of the striate cortex, the animals
may still take longer to pretrain before formal testing can commence (Schneider,
 
 
 
226
 
 
 
 
 
1970). In man there is evidence of age-related sparing of sensory function.
Rudel et al (1974) noted minimal impairment in brain damaged children on
somaesthetic thresholds but these children were still impaired on tactile object
recognition. Elementary motor function in children may also show greater
savings following early massive unilateral injury (Cairns and Davidson, 1951),
but such abilities as are preserved, are rudimentary. In man following early
unilateral damage that is extensive enough to destroy large areas of the striate
cortex, the visual field defects are similar to those produced in adults with similar
pathology (Paine, 1960). In this case it might be expected that savings would be
possible given the existence of a second, phylogenetically older, visual structure
in the midbrain. Where lesions are more restricted, however, savings on visual
(in terms of shrinkage of the size of scotoma), somatosensory and motor
functions are age-related and show relatively better recovery even when damage
occurs early in the third decade of life as compared to later (Teuber, 1975).
Without doubt however, the most outstanding examples of functional recovery
are those which occur in the areas of language and spatial skills in man following
early injury.
 
The explanation of the functional recovery that does occur following early
brain damage is not straightforward. As discussed above, part of the restitu-
tional capacity may lie in mechanisms that enable individual neurones to
withstand injury so the functional extent of a lesion may be less than in the
mature system. It may also lie in neural regeneration per se, although Schneider
(1979) has provided evidence that such anomalous regeneration, when it occurs,
may actually result in greater behavioural deficit. In the case of the somatosensory and motor systems, while the greater volume of neural circuitry is
concerned with analysis and control of the contralateral side of the body,
ipsilateral pathways do exist. In the case of the motor system there is even
evidence that hypertrophy of ipsilateral pathways occurs after early hemi-
spherectomy (Hicks and D’Amato, 1970). These ipsilateral pathways may
assume greater functional importance in the case of unilateral brain damage and
sustain the limited behavioural savings that occur. The continued development
of linguistic and spatial skills after early brain damage are however of a different
order. It has been suggested that the survival of these skills in one hemisphere is
due to the fact that the necessary processors exist initially in each hemisphere but
that during development one hemisphere suppresses the influence of the other.
This suppression of the influence of the contralateral hemisphere may be a
necessary prerequisite for the development of higher cognitive skills, since
processing space may be at a premium. When both language and spatial skills
are acquired by only one hemisphere (following early hemispherectomy) neither
skill reaches its full potential (Teuber, 1975). Where the corpus callosum is
absent during development, and normal interhemispheric communication is
consequently impossible, a rather bizarre pattern of cognitive development is
 
 
 
DETERMINATE AND PLASTIC PRINCIPLES
 
 
227
 
 
seen. In such cases of callosal agenesis it appears that either performance IQ or
verbal IQ becomes pre-eminent despite the existence of functional capacity in
two hemispheres (Dennis, 1977). Extreme discrepancies do not in these cases
appear to be predictable from age of the subject at the time of testing, sex, type of
agenesis, handedness or specific neurological signs. It appears rather that a
mechanism which enables a normal balance of cognitive skills to occur is absent.
In the normal individual then the existence of two intact hemispheres may not be
sufficient for normal cognitive growth. Some additional mechanism which
ensures that unnecessary duplication does not occur and enables an efficient use
of available processing capacity seems to be necessary. The consequence of the
presence of such a mechanism during development is that usually specific skills
become established predominantly in one hemisphere or the other and once
they are so established there is little opportunity to recapitulate the process. The
failure of the adult brain to fully re-establish linguistic or spatial skills following
damage is a consequence of the presence of a mechanism (whose effector path is
the corpus callosum) that enables a balanced and complete cognitive growth to
occur.
 
 
Conclusions
 
Consideration of the evidence concerning the outcome of brain injury in the
developing nervous system leads to the inescapable conclusion that age at the
time of injury is a critical factor in determining both the initial syndrome and the
pattern of adaptation that follows. Certain abilities such as early maturing
spatial skills, language and various elementary sensory and motor functions may
show relative recovery, the extent being determined by the location and size of
the lesion. Other abilities, particularly those involving fine motor coordination,
late maturing spatial skills as well as the overall level of intellectual attainment,
may show more profound defects. These latter effects may be partly ascribed to a
lower overall processing capacity, but in the case of general intellectual
attainment, more profound effects are seen following damage to the right
hemisphere before (compared with after) one year of age, a factor which argues
that volume of tissue damaged is not the sole consideration. Differing patterns of
language disturbances are also seen in children depending on the child’s age at
the time pathology develops. These considerations, which emphasize that
damage is occurring within a system whose state is continually changing, present
particular difficulties for those with interests in the outcome of early brain
damage. As Teuber and Rudel (1962) point out: ‘Whether we are working with
infrahuman forms or with children, we must define (1) those aspects of
behaviour in which the effects of early injury appear only with a delay, as
development progresses; (2) those other aspects of performance in which there
will be impairment at all ages; and finally, (3) those aspects of performance
 
 
 
228
 
 
 
 
 
where there is an immediate effect which, however, disappears as development
proceeds’. The interpretation of research which finds age-dependent differences
is often far from clear. Reports that the patterning of IQ subtests is insensitive to
the location of injury in young children may be a genuine indication of an age-
dependent difference, despite the fact that such tests were not specifically
designed to assess brain damage. However, since it is found that in adults that IQ
tests are only indicative of lesion laterality in the acute but not the chronic phase
(Fitzhugh et al ., 1962), the failure to find specific indication of lesion location in
children with infantile injury may be attributable to the interval between injury
and testing rather than an age-related difference in functional organization.
 
The emergence of specific functional deficit following early brain injury
stresses the importance of critical structures for cognitive achievement. Just how
extensively the predesignated structures of the nervous system determine the
detailed characteristics of cognitive development remains to be seen. It is only
relatively recently that explorations of the specific patterns of loss, as measured
by specially developed tests, have begun. It is also apparent that much of the
research has been concerned with hemispheric asymmetry where the structural
similarity of the hemispheres means that the capacity for interhemispheric
reorganization may lead to an over-valuation of the plastic capacities of the
developing brain. Cases where bilateral loss of a structure is involved give one
much less confidence in the restitutional capacity of the young brain. Bilateral
frontal lobe damage in childhood appears to have effects at least as serious as
equivalent damage in adults (Russell, 1959; Ackerly, 1964) although the paucity
of research in this area and the anecdotal nature of some of the findings make
conclusions tentative. There is a scarcity of unequivocal evidence that areas of
the brain not normally involved in the development or performance of specific
functions may assume those functions when other areas are damaged. Goldman
and Lewis’s (1978) demonstration that the dorsolateral prefrontal cortex in the
macaque may assume some of the functional capability of the orbitofrontal
cortex, provided damage occurs early in development, remains one of the few
clear demonstrations of such effects in the CNS of primates following bilateral
lesions. However, the fact that the normal limits of plasticity and the extent to
which they may be influenced by specific experience remain undetermined, gives
developmental neuropsychologists particular problems in understanding the
precise nature of brain-behaviour interrelations. Nevertheless, the fact that
plasticity is a real phenomenon and may be influenced by specific experience
(Goldman and Lewis, 1978) gives some hope that understanding of its nature
may lead to more effective remediation regimes designed to capitalize on its
characteristics.
 
Acknowledgements
 
I am grateful to H. D. Ellis and E. A. Salzen for advice and discussion during preparation of this
manuscript.
 
 
 
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229
 
 
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CHAPTER EIGHT
 
 
SEX DIFFERENCES IN BRAIN DEVELOPMENT:
PROCESS AND EFFECTS
 
MIRANDA HUGHES
 
 
Introduction
 
Identifying the neural mechanisms which underlie particular behavioural and
cognitive functions has become a fundamental aspect of psychological research,
and in recent years considerable progress has been made in understanding the
way in which both pre- and postnatal hormones can affect brain differentiation.
The notion that prenatal hormones which are differentially produced by males
and females may have irrevocable effects on the brain as well as on physical
morphology is politically provocative; nonetheless, improving our knowledge of
the neural substrates of behaviour ought also to facilitate our understanding of
how postnatal environment exerts its influence. Thus, to find sex differences in
brain differentiation, and to link these to sex differences in cognitive ability and
behaviour, does not necessarily imply biological determinism; rather, it
enhances our understanding of the raw materials which educational and cultural
pressures may mould in a variety of ways.
 
This chapter discusses the way in which pre- and postnatal hormones affect
brain differentiation, and it is argued that the long-term effects of the early
hormone environment may predispose any individual to certain ‘masculine’ or
‘feminine’ type behaviours. However, different aspects of our behavioural
repertoire are certainly under different degrees of hormonal influence and
human behaviour is not clearly sexually differentiated. As Money (1977a) put it
so cogently ‘... the only irreducible sex differences are that women menstruate,
gestate and lactate, and men impregnate ... most sexually dimorphic behaviour
 
 
233
 
 
 
234
 
 
 
 
 
as we know it is the product of cultural history and not of some eternal verity
programmed by non-cultural biology. 5 (pp. 32, 33).
 
Following Pfeiffer’s (1936) innovative and now classic work, the precise role of
prenatal hormones in the development of the hypothalamic mechanisms which
control hormone release at puberty, and which are responsible for the development of sexually differentiated physical characteristics, is well established
(Harris, 1964, 1970). The well documented cases of children exposed to
abnormal levels of particular steroids in utero (Money and Ehrhardt, 1972), and
the work of Dorner (1979) on human homosexuality, have subsequently raised a
number of questions concerning the effect of hormones on a wide range of
behaviour. The line of reasoning seems to be that if (a) some neural mechanisms
(e.g. for gonadotropin release at puberty) are determined by the role of prenatal
hormones, and (b) foetuses exposed to abnormal levels of types of particular
hormones behave in specific and atypical ways, then it follows that (c) just as the
prenatal hormonal environment has ‘wired-up 5 the brain in such a way as to
determine the expression of certain endocrine functions, so too can it predispose
an organism to specific behaviour patterns. A closer examination of the three
stages of this argument should facilitate the development of a conceptual
framework within which to extend our understanding of the variety of
expression in human abilities and behaviour.
 
 
Prenatal sex differences in development
 
Distinctively male or female development begins at around the seventh week
after conception when the initially bi-potential embryonic gonad differentiates
to form either a testis (in the case of a male) or an ovary (in the case of a female).
This differentiation of the gonads is determined by the genetic sex of the zygote
(46XY in the male; 46XX in the female); where there is no second sex
chromosome as in Turner’s syndrome (45X) the gonads are undifferentiated at
birth, although germinal follicles may have been present in the early foetal stages
(Scott, 1978). Jost (1979) tentatively suggests that there may be a specific
membrane protein controlled by a locus on the Y chromosome (the H-Y
histocompatibility antigen) which is responsible for the differentiation of testes,
and whose individuals who do not produce this antigen will form an ovary. Once
a testis has been formed the release of a substance (probably a foetal protein)
known as Mullerian inhibiting substance (MIS) induces the regression of the
Mullerian ducts, and secretion of androgenic hormones enables the development of the male reproductive tracts and genitalia. In the absence of testicular
hormones female development occurs; ‘...in mammals and birds body sex
shows a basic developmental trend corresponding to that of the homozygous
sex. Characteristics of the heterozygous sex have to be actively imposed by the
secretions of the corresponding gonads 5 (Jost, 1979, p. 8). Thus, the appearance
 
 
 
SEX DIFFERENCES IN BRAIN DEVELOPMENT 235
 
of Turner’s syndrome infants is unequivocally female, and that of Klinefelter’s
syndrome infants (47XXY and 48XXXY) is unequivocally male.
 
Sexual differentiation does not, however, always proceed entirely smoothly,
and Scott (1978) has provided a useful classification of some ‘intersex’ conditions. He suggests that there are four basic processes which may distort normal
sexual differentiation: (i) chromosomal intersex, in which extra or missing sex
chromosomes affect development; (ii) gonadal intersex, in which the gonadal
tissue is at variance with the chromosomal constitution of the individual; (iii)
partial masculinization of chromosomal and gonadal females, due either to a
disorder of adrenal functioning or the exogenous administration of steroid
hormones to the mother; and (iv) incomplete masculinization of chromosomal
and gonadal males which may occur either because an individual is insensitive to
the androgen being produced by the testes or because there is some failure in
androgen production. These medical conditions have often been described as
‘nature’s experiments’ because they shed light on the various ways in which
hormones affect development.
 
In Turner’s syndrome, the missing chromosome may be either an X or a Y:
evidence for this comes from a report by Leujeune (1964) of monozygotic twins,
one of whom was a normal male of 46XY karyotype, the other of whom was
born with 45X karyotype (and therefore a female phenotype). Turner’s
syndrome females are typically short in stature, and require oestrogen therapy at
puberty to effect normal breast development. There may be a range of other
physical stigmata present (e.g. shield chest, neck webbing, low-set ears), but
general intelligence is not significantly affected (Money and Ehrhardt, 1972).
The streak gonads are often entirely non-functional, but they may contain some
ova in which case pregnancy is possible.
 
There are approximately two cases per thousand of males with a 47XXY
karyotype, and a similar number with 47XYY karyotype. The former often have
small testes and prostates and diminished body and facial hair; they may also
show some breast development at puberty, and are frequently infertile. Males
with 47XYY karyotype have a tendency to be taller than average, but otherwise
display no specific physical abnormalities. It has been estimated that both
47XXY and 47XYY males are over-represented in mental or penal institutions
during late adolescence or adulthood. Differences between these groups in
deviant behaviour are not significant; however, their crimes are more likely to be
sex or property offences than those of their delinquent peers (Meyer-Bahlburg,
1974).
 
Cases of gonadal intersex (hermaphroditism) show widely varying arrangements of gonadal tissue and genitalia. Scott (1978) suggests that there may be an
interchange of genetic material between the X and Y chromosome before the first
meiotic division in the primary spermatocyte, which could lead to widely
varying sexual differentiation according to the cells in which the Y chromosomal
 
 
 
236
 
 
 
 
 
material is active. Such cases clearly do not provide a homogeneous subject
sample, but are nonetheless interesting individually.
 
The partial masculinization of females and the incomplete masculinization of
males illustrate clearly the role of steroids in the development of sex-related
physical characteristics. The former of these conditions is usually due to
congenital adrenal hyperplasia (CAH) which occurs as a result of an enzymatic
deficiency in the adrenal steroid metabolic pathways. The most common form of
CAH is 21-hydroxylase deficiency which results in a build-up of 17-hydroxypro-
gesterone, the metabolic derivatives of which have a virilizing influence on the
female foetus. Male infants appear normal at birth (although puberty may be
accelerated by as much as ten years if the condition is not diagnosed and
treated), but female infants have masculinized external genitals. The female
internal organs are normal, and with appropriate medical treatment (including
surgery to feminize the genitals) these girls may menstruate at puberty and
eventually bear children.
 
Masculinization of females may also arise from the influence of steroid
hormones administered to the mother during pregnancy to prevent miscarriage.
Ehrhardt and Money (1967) report ten such cases, and Scott (1978) describes an
individual case following the administration of norethisterone to the mother. As
in the CAH cases, masculinization is apparently restricted to the external
genitalia, and can be corrected surgically.
 
There are three possible defects of the androgenization process which can give
rise to the incomplete masculinization of the male: defective androgen production, defective Mullerian regression, and androgen insensitivity. The last of
these is the best documented and is often known as ‘testicular feminization’. In
this condition infants with a normal male 46XY chromosome complement are
born with a female phenotype. Since Mullerian regression has occurred
normally, there is a short blind vagina, but the external genitalia are unequivocally female. At puberty, there is spontaneous breast development, although
pubic hair tends to be scant. The condition occurs despite normal steroid output
from the testes, when the receptor cells fail to respond to the androgens which
are present.
 
Individuals with complete androgen insensitivity may be quite oblivious to
their condition until puberty, when they seek medical advice for amenorrhea.
However, there are incomplete forms of androgen insensitivity: for example,
where there is a failure to convert testosterone to the more potent 5a-
dihydrotestosterone there may be incomplete masculinization of the external
genitalia. Imperato-McGinley et al (1974) described some such cases in which
the affected infants are given a female assignment at birth, but at puberty have to
undergo a gender re-assignment because ‘anabolic events at puberty, in
particular the increase in muscle mass, the growth of the phallus and scrotum,
and the voice change, appear to be mediated by testosterone’ (p. 1214).
 
 
 
SEX DIFFERENCES IN BRAIN DEVELOPMENT
 
 
237
 
 
All of these ‘intersex’ conditions have considerable interest for psychology, in
that they provide an opportunity to examine the possible behavioural effects of
the prenatal hormones. It is certainly true that the prenatal hormonal environment affects cell differentiation in the brain; the speculations which require
critical examination are those concerning the behavioural implications of such
hormonal effects.
 
 
Hormonal action
 
The role of prenatal hormones in the development of the internal sex organs and
genitalia is clearly established. If these hormones exert equally critical influences
on brain differentiation, one would expect to find different patterns of neural
networks in male and female brains. It is instructive therefore to examine the
mechanism whereby hormones exert their influence so that sex differential
developmental processes can be appropriately evaluated, and any anomalies of
normal development can be interpreted.
 
The steroid hormones include the male sex hormones (androgens), the female
sex hormones (oestrogens and progestins), and the hormones secreted by the
adrenal glands (corticosteroids). Structurally, they resemble one another quite
closely but differ radically in function. Their common core structure consists of
four interconnected carbon rings. The pattern of bonding and the different side
groups affect the overall shape of each molecule, and it is these subtle differences
in shape which enable the hormones to attach themselves to specific target cells.
 
Hormones act directly on genetic mechanisms, so that when gene action is
blocked (for example, by the action of certain antibiotics) hormones become
powerless to exert their characteristic effect. A single hormone can activate an
entire set of functionally related but otherwise quite separate genes, and
hormonal specificity is dependent on the functional integrity of the target cells as
much as on the hormone itself. The cytoplasm of target cells contains specific
intracellular receptor proteins which accumulate and retain the hormone (this in
contrast to the receptor mechanism of say, amines, for which the receptor site lies
in the cell membrane). The steroid hormones then give rise to an increase in
RNA synthesis, and can also effect the synthesis of a new variety of messenger
RNA; these RNA molecules direct the formation of new protein molecules in the
cytoplasm of the cell which enable the target cell to make its functional responses
to the hormone.
 
During development the presence of male hormones will (in general) have a
masculinizing effect on a genetic female. However, in experiments on rats it was
found that whilst testosterone increased the amount of RNA produced in the
liver cells of both males and females, in the female not only was there an
increased amount of RNA, but a new type of RNA was being produced; this
finding does suggest that even when male and female developing embryos are
 
 
 
238
 
 
 
 
 
exposed to similar hormonal environments, the consequences need not
necessarily be identical (Davidson, 1965). There has been some attempt to
discover whether sex differences in brain differentiation are mediated by sex
differences in cytoplasmic receptors. Data from Maurer (1973) and from Whalen
(1974) show that there was selective cytoplasmic binding of oestrogen in the
anterior hypothalamic-preoptic area (of rats), in the median eminence, but not in
the cortex; however, the sex differences were not striking \ .. it seems unlikely
that the small difference in nuclear retention that we found can account for the
large differences existing between males and females in their behavioural
responses to oestrogen’ (Whalen, 1974, p. 278).
 
 
Sex differences in brain differentiation
 
Pfeiffer (1936) was the first to establish that sex differences in the reproductive
endocrinology of rats were determined by the hormone environment at a specific
stage of development. He demonstrated that if a male rat is castrated within 3
days after birth and is subsequently (in adulthood) given ovarian grafts, he will
respond to endogenous hormones with a surge of luteinizing hormone (LH)
which is sufficient to produce corpora lutea in the ovarian graft. When the
ovaries of newborn females were replaced with testes, many of these females
failed to show any sign of oestrous cycles when they became adult, but entered a
state of constant vaginal oestrus. However, female rats, which were ovari-
ectomized at birth and subsequently had received ovarian implants, showed
normal oestrous cycles and formation of corpora lutea. Male rats in which the
testes were transplanted into the neck region at birth, and which received
ovarian implants as adults, showed no capacity to form corpora lutea in the
ovarian grafts. Pfeiffer concluded (erroneously) that the pituitary gland becomes
sexually differentiated; subsequent experiments (see Harris, 1964, 1970) made it
clear that in fact permanent control by the hypothalamus over the pituitary was
established by the presence or absence of testosterone in a critical neonatal
period. In the absence of testosterone a pattern of cyclic release of follicle
stimulating hormone (FSH) and LH by the pituitary was established; when
testosterone was present, release of hormones was tonic. Reznikov (1978) states
that the critical periods for the sexual differentiation of the brain centres which
regulate gonadotropin release \ .. occur in rabbits during the period of 19-23
days, and in guinea pigs at 36-38 days of pre-natal life, in rats, mice and
hamsters, in the course of the first five days after birth. In the case of humans, the
most probable period of sexual differentiation is considered to be the second
trimester of pregnancy. It should be emphasised that experimental influences
exerted outside the “critical” period are incapable of moderating the sex-
specifying parameters of differentiation of the brain’ (p. 127).
 
Barraclough and Gorski (1961) demonstrated that cyclic gonadotropin
 
 
 
SEX DIFFERENCES IN BRAIN DEVELOPMENT
 
 
239
 
 
release in female rats is regulated from a specific centre in the pre-optic
hypothalamic region, whereas tonic gonadotropin response is regulated from
the hypothalamic ventromedial arcuate region. Bari Kolata (1979) reviews the
recent evidence that (in rats) it is the aromatization of testosterone to oestrogen
which is crucial in the sex differences which occur during brain differentiation:
when testosterone reaches the brain cells of newborn male rats it is converted to
oestrogen and dihydrotestosterone but newborn female rats’ brains are protected from the effects of endogenous oestrogen by a-fetoprotein (a protein made
by the fetal liver) which binds oestrogen and thus prevents it from reaching the
developing brain. However, animals whose critical period for brain differentiation ends before birth (such as humans) have a-fetoproteins which do not bind
oestrogens, and it is not yet clear what mechanisms might protect those animals’
brains from the effects of oestrogen.
 
 
Behavioural effects of sex differences in brain differentiation
(i) Sexual behaviour
 
The effects of pre- and perinatal hormones on the sexual behaviour of infrahuman species are reviewed carefully by Hoyenga and Hoyenga (1980), and the
interested reader is referred to their text for a detailed list of primary sources. The
evidence that early hormones are critical in determining sexual behaviour is
unequivocal: neonatal castration of male rats (i.e. deandrogenization) increases
all types of female sexual behaviours; and the prenatal androgenization of
female rats increases the incidence of mounting and decreases the incidence of
lordosis (the female sexual response consisting of concave arching of the back
with simultaneous raising of the head and hind-quarters). Comparable evidence
is available from primate studies. However, the perinatal administration of
androgen to a female rat does not entirely masculinize her complete repertoire of
sexual behaviour, any more than the castration of a male entirely suppresses all
male-type responses.
 
Whalen (1974) proposed an orthogonal model of sexual differentiation in
which he suggested that ‘during development hormones can defeminize without
masculinizing and masculinize without defeminizing, and that hormones can
defeminize one behavioural system (e.g. mating) while masculinizing another
system’ (p. 469). This conception is not really satisfactory, for if one considers
any specific aspect of sexual behaviour (such as lordosis) it is difficult to see how
‘masculinization’ does not also imply ‘defeminization’; however, it does try to
deal with the data which indicates that lordosis in the female is not necessarily
inhibited by perinatal administration of testosterone, even though she also
exhibits increased incidence of mounting. In the same article Whalen raises some
important criticisms of the naivety of the behavioural analysis which has often
been employed in studies of sexual behaviour, and similar criticism is reiterated
 
 
 
240
 
 
 
 
 
by Beach (1979). Responses such as lordosis can be only partially completed,
and neonatally androgenized females do exhibit weak or partial lordosis
responses with moderate frequency. Similarly, mounting is not always accompanied by intromission and ejaculation. A fmer-grained categorization of the
behavioural units which comprise ‘sexual behaviour’, and due attention to
controlling for the stimulus conditions in which it occurs, might facilitate our
understanding of its general structure, and thus enhance our knowledge
concerning the differential effects of various hormones. Beach suggests that both
male and female brains have the appropriate neural substrates for homotypical
and heterotypical sexual behaviour, and that sexual differentiation of the brain
serves to alter the probability of a particular response being elicited in a given set
of stimulus conditions. Thus, demasculinization does not eradicate the possibility of a male type response, it simply reduces its probability of occurrence.
Figure 8.1 shows the critical period during which sexual differentiation of the
brain occurs in rats. The degree to which the behaviour of the female rat is
masculinized is dependent both on dosage and on timing of testosterone
administration.
 
 
 
Figure 8.1 The effects of perinatal testosterone or castration on neonatal rats.
 
 
 
 
 
 
 
 
 
 
 
SEX DIFFERENCES IN BRAIN DEVELOPMENT
 
 
241
 
 
There is some interesting evidence from Dorner’s laboratories (Dorner, 1977,
1979) that human sexual behaviour may be affected by the prenatal hormonal
environment. ‘An androgen deficiency in genetic males during a critical period of
brain organization gives rise to predominantly female differentiation of the
brain. This androgen deficiency in early life can be largely compensated by
increased hypophyseal gonadotropin secretion in later life. Thus, the predominantly female-differentiated brain is post-pubertally activated by an approximately normal androgen level, leading to homosexual behaviour’ (Dorner, 1979,
p. 87). The evidence from which this conclusion is derived comes partly from an
experiment in which adult males were given an intravenous oestrogen injection:
in homosexual males there was a subsequent rise in LH values above initial
levels (a response which would be normal in females), whereas in bisexual and
heterosexual males no such rise was detected. Goy and McEwen (1980) express
some discomfiture with these data, and in particular point to evidence of time-
dependent partial dissociation between the differentiation periods of central
nervous centres regulating gonadotropin secretion and those responsible for
sexual behaviour. However, Dorner (1977, 1979) clearly believes that the
evidence of a relationship between prenatal hormones and adult sexual
behaviour is now sufficiently strong to contra-indicate the prescribing of any
androgenic or anti-androgenic substances to pregnant women, and recent data
on females with CAH may tentatively support this view. In contrast to earlier
findings which suggested that CAH females were no different from normal
controls in their heterosexual interests and behaviour (Ehrhardt et al ., 1968a;
Ehrhardt et a/., 19686) a more recent investigation by Money and Schwartz
(1977) has suggested that early treated CAH females may be delayed in
establishing dating and romantic interests. In addition, they found that in their
sexual fantasies CAH females showed an increased rate of awareness of
bisexuality relative to controls (although this did not necessarily reflect actual
experience). It is plausible that these more recent data reflect a less prescriptive
social climate than that which prevailed during the early 1960s when the original
data were presumably collected, and one can only conclude that the nature of the
biological, cognitive and social factors which regulate human sexual behaviour
are by no means well established. This area remains wide open to debate.
 
(ii) Non-sexual behaviour
 
The effects of pre- and perinatal hormones on animals are not restricted to
endocrinology and sexual behaviour. Levine (1966) cites evidence which
demonstrates that female rats who have been injected as neonates with
testosterone show male-type behavioural responses in an open field; and that
female rhesus monkeys injected with testosterone in utero show levels of rough
and tumble play which are approximately equivalent to those of normal male
monkeys. Goy (1968, 1970) reports that initiation of play and pursuit play are
 
 
 
242
 
 
 
 
 
greater in neonatally androgenized female monkeys than in normal females, and
a number of workers have reported effects of neonatal hormones on activity
(Gray et al ., 1975; Stewart et al ., 1975), exploration (Quadagno et al ., 1972;
Gummow, 1975), and learning (Beatty and Beatty, 1970; Dawson, 1972;
Dawson et al ., 1973). Quadagno et al. (1977) have reviewed the extensive
literature on the effects of perinatal hormones on non-sexual behaviours with
particular reference to energy expenditure, maternalism and learning, and they
are able to conclude that the effects of early hormones on the behaviour of infrahuman species are well established.
 
McEwen (1976) and Goy and McEwen (1980) describe the experimental data
which have led to the identification of specific neural pathways that are
established by the influence of sex hormones and are sexually differentiated, and
which underlie sex differences in behaviour. The work of Raisman and Field
(1973) represented an important breakthrough in this field: they found that
adult female rats have more dendritic spine connections in the preoptic area than
males, but that males castrated within 12 hours of birth have spine connections
equivalent *o those of the female. They demonstrated that those animals which
show frequerd lordosis have different patterns of synaptic connectivity than
animals with a limited capacity for lordosis. Various other studies have also
shown that the brain of a male rat deprived of androgen and the female exposed
to androgen will take on heterotypical characteristics: for example, the size of
the cell nuclei in the preoptic area is positively correlated with the degree of
lemaleness’ in the rat’s sexual behaviour (Dorner and Staudt, 1968, 1969); both
serotonin levels (Ladosky and Gaziri, 1971) and RNA metabolism (Clayton et
al ., 1970) are also affected. Litteria and Thorner (1974) and Phillips and Deol
(1973) report sex differences in the cerebellum and septum which can be reversed
by the presence or absence of androgens. However, even if these differences do
indeed underlie the observed differences in behaviour (as seems plausible) and
we assume that similar mechanisms of differentiation occur in humans, it is
nonetheless unlikely that human behaviour would be so strongly determined by
neural networks (particularly in the face of conflicting socialization).
 
 
Hormonal anomalies in human development
 
The data from the above animal studies provide sufficient evidence for the
assertion that hormones are critical in determining patterns of brain differentiation, and suggest that pre- and perinatal hormones may also exert long-term
effects on behaviour patterns. It is instructive then, to consider the effects of early
hormones on human behaviour insofar as this can be achieved within the
limitations of ethical considerations (see Reinisch and Gandelman (1978) for an
interesting discussion of these issues). It has already been noted that prenatal
hormones affect the development of sex-typical physical characteristics, and
 
 
 
SEX DIFFERENCES IN BRAIN DEVELOPMENT
 
 
243
 
 
individuals with anomalous genital development at birth, or who present with
related problems at puberty (e.g. amenorrhea in patients with testicular
feminization), have been studied by psychologists interested in the possible
effects on hormones on behaviour.
 
Two clinical syndromes can be regarded as close counterparts of experimental
anti-androgenization (or demasculinization) in animals: Turner’s syndrome and
testicular feminization due to androgen insensitivity. In Turner’s syndrome the
missing chromosome may be either an X or a Y, and if a few androgen-secreting
cells remain in the gonadal streak tissue there may be a mild degree of labial
fusion and an enlarged clitoris. Some individuals have a 45X/45XY mosaic
karyotype: they have testes, but these are not properly formed and are at high
risk for cancer (Money, 1911b). Thus, deandrogenization in Turner’s syndrome
is due to a failure of the gonads to manufacture androgens; in contrast, other
testicular feminization syndromes are a result of the failure of the target organ
cells to take up and utilize the androgens which are secreted from testes in
foetuses with the normal 46XY karyotype.
 
The behaviour of girls and women with either Turner’s syndrome or testicular
feminization is unequivocally feminine. In the case of Turner’s syndrome there
seems even to be a tendency of extreme conformity to female sex stereotypes:
they are known to fight less, to be less athletic and to be more interested in
personal adornment than control comparisons (Money and Ehrhardt, 1972);
and Theilgaard (1972) reported that women with Turner’s syndrome preferred
to wear very feminine-style clothing and jewellery. All but one of the 15 girls in
the group studied by Money and Ehrhardt (1972) had played exclusively with
dolls, and most of them expressed a very strong interest in maternalistic activities
associated with child care. In their anticipation and imagery of romance and
motherhood, Turner’s syndrome females were found to be no different from their
control comparisons. From these data, one may infer that differentiation of a
feminine gender role is not dependent on the presence of prenatal gonadal
hormones, nor does it require the presence of a second X chromosome. Indeed,
Money and Ehrhardt are prepared to assert that ‘a feminine gender identity can
differentiate very effectively without any help from prenatal gonadal hormones
that might influence the brain and perhaps, in fact, all the more effectively in
their absence’ (p. 108).
 
Babies born with the testicular feminization syndrome look like absolutely
normal females, although these females tend to be of above average height
(Money, Ehrhardt and Masica (1968) quote a mean height of 5 feet 1\ inches for
their sample of ten patients). Diagnosis of their condition normally follows
referral for primary amenorrhea so data regarding behaviour in early childhood
are necessarily based on retrospective report (which may be influenced by
knowledge of their condition). Even with this caveat in mind the data reported
by Money et al (1968) and Money and Ehrhardt (1972) do seem to provide
 
 
 
244
 
 
 
 
 
strong evidence for the unequivocal differentiation of female gender role in these
patients. They reported playing primarily with dolls in early childhood and
having dreams and fantasies which reflected the normal sex-role stereotypes of
marriage and motherhood. With one exception these women rated themselves as
fully content with the female role, and at adolescence they conformed with the
normal patterns of heterosexual behaviour. Most of them expressed positive
enjoyment in adopting ‘feminine’ styles of dress and personal adornment. ‘Babies
with the androgen insensitivity syndrome who are consistently reared as girls
have no uncertainties about themselves as girls, women, wives, sexual partners,
and mothers by adoption ... they grow to be womanly in their behaviour, in
their erotic mental imagery, and in their self-perception, even when they know
the medical terminology of their diagnosis’ (Money, 1977a, p. 262).
 
Reifenstein’s syndrome resembles that of complete androgen insensitivity
except that there is partial masculinization of the genitalia during foetal life and
the neonate is thus sometimes classified as a male. At puberty the development of
secondary sex characteristics nevertheless proceeds as described above.
According to Money and Ogunro (1974) those infants assigned as males did not
show any preference in childhood for female-type activities (doll play etc.) and
made concerted efforts to compensate for their relative inferiority in athletic
pursuits. At puberty, their breasts had to be surgically removed; in adulthood
their physiognomy is beardless and unvirilized, and because of their extremely
small, surgically repaired genitalia they may encounter some difficulty establishing a sex life (none reported homosexual preference). On the whole, gender
identity conforms with socialization and there seems to be no evidence from
these cases of any biologically based behavioural imperative for feminization.
These cases may reflect the experimentally induced ‘demasculinization’ without
accompanying ‘feminization’; and as far as we can tell from these few cases the
social environment is a paramount factor in influencing preferred activity and
gender identity.
 
The form of male pseudohermaphroditism described by Imperato-McGinley
et al (1974) results from a 5a-reductase deficiency which leads to incomplete
differentiation of the external genitalia at birth, and thus a female sex assignment
is often made. At puberty, however, differentiation of male characteristics occurs
and sex re-assignment is necessary. A recent report by Savage et al (1980)
confirms the rather surprising finding that this gender-role transition is made
relatively easily and they conclude ‘... that exposure of the brain to androgens
during foetal life and thereafter appears to have had more effect on determining
gender identity than the pre-pubertal sex of rearing’ (p. 404).
 
In the light of this conclusion it is interesting to consider the effects of the
masculinization of a female foetus. These have been documented in two clinical
syndromes: progestin-induced hermaphroditism (PIH) and the adrenogenital
syndrome (CAH). PIH occurred following the administration of synthetic
 
 
 
SEX DIFFERENCES IN BRAIN DEVELOPMENT
 
 
245
 
 
progestins to pregnant mothers with histories of miscarriage; these steroids were
devised as substitutes for the pregnancy hormone, progesterone, but because
their chemical structure was similar to androgen, they exerted an unexpected
masculinizing effect on a female foetus (Walker and Money, 1972). Once this
effect was discovered (in the early 1950s) the use of these hormones was
discontinued; however, the subsequent development of girls born with PIH has
been studied (Ehrhardt and Money, 1967; Money and Ehrhardt, 1972). If the
external genitalia were surgically feminized shortly after birth, no further
surgical or hormonal treatment was required; this is in contrast to girls born
with CAH who require constant maintenance on cortisone to prevent continuing postnatal masculinization and accelerated pubertal development. Table
8.1 summarizes some of the data obtained on the reported behaviour of these
cases. Basically, there is little difference between that of the PIH and CAH girls,
but both these groups differ significantly from control comparisons on measures
of tomboyism, athletic skills and preference for boys’ toys (e.g. cars, guns etc.).
Perhaps, as a result of these interests, it is not surprising that these girls also
prefer male playmates.
 
The accuracy of assessment of behaviour in these cases is difficult to evaluate
and Ehrhardt and Baker (1974) are clearly aware of this when they discuss, in
some detail, exactly how the interviews with patients and their parents were
conducted. It is important to be aware'that no observations were made of the
 
 
Table 8.1 Behavioural effects of prenatal exposure to androgens*
 
 
Childhood behaviour
 
PIH
 
CAH
 
Tomboyism
 
above average
 
above average
 
Athletic interests and skills
 
above average
 
above average
 
Preference for male playmates
 
above average
 
above average
 
Preference for ‘functional’ clothing
 
above average
 
above average
 
Preference for toy cars, guns etc. over dolls
 
above average
 
above average
 
Anticipation of future
 
Priority of career over marriage
 
above average
 
above average
 
Heterosexual romanticism
 
normal
 
normal
 
Anticipation of pregnancy
 
normal
 
Less frequently reported
than controls
 
Dissatisfaction with female role
 
no
 
no
 
Sexual behaviour
 
Childhood-shared genital play/copulation
play
 
normal
 
normal
 
Adolescent boyfriend and dating
 
normal
 
normal
 
Bisexual/homosexual fantasy
 
(data not available)
 
above average
 
Bisexual/homosexual behaviour
 
no
 
within normal range
 
 
*Data adapted from Ehrhardt (1977); Ehrhardt and Baker (1974); Epstein and Money (1968);
Ehrhardt and Money (1967); Money and Ehrhardt (1972); Money and Schwartz (1977).
 
 
 
246
 
 
 
 
 
children and that reliability was assessed purely in terms of the concordance
between the mother’s and child’s reports. Even so, these data do seem to reflect a
tendency for increased activity in females who have been exposed to abnormally
high levels of androgen in utero ; and compatible with these tomboyish interests,
these girls also seem less interested than control comparisons in personal
adornment and maternal behaviours. Their gender identity is nonetheless
entirely female (although 35% of them said they would not mind being a
boy).
 
It appears then, that the effect of prenatal androgens on gender identity
cannot be as imperative as Imperato-McGinley et al. (1974) and Savage et al
(1980) suggest; it is more likely that the activational effects of circulating male
hormones at adolescence are crucial to the satisfactory transition to the male
gender role for these male pseudo-hermaphrodites. However, the surmise that
the behavioural development of CAH and PIH females is in some way
analogous to that of prenatally androgenized monkeys (Goy, 1968) is certainly
supported by the available data. Furthermore, it is interesting to note that whilst
the excess of androgens may be contributing to a masculinizing effect on some
behaviours it does not have a global ‘defeminizing’ effect. Indeed, a sample of
late-treated CAH patients described by Ehrhardt, Evers and Money (1968)
conform strongly to female sex stereotypes in their careers and/or marriages. In
fact, the influence of prenatal androgen exposure is probably limited to a specific
effect which in some way creates a predilection for physical energy expenditure;
associated preferences for functional clothing and male playmates may be no
more than a reflection of this basic trait. This conclusion is confirmed to some
extent by the finding that males with CAH are no different from a comparison
group of unaffected male siblings except that they are more frequently (80 % of
CAH males: 20% sibs) reported to engage in intense energy expenditure
(Ehrhardt and Baker, 1975).
 
In two studies (Zussman et al. (1975) cited in Goy and McEwen, 1980;
Ehrhardt et al, 1975) which considered the effects of prenatal progesterone on
childhood behaviour (not the androgenic progestins which caused PIH),
subjects were found to exhibit lower energy levels and a tendency to prefer
‘female type’ clothing styles. They suggest that non-androgenic progestins may
actively counteract androgen effects in utero in both males and females.
 
During childhood, then, the major behavioural effect of prenatal androgenic
hormones is on activity level: when the foetus has been exposed to androgen,
he/she will subsequently display a predilection for high levels of physical energy
expenditure (and these effects appear to be dose-related). These results are
consonant with the findings on the effects of androgens in rodents and primates
(Quadagno et al , 1977), and they do suggest that these hormones have an
organizing effect on brain differentiation which will usually be sexually
dimorphic.
 
 
 
SEX DIFFERENCES IN BRAIN DEVELOPMENT
 
 
247
 
 
Personality
 
Reinisch (1977) argues that prenatal exposure to (non-androgenic) progestin
also has long-term effects on personality, and her data confirm the earlier
suggestions of Ehrhardt and Money (1967) that progestin-exposed subjects
show high levels of self-assertive independence and self-reliance. Twenty-six
subjects, whose mothers had been administered a minimum dosage of 40 mg
progestin for at least four weeks during the first trimester of pregnancy, were
tested on age-appropriate Cattell Personality Questionnaires. They exhibited
high scores on individualistic, self-assured and self-sufficient factors relative to
sibling controls. In contrast, subjects exposed to high oestrogen levels in utero
were found to be more group-dependent and group-oriented than a sibling
control group.
 
An investigation by Yalom, Green and Fisk (1973) also attempted to evaluate
the long-term effects of prenatal oestrogens on personality. Because diabetic
women produce lowered levels of oestrogen and progesterone during pregnancy
they are sometimes prescribed supplemental doses of these hormones; Yalom et
al. studied the male children of diabetic mothers who had received high
oestrogen doses, and compared them to a control group of children with normal
mothers and a group of children of untreated diabetic mothers. At the age of 6
the boys who had been exposed to the highest levels of oestrogen were rated by
their teachers as being less assertive and less athletic than their male peers. By
the age of 16 a whole range of behaviours seemed to be related (albeit weakly)
to the level of oestrogen exposure: athletic coordination, competitiveness,
assertiveness, aggression, and global measures of ‘masculinity’. The children of
diabetic mothers who had not received oestrogen supplements were consistently
more masculinized than the control group of sons of normal mothers, and the
children of mothers who had received supplemental oestrogen were the least
masculine. It is possible that some of these effects may be due to differing levels of
activational hormones in these boys since the development of the testes and
output of testicular hormones are likely to have been affected (Zondek and
Zondek, 1974).
 
 
Cognitive ability
 
In an exhaustive review of psychological sex differences, Maccoby and Jacklin
(1974) concluded that males show superior visuo-spatial and mathematical
abilities relative to females. Females though, are better at some verbal skills: they
are more fluent, they are better readers and spellers, and their speech is more
comprehensible than that of males (Harris, 1977). The extent to which these
differences reflect underlying differences in neural organization has been a
matter of considerable debate (Archer, 1976) since the influence of differential
 
 
 
248
 
 
 
 
 
socialization in the development of sex-typed abilities is difficult to evaluate.
Males and females show similar rates of early babbling (Moss, 1967; Lewis,
1972), but by six months of age girls receive more physical, visual and vocal
contact with their mothers (Goldberg and Lewis, 1969; Messer and Lewis, 1972).
Infant boys are encouraged more than girls to explore and to be independent of
their mothers (Baumrind and Black, 1967; Hoffman, 1972). McGuinness (1976)
argues convincingly that sex differences in cognitive abilities may develop from
fundamental differences in auditory and visual acuity—from an early age
females show lower auditory thresholds and superior pitch discrimination
compared to males, and the sex difference increases with higher frequencies and
with age (McGuinness, 1972); males have superior foveal vision, greater sensitivity to light and longer photopic persistence.
 
The aspect of spatial ability in which males most consistently excel is the
capacity to rotate mentally three-dimensional images, or to redefine visual
images into new planes; males thus perform better on mathematical problems
which require spatial visualization (Fennema and Sherman, 1977; Petersen,
1979) and which involve the ability to ‘break set’ and restructure (Garai and
Scheinfeld, 1968; Hutt, 1972a, b). Until adolescence, the majority of studies show
no sex differences in quantitative skills, but males move ahead after this point
and show consistently superior performance (Maccoby and Jacklin, 1974).
 
If these sex differences in cognitive abilities are subserved by the neural
organizing effects of androgens in utero, a sample of females exposed prenatally
to androgen would be expected to show a male pattern of abilities. Similarly, if
enhanced oestrogen levels affect the neural substrates of verbal behaviour then
males exposed to supplemental oestrogen in utero would show a female pattern
of abilities. In fact, neither of these hypotheses is substantiated by the available
data.
 
Ehrhardt and Money (1967) report identical mean verbal and performance IQ
scores for a PIH sample of ten females (mean verbal IQ = 125, s.d. = 11.4; mean
performance IQ = 125, s.d. = 12.5). Although Perlman (1971) (cited in Reinisch
et al., 1979, and in Baker and Ehrhardt, 1974) found that CAH girls performed
significantly lower than their matched controls on Verbal and Comprehension
sub-tests of the Wechsler IQ scale, they also scored lower on Block Design.
However, the scores of CAH girls on the Healy Pictorial Completion Test were
comparable to those of CAH and normal boys; Perlman suggests that this result
may reflect the higher activity levels of the CAH girls which would have made
them more familiar with the kinds of situations depicted on the test. Baker and
Ehrhardt (1974) report no statistically significant difference on perceptual or
verbal factors between AGS patients and sibling control comparisons, although
the trends were in the expected direction (i.e. CAH females performed slightly
less well on the verbal sub-tests of the WISC than their unaffected female
siblings, but slightly better on the perceptual sub-tests).
 
 
 
SEX DIFFERENCES IN BRAIN DEVELOPMENT
 
 
249
 
 
Curiously, patients exposed to prenatal androgen do seem to have above
average IQ scores, but close examination of the relevant data reveal this finding
to be due to factors other than the androgenic influence. Baker and Ehrhardt
(1974) tentatively suggest that the recessive genetic trait for CAH may somehow
be linked to another trait which favours postnatal intellectual development, and
this notion is supported by the finding that the IQ levels of CAH patients do not
differ significantly from those of their parents and siblings which are also higher
than normal. The elevated IQ of the PIH group (Ehrhardt and Money, 1967)
can be ascribed to social class factors among the parents: six of the nine families
involved in this study had at least one parent who was a college graduate. Thus,
there is no substantial evidence to link prenatal androgens with enhanced IQ
scores.
 
Dalton (1968, 1976) suggested that prenatal progesterone (not of the
androgenic type) increased intellectual achievement, but these data were not
replicated in a study reported by Reinisch and Karow (1977) and have been
discredited on statistical and theoretical grounds (Lynch et al ., 1978; Lynch and
Mychalkiw, 1978).
 
The only study to consider the effects of prenatal oestrogen on cognitive
ability is that of Yalom et al. (1973). These (male) subjects were administered the
Embedded Figures Test to evaluate their spatial ability: those boys who had
been exposed to supplemental oestrogen in utero showed slightly inferior
performance relative to the two comparison groups, but this result did not reach
statistical significance.
 
Other hormonally anomalous clinical conditions in no way implicate the role
of prenatal hormones in determining the future patterns of intellectual abilities.
Patients with testicular feminization show the typical female pattern of lower
spatial than verbal ability: in the study reported by Masica et al. (1969) a sample
of fifteen cases had a mean Wechsler verbal score of 111.8 and a mean
performance score of 102.3. Since their exposure both to hormones and
socialization is equivalent to that of genetic females, one can conclude from these
data simply that superior male visuo-spatial abilities are not genetically
determined from a locus on the Y chromosomes. For some time it was thought
that spatial ability was partly determined by a locus on an X-linked gene
(O’Connor, 1943; Stafford, 1961), but recent data indicate that the pattern of
spatial abilities within familial groups is better explained by a model of an
autosomal dominant gene which has reduced penetrance in females (Fain, 1976,
cited in Vandenberg and Kuse, 1979). Whether this mechanism might influence
brain differentiation must be purely speculative, and there is, as yet, no evidence
to this effect.
 
Turner’s syndrome females have IQ scores within the normal range (Money,
1964; Shaffer, 1962), but also tend to show specific deficiencies in spatial ability.
Shaffer (1962) quotes a mean verbal IQ of 106, but a mean performance IQ of 88.
 
 
 
250
 
 
 
 
 
Alexander, Ehrhardt and Money (1966) showed that Turner’s syndrome females
experienced great difficulty on a visual memory test which requires the
reproduction of angulated shapes, and Theilgaard (1972) reported that they
performed badly on an embedded figures task.
 
It is reasonable to speculate from these data that androgens play some role in
facilitating spatial ability. Since Turner’s syndrome females produce no
androgens, and testicular feminized patients are insensitive to their effects,
spatial ability is thus slightly impaired. In the oestrogen-exposed patients, the
testes may have been producing less androgen than normal (Zondek and
Zondek, 1974). The data from the CAH patients indicate that it is not the
prenatal hormonal environment which is crucial, so the effect of androgens on
spatial ability appears to be activational rather than organizing. This conclusion
is supported by data from Petersen (1979) which indicate that females with
androgynous somatic characteristics have better spatial ability than their more
‘feminine’ peers.
 
Similarly, the effect of oestrogens on verbal ability may also be an activational
one. Dawson (1972) reports a study of West African males feminized by
kwashiorkor-induced endocrine dysfunction. In severe cases of kwashiorkor the
liver becomes unable to inactivate the normal amount of oestrogen which the
male produces, and Dawson found that males with this condition had
‘significantly lower spatial ability and a more feminine field-dependent cognitive
style than controls. In addition these subjects had significantly lower numerical
and higher verbal ability compared to normal males’ (p. 24). Presumably
though, these males had had equivalent gestational experiences to their
controls and so the prenatal hormonal environment is not implicated in these
results.
 
 
Sex differences in postnatal brain development
 
The human brain is not fully mature until around sixteen years of age. The main
‘growth spurt’ of the human brain begins during the last trimester of pregnancy
and continues into the second year of life. During this period there is an increase
in the number of glial cells (from which myelin is derived) and hypertrophy of all
cells, specifically in the form of increased axonal terminal and dendritic
branching (i.e. interneuronal connections). Most cortical areas are fully
myelinated by the child’s third year but myelination of the reticular formation
the cerebral commissure, and the intracortical association areas may continue
into the second and third decades of life (Marshall, 1968).
 
Recent evidence suggests that sex differences in brain development are partly
reflected in sex differences in hemispheric specialization (Hutt, 1979a; McGlone,
1980). For example, Witelson and Pallie (1973) in a study of infants up to 3
months old, reported that the increased size of the left (relative to the right)
 
 
 
SEX DIFFERENCES IN BRAIN DEVELOPMENT
 
 
251
 
 
temporal planum (the posterior surface of the temporal lobe, including part of
Wernicke’s area which subserves language) was significant in neonate females
but not in males (although a significant difference was found for slightly older
(20-90 days) males). Buffery and Gray (1972) cite evidence that in four-year-old
girls the degree of myelination in the temporal planum is greater than that for
four-year-old boys and they suggest that this may account for the female
precocity in language development.
 
Witelson (1976) describes an experiment which suggests that in boys, the right
hemisphere is specialized for spatial processing from as early as six years of age,
whereas females show evidence of bilateral representation. She suggested that
this specialization might subserve superior spatial skills in males. Levy (1969)
postulated that bilateral representation of language in females could interfere
with the development of spatial processing abilities in the right hemisphere—
thus the cerebral organization which is presumed to give females an advantage
in language development and verbal abilities may serve also to impede their
development of spatial skills.
 
Waber (1976) has argued that lateralization is a function of maturation rate
rather than sex. On the whole, girls mature faster than boys and generally
display a greater tendency towards bilateral representation of skills. However,
late-maturing adolescents of either sex are more likely to be strongly lateralized
than their early-maturing peers, and are also more likely to show evidence of
superior spatial skills.
 
The evidence for the existence of anatomical substrates which would underlie
lateralization processes is both limited and confusing. For example, Wada et al
(1975) did not replicate Witelson and Pallie’s (1973) findings on sex differences
in cerebral asymmetry in infants: they report that both male and female infants
tend to have a larger temporal planum in the left hemisphere than in the right,
yet adult females are more likely than males to show the reverse pattern of
asymmetry. It is possible that the anatomical asymmetry reported for adult
females is a reflection of the greater plasticity of localization of function in
females than in males. In a recent report (Hughes et al , 1980), females performed
faster on a task which had both a verbal and a visuo-spatial component, whereas
both sexes performed at the same speed on the verbal task alone. The authors
interpret this finding as reflecting the ability of females to process both aspects of
the task in one hemisphere; in males additional time is needed to complete the
combined task because information has to be transferred between hemispheres.
However, this sort of speculation awaits support or rebuttal from further
anatomical evidence.
 
The ontogeny of hemispheric specialization and lateralization is simply not
yet adequately charted. It is not known whether (or how) the environment might
modify lateralization and thus we cannot know whether the data of Wada et al.
(1975) from adult females are the result of endogenous, hormonally-mediated,
 
 
 
 
252
 
 
 
 
 
changes or a reflection of educational experience. Tomlinson-Keasey and Kelly
(1979) report that lack of early hemispheric specialization is predictive of better
reading skills, and that right hemisphere specialization is positively associated
with mathematical skills —data which confirm stereotypic achievements (i.e.
females tend to be less lateralized and are better readers, males show a greater
tendency towards lateralization of spatial skills in the right hemisphere and are
better at mathematics). However, the nature of these relationships needs to be
carefully explored.
 
Witelson (1977) argues that the functional neural substrates for the lateralization of particular abilities may show a plasticity during development which is
lost in adulthood, and that this plasticity may reflect a susceptibility to
environmental influences. However, experimental support for such an idea is still
thin. It may be that lateralization predisposes cognitive strategies and atten-
tional biases rather than specific skills. The female precocity in language
development leads to a preferential use of language as a processing mode and
consequent inferior performance in visuo-spatial skills (McGlone and Kertesz,
1973). Bryden (1979) offers a review of experimental data which serve as a useful
reminder that sex differences in cerebral organization are not clearly defined: the
degree of overlap between the sexes is often substantial, and seems to vary as a
function of the experimental paradigm.
 
 
How different are sex differences?
 
It is easy to fall into the habit of discussing sex differences in ability and
behaviour as though these represented absolute differences between two quite
distinct populations. A sex difference in mean scores on a particular ability tends
to deflect our attention from the within-group variances which indicate how
much the groups overlap. Even when there is a statistically significant difference
between the mean scores of males and females on a test the majority of both
sexes may score within the same range.
 
The interpretation of a report of sex differences will depend on whether one is
concerned with socio-political/practical issues (in which case differences are
often trivial and meaningless) or with scientific/theoretic issues (in which case
small but consistent differences may yield important insights). Thus, consistent
reports of sex differences in verbal and visuo-spatial skills have raised interesting
hypotheses both about hemispheric specialization, and the role of prenatal
hormones; they do not, of course, provide any justification for boys to do badly
when studying modern languages or for girls to abandon mathematics education
at the earliest possible opportunity.
 
It may be that sex differences in certain skills are a result of long-term
evolutionary pressures. For example, Hutt (1972 b) argued that athletic and
visuo-spatial skills in males maximize hunting success and thus increase the
 
 
 
SEX DIFFERENCES IN BRAIN DEVELOPMENT
 
 
253
 
 
probability of survival, whereas the socially communicative abilities and
superior manual dexterity of the females have evolved as an adaptive consequence of her predominantly nurturant role in caring for dependent infants.
Yet characteristics with a presumed evolutionary adaptive basis are not fixed for
every individual: cultural pressures will influence the expression of abilities, and
‘typical’ sex differences are simply not found in some cultures. For example,
cross-cultural studies of field-independence (presumed to be related to visuo-
spatial skills) reveal no sex differences in Eskimo and Zambian cultures
(MacArthur, 1967; Siann, 1972). McGuinness (1976) argues that \ .. the fact that
boys do learn to read and write fluently, suggests that though initial processes
may be guided by certain sensory differences, there is no reason to assume that
these differences must remain. Parents insist that boys learn to speak, read and
write but no such insistence induces the females to learn about spatial-
mechanical relationships’ (p. 144).
 
In our own culture then, there is an attempt to educate males in heterotypical
skills whereas the converse is not true for females. Even in homes where parents
believe that they do not discriminate between male and female children
Rheingold and Cook (1975) found that \ .. the rooms of boys contained more
animal furnishings, more educational art materials, more spatio-temporal toys,
more sports equipment and more toy animals. The rooms of girls contained
more dolls, more floral furnishings, and more “ruffles’” (p. 461). The agents of
socialization are evidently insidious, and may tend to exaggerate sex differences
in proclivities for particular forms of behaviour. Even if we accept that
evolutionary pressures have resulted in sex differences in neural organization
which may differentially predispose males and females to specific abilities, we
must stand this against our knowledge that the plasticity of the human brain will
probably enable us to modify the expression of those abilities. This in turn
implies a responsibility of educators and caretakers to provide an appropriate
range of educational opportunities and role exemplars for their male and female
charges.
 
At present, not only are females less likely to be given the wide range of toys
that males have, their role is also under-represented by the literature and
television media: \ .. females were under-represented in the titles, central roles,
pictures, and stories of every sample of books we examined ... Even when
women can be found in books, they often play insignificant roles, remaining both
inconspicuous and nameless’ (Weitzman et al ., 1972). Sternglanz and Serbin
(1974) made a study of T.Y. programmes with high popularity ratings, and
found that half of these programmes did not portray any female roles: of those
that did, the authors comment ‘female children are taught that almost the only
way to be a successful human being if you are a female is through the use of
magic’ (p. 714). Exposed to these kinds of socialization pressures it comes as little
surprise that females tend to be diffident about their own ability and are
 
 
 
254
 
 
 
 
 
particularly unwilling to tackle those skills which they perceive as falling within
the male domain (Hutt, 1979b; Byrne, 1978).
 
Socialization experiences in our culture thus tend to exaggerate a dichotomy
of roles and abilities between males and females. Historically, this socialization
has acted to repress the female more than the male (her rights to be educated and
to vote have, after all, been won only comparatively recently), but there is little
doubt that a deliberate educational policy could serve to increase the range of
both male and female behaviour. Goy and McEwen (1980, pp. 60-61) present
some interesting evidence that female attachment to an infant may be innate (its
expression being in part activated by elevated hormonal levels during pregnancy
and birth), whereas male attachment is socially learned. This in no way implies
that males are unable adequately to perform parenting behaviours, but its
expression may be subserved by different neural mechanisms. There is no reason
to believe that the expression of sex typical intellectual abilities is any less
modifiable.
 
 
Towards a model of human sex differences
 
Waddington’s (1957) notion of‘canalization’ in an epigenetic landscape provides
a useful conceptualization for understanding differences in the degree of sexual
dimorphism in behaviour. Waddington suggested that, for all members of a
species, a set of target physical characteristics (eyes, arms, legs etc.) is defined by
the genotype and, despite underlying genetic variability, genetic processes
operate together to ensure that these targets are achieved. He depicts the
development pathways of the phenotype as a ball rolling through a set of valleys
(the epigenetic landscape); the valleys can vary in steepness and thus vary the
opportunity of the phenotype to deviate from a given course—-the steepness of
the valley reflects the degree of canalization. At certain critical points in
development, when the phenotype is undergoing rapid change, it is susceptible
to certain environmental or genetically induced stresses. For example, the
embryological development of the arms takes place around 38-48 days
(postmenstrual); this development is strongly canalized (i.e. all normal humans
have arms) but a teratogen, such as thalidomide, taken by the mother during this
critical period will inhibit this normal phenotypic development and the foetus
will eventually be born either with no arms at all or severe under-development
(see Fishbein, 1976, pp. 46-47). The development of some other physical
characteristics may also be affected by thalidomide during this period, but in
general each character has its own critical period.
 
As we noted earlier, the basic developmental trend of the body’s sexual
characteristics is in a direction corresponding to that of the homozygous sex (i.e.
the female). This trend is canalized to develop a female foetus from the zygote
which is formed at conception. However, if the embryonic gonad differentiates to
 
 
 
SEX DIFFERENCES IN BRAIN DEVELOPMENT
 
 
255
 
 
form a testis, then phenotypic development is deflected from the female pathway
when the testis begins to secrete MIS and androgens. Differentiation of neural
networks occurs in the same manner: there will be critical periods when the
presence or absence of biochemical agents (usually hormones) will affect the
development of RNA which is specific to particular structures. Behaviours which
depend on these specific neural anlagen for their expression will subsequently be
affected. The specificity of hormonal effects is well illustrated by some data
presented by Short (1979): certain aspects of male-type sexual behaviour were
exhibited by ewes which were androgenized late during gestation (days 50-100,
or 70-120) and had essentially female external genitalia; androgenization during
early gestation (days 30-80) resulted in complete masculinization of the external
genitalia which was not accompanied by male type sexual behaviour. Masculinization of urination behaviour could be effected by androgenization at a
relatively late period of gestation after it was no longer possible to masculinize
sexual behaviour. The positive feedback effect of oestrogen on LH, which is
normally exhibited only as a female characteristic, was sometimes abolished by
androgenization but ‘gave no clue whatsoever to the type of sexual behaviour to
expect from the animal’ (p. 258). This example illustrates clearly that the degree
of masculinization of behaviour cannot be inferred from physical characteristics.
The developmental pathways for specific behaviour patterns are also
independent of one another, so the masculinization of sexual behaviour does not
necessarily imply masculinization of activity levels.
 
If we return to the image of males and females rolling through their
(sometimes overlapping) epigenetic landscapes, it is possible to visualize the way
in which different levels of canalization will result in different degrees of sexual
dimorphism in the eventual expression of behaviour. If the pathway for a
particular neural substrate is very steep, it will be difficult to deflect the
phenotype from its developmental path—thus most genetic females will manifest
the appropriate ‘female’ behaviour pattern and most males will not. If the
pathways are gentle then the phenotypes may be spread more thinly, and a linear
male-female dimension may be evident in the subsequent behavioural pattern.
The neural mechanism which mediates gonadotropin release is clearly strongly
canalized, other neural substrates in humans are less strongly canalized and
therefore enable the expression of greater variability in behaviour and skills.
 
The critical periods for development of the neural networks which underlie
particular behaviours may vary in length; they may overlap in time, but they are
independent of one another. Thus, in the female rat, by the judicious administration of neonatal testosterone, it is possible to decrease the incidence of lordosis
but not to increase the incidence of mounting. Armstrong’s attempt (cited in
Jost, 1974) to relate sexual orientation to body type is therefore quite
misconceived: there is no reason to believe the homosexuals will have heterotypical body characteristics.
 
 
 
256
 
 
 
 
 
There are three ways in which hormones can act on the brain to produce sex-
differentiated effects. The prenatal hormones organize neural networks in
distinctively male and female patterns; they also have a critical role in the
development of physical characteristics. Postnatally, the output of gonadotropic
hormones can activate these neural networks (for example, in the control of the
menstrual cycle). Alternatively, sex-related hormones may have independent
effects: an example of this is the yawning behaviour of rhesus monkeys which is
normally displayed more frequently by males than by females, but which can be
increased in the female by the administration of exogenous testosterone (Goy
and McEwen, 1980).
 
In humans, evolution has operated to permit a high degree of behavioural
phenotypic plasticity, which would in turn imply weak canalization of the neural
mechanisms which subserve particular behaviours and abilities. (This may
account for the conflicting findings in anatomical studies of hemispheric
asymmetry, supra). This phenotypic plasticity enables individuals with very
different genotypes to exhibit similar or identical behaviour. In the expression of
human behaviour and ability then, phenotypic plasticity and not biological
canalization may produce conformity of behaviour within a same-sex group.
Evidence for sexually differentiated canalization of a particular behaviour
requires that its manifestation be virtually universal and not restricted to a single
cultural group: the only behaviours which fulfil this stringent requirement are
indeed ‘menstruation, gestation and lactation’ in females and ‘impregnation’ by
males. The weaker canalization of sex-related brain differentiation in humans
relative to infra-human species would also lead us to expect less sexual
dimorphism of behaviour in humans; this should always be borne in mind when
extrapolating from animal to human studies. Rodent studies have been crucial in
extending our knowledge of brain differentiation—they do not necessarily tell us
much about human behaviour.
 
The identification of the brain areas and mechanisms which subserve
particular behaviours or the articulation of specific cognitive skills in humans is
by no means straightforward. Whilst it has been possible in infra-human species
to identify the critical period of development for the expression of certain
behaviours (vide Short, 1979) this has not been possible in humans and probably
(for ethical reasons) never will be. Nor is it entirely clear how circulating
hormones affect human behaviour: studies of the menstrual cycle produce
conflicting evidence (Hutt et al ., 1980), and studies of sexual behaviour (e.g.
Bancroft and Stakkebaek, 1979) or cognitive ability (Peterson, 1979) have not
yielded definitive conclusions. We also lack evidence on the way in which
educational experience affects brain development. Thus, information which is
vital to a definitive model of the effects of brain differentiation on psychological
sex differences is not available. Nonetheless, the existing evidence leaves no
doubt that brains of males and females differ as a function both of the prenatal
 
 
 
SEX DIFFERENCES IN BRAIN DEVELOPMENT
 
 
257
 
 
environment and subsequent maturational effects. These differences may well
underlie the predilection for males and females to act in particular ways, but
they cannot be seen as constituting a biological imperative.
 
 
Summary
 
De facto sex differences in ability and behaviour are not as dimorphic as sex
differences in physical characteristics. The major influences on sex differential
brain development are the sex steroids (androgens, oestrogens and progestins),
but their effect on behaviour is attenuated by the weak canalization of the
human behavioural repertoire. Sexual differentiation of the brain may create
predilections for particular behaviour and specific cognitive strategies, but it
does not constitute a biological imperative for psychological sex differences.
 
 
Acknowledgements
 
I would like to thank Dr. J. E. Blundell and Professor J. Scott for their helpful comment and criticism
during the preparation of this manuscript. I am also grateful to Derrick Pritchatt who translated the
article by Reznikov (1978).
 
 
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Chapter Six - Asymmetry of Cerebral Hemispheric Function During Development

Andrew W. Young

Introduction

The cerebral cortex of the human brain is divided into two cerebral hemispheres. The hemispheres are connected to the body by nerve tracts mediating sensation and movement, whose principal organization is contralateral. In other words, the left hemisphere is primarily responsible for sensation and movement of the right side of the body, whilst the right hemisphere is primarily responsible for sensation and movement of the left side of the body. It should be noted that in both cases there are ipsilateral nerve connections between the left hemisphere and the left side of the body, and between the right hemisphere and the right side of the body. The contralateral nerve fibres predominate, however, and the precise role of the ipsilateral fibres is not fully understood.

This ‘crossed’ arrangement of the nervous system is found in many species (Dimond, 1972), though why it evolved is not known. In addition to the ipsilateral and contralateral connections to the body, the cerebral hemispheres are connected to each other by bundles of nerve fibres. In man, the principal interhemispheric connections are mediated through the corpus callosum and the anterior commissure (Seines, 1974; Gazzaniga and Le Doux, 1978).

The fact that most people show a preference for the use of the right hand for a number of activities was noted in ancient times, and has been much discussed ever since. Although individual members of other species may also exhibit lateral preferences, they tend to be less marked than those observed in most humans, and when preferences are found they average out across individual animals at about 50 % left preference and 50 % right preference. In contrast, no more than 10% of humans are left-handed (Hardyck and Petrinovich, 1977), though the precise figure obtained depends on the strictness of the criteria used.

During the nineteenth century it was discovered that the majority of adults who suffer serious speech disturbances after unilateral (one-sided) brain injury do so following damage to the left cerebral hemisphere. As well as expressive language (speech and writing), language comprehension was also found to be more likely to be disturbed following left rather than right hemisphere injury. The possibility of a connection between the involvement of the left cerebral hemisphere in both language and right hand preference was quickly seen, and led to the conception of the left hemisphere as being typically the dominant hemisphere and the right hemisphere as minor or non-dominant. This idea held sway in some quarters until quite recently, though not without opposition. Over the last thirty years, however, a convincing body of evidence for right hemisphere superiorities has accumulated (Joynt and Goldstein, 1975), and it would now seem that the cerebral hemispheres each have their own different functions.

Asymmetric organization, then, is typical of certain cerebral hemispheric functions in the adult human brain. The left cerebral hemisphere is specialized for functions of language and speech, and also controls movement of what is for most people the preferred hand, whilst the right hemisphere is superior for a collection of functions that are often rather loosely characterized as non- linguistic and visuo-spatial. These include the perception and memory of nonlinguistic auditory and visual patterns (such as environmental sounds and people’s faces), and spatial ‘reasoning’ (such as when working from an engineering plan). It is not, at present, clear whether functional asymmetries are also typically found in the brains of non-human animals. They have been found in some cases (e.g. Nottebohm, 1970; Dewson, 1976; Trevarthen, 1978), which suggests that the phenomenon may he more widespread than was thought on the basis of studies of lateral motor preferences.

The existence of functional asymmetries between the cerebral hemispheres of the adult human brain raises interesting ontogenetic questions as to how functions are organized in infancy and childhood. For instance, it can be asked whether asymmetry of cerebral hemispheric function is present in infancy, which will be regarded here as the period from birth until two years of age, or whether it develops gradually from an initial bilaterally symmetric organization.

Although such questions are of considerable theoretical and practical importance, they have proved very difficult to answer satisfactorily. The functions being investigated are obviously very complex, and the available methods of investigation are rather indirect. In consequence, conclusions need to be drawn carefully and cautiously. This has not always been done.


The present chapter is intended to examine our knowledge of asymmetry of cerebral hemispheric function during development. In doing this, no attempt will be made to select only those results that fit a preconceived pattern, or to hide where the gaps in our knowledge lie. In some cases, however, criticisms will be- made of studies that exhibit obvious or characteristic deficiencies. This can create a rather negative impression, but it is necessary in order that the results of unsound studies may be discounted and, it is hoped, in order that such pitfalls are avoided in future investigations.

An excellent review of the development of hemispheric function has been published by Witelson (1977a). The present chapter differs from Witelson not only by including more recent material but also in emphasizing more strongly the importance of studies of the development of normal children and the importance of using methods that are themselves properly researched and understood. The potential value of the application of techniques deriving from experimental psychology to enable a degree of precision in pinpointing the sources of obtained laterality effects will also be stressed.


Organization of function in the adult brain

Before examining the available evidence concerning asymmetry of cerebral hemispheric function during development, it is necessary to clarify certain important features of the organization of cerebral hemispheric functions in the adult brain. It needs to be made clear that some functions are more asymmetrically organized than others and, although this chapter will concentrate on the asymmetrically organized functions, it must not be forgotten that there are many functions that are quite symmetrically arranged (Trevarthen, 1978).

The most marked asymmetry seems to occur for the production of speech, which is almost exclusively under the control of the left hemisphere (Searleman, 1977). The right hemisphere is usually mute or only capable of highly stereotyped utterances. The motor asymmetry involved in the production of speech is much more marked than other motor asymmetries, and left hemisphere control of speech production is found in nearly all right-handed adults, and many left-handers (Goodglass and Quadfasel, 1954; Branch et al , 1964). Hence, left cerebral control of speech production is more common than right- handedness. This point has important implications for developmental theories, since it renders untenable the view that the ontogeny of hemispheric specialization for speech production arises from an increasing and generalized dominance of the left hemisphere consequent on the development of right hand preference.

For the purposes of the present review, the interesting questions raised by the existence of interindividual differences in organization of cerebral function will be ignored, since they have not been studied developmentally, and the pattern of organization of function found in the majority of right-handed adults will be regarded as typical. Neither will detailed consideration usually be given to differences between studies in the criteria used for sampling from possible subject populations, since most studies have used subject groups of reasonable size drawn from populations in which the ‘typical’ pattern of organization could be expected to predominate in adulthood.

Despite its being almost completely lacking in the ability to express itself through speech, the right hemisphere does seem to have some capacity to understand language (Searleman, 1977). Zaidel’s (1976, 1978, 1979) studies, especially, have revealed an extensive auditory and a rather more restricted visual comprehension vocabulary, and some syntactic competence as well. It does not, however, appear to be the case that the right hemisphere’s vocabulary is merely an impoverished version of that of the left hemisphere. Instead, the right hemisphere is relatively capable of understanding concrete, imageable words (Searleman, 1977; Marcel and Patterson, 1979) and poor at understanding abstract words.

There is evidence, then, indicating that there are qualitative differences between the language abilities of the left and right hemispheres of the adult brain. The position is much less clear with regard to those abilities for which the right hemisphere shows superiority. These have been comprehensively reviewed by Joynt and Goldstein (1975). For the sake of simplicity, they will be loosely grouped here into ‘perceptual’ and ‘spatial’ abilities.

Although real, these right hemisphere superiorities are often not large, and in many cases would seem to represent quantitative rather than qualitative differences to left hemisphere abilities. In the case of nonlinguistic visual and auditory perceptual superiorities, for instance, both the left and right hemispheres are able to carry out the processes concerned, but the right hemisphere is in some way more efficient. This is one reason why the term ‘superiorities’ is used here with reference to the right hemisphere, rather than ‘specializations’. There is no sense in which the left hemisphere might be regarded as blind or deaf. This point is emphasized by Gazzaniga and Le Doux (1978), who regard the existence of right hemisphere superiorities as a side-effect of the left hemisphere’s language specializations. The only known case in which a claim for a qualitative perceptual superiority of the right hemisphere might be made is that of face recognition, but the evidence indicating that this may be a qualitative rather than a quantitative right hemisphere superiority is far from convincing (Ellis, 1975).

Certain complex spatial tasks, such as finding one’s way about and dressing, are more adversely affected by right than by left hemisphere brain injuries (Joynt and Goldstein, 1975). Similarly, in normal people, although there does not seem to be any difference in basic tactual perceptual abilities between the left and right hands, left hand (and hence presumably right hemisphere) superiorities can be shown for tasks with a degree of ‘spatial’ complexity (Corkin, 1978) such as identifying the direction of raised lines felt by touch (Varney and Benton, 1975). Present knowledge of what is involved in such spatial abilities is, however, so rudimentary that it cannot be stated with certainty whether qualitative or quantitative superiorities are involved. Le Doux, Wilson and Gazzaniga (1977) and Gazzaniga and Le Doux (1978) maintain that to the extent that such tasks demand active manipulation of materials (which most do) qualitative interhemisphere differences do arise. They attribute such differences to an involvement of the inferior parietal lobule of the left hemisphere in linguistic at the expense of manipulospatial functions. On this view the right hemisphere is superior for manipulospatial functions only to the extent that the left hemisphere’s language specializations have led to its being deficient in manipulospatial functions.

This brief summary of our knowledge of interhemisphere differences in the adult brain gives some idea of the complexity of the phenomenon of cerebral asymmetry, and how little is understood as to its true nature. Many people have found it convenient to adopt summary dichotomies to describe the functions of each hemisphere, such as left-dominant right-minor, left-verbal right-visuo- spatial, or left-analytic right-holistic. Such descriptions should be treated cautiously. In many cases they distort what is known by ignoring the extent to which duplication and symmetry of function actually does take place, and the extent to which the cerebral hemispheres work together as an integrated system.


The concept of lateralization

Although the investigation of asymmetry of cerebral hemispheric function during development is seriously hampered both by our lack of knowledge of hemisphere function in the adult brain and by the indirect nature of the methods suitable for work with children, quite comprehensive theoretical statements have been attempted. The most well known of these is that of Lenneberg (1967).

Lenneberg’s principal concern was with language functions, but he also discussed the development of hand preference. He did not really offer a new theory of the ontogeny of cerebral asymmetry, but he did give what was already a widely accepted view its most thoroughly documented and complete expression. Although there are slight changes in emphasis at different points in the book, the main point of Lenneberg’s theory is contained in the view that the extent of lateral asymmetry of organization of particular functions in the left and right cerebral hemispheres is not a fixed characteristic of the human brain, but increases during development in a quite regular manner. In other words, some hemispheric functions are claimed to be progressively lateralized. During the first years of life the cerebral hemispheres are seen as perfectly equipotential for language acquisition, in the sense that either could acquire language with equal facility if the other were injured, and there is no asymmetry of function.


Functional asymmetry begins to emerge toward the end of the second year, but it is not marked, and the right hemisphere is involved as well as the left in language acquisition. The degree of asymmetry increases throughout childhood, reaching the adult level at puberty. As the extent of lateralization of function increases and the right hemisphere’s involvement in language functions falls behind that of the left hemisphere equipotentiality declines, so that the final organization is relatively fixed.

Although directed toward asymmetries of language and hand preference, this type of theory can easily be extended to include the ontogeny of right as well as left hemispheric functional superiorities, though there have been disagreements as to whether the functions of the two hemispheres lateralize concurrently or with one leading the other (e.g. Corballis and Morgan, 1978). There have also been disagreements about the precise age at which lateralization is completed. Krashen (1973) has suggested completion by age five instead of by puberty, whereas Brown and Jaffe (1975) suggest that the process continues into old age. As none of these theories disagrees over the usefulness or the validity of the concept of lateralization they are all regarded here as fundamentally similar to Lenneberg’s position.

Lenneberg’s theory has many attractive features. Many parents feel that it is difficult to tell at first whether a child will be left- or right-handed. The theory brings together a very wide range of observations, and people always seem to have liked theories that postulate general ways in which children and adults differ. None of these, however, is a very good reason for accepting Lenneberg’s position, and during the last ten years it has become clear that his theory is quite wrong. In order to understand why this is the case, it is necessary to look in detail at the available evidence from the developmental studies that have been carried out. These will be grouped into three general types; studies of neuroanatomical asymmetries, studies using noninvasive methods with normal children, and studies of the consequences of cerebral injuries sustained at different ages.

Developmental studies

Neuroanatomical asymmetries

Our understanding of functional cerebral asymmetries may be at present limited, but knowledge of any corresponding neuroanatomical asymmetries is very scant indeed. None the less, neuroanatomical asymmetries do exist. The most thoroughly researched is the asymmetry of the planum temporale in the posterior region of the superior surface of the temporal lobe (Geschwind and Levitsky, 1968). The planum temporale of the left temporal lobe, which forms part of an area of known importance in language functions, is larger than or equal in size to the planum temporale of the right temporal lobe in approximately 90 % of adults.


Is such an asymmetry present in the brains of babies? It is quite clear that the answer is yes. Studies by Teszner et al (1972), Witelson and Pallie (1973) and Wada et al (1975) have demonstrated differences in the relative sizes of the left and right planum temporale of the foetal, newborn and infant brain. Opinions differ as to whether the degree of this neuroanatomical asymmetry increases between infancy and adulthood. This is hardly surprising, since it is by no means clear which measurements should be used to effect such a comparison. There is no disagreement, however, that the nature of the neuroanatomical asymmetry does not differ between infants and adults.

It is clear, then, that if functional asymmetries are found in the infant brain, this would not conflict with neuroanatomical knowledge. Similarly, the existence of neuroanatomical hemispheric asymmetries in the newborn makes it difficult (though not impossible) to believe in the complete equipotentiality of the cerebral hemispheres for language functions. On the other hand, as Witelson (1977a) points out, the existence of a neuroanatomical asymmetry is not in itself sufficient to imply that the cerebral hemispheres function asymmetrically in infancy. It may only represent the structural bias underlying later developing functional specializations. Because neuroanatomical findings are ambiguous in this way, it is necessary to look at results deriving from other methods.

Noninvasive methods

A number of methods have been devised in an attempt to study functional asymmetries in the normal, intact brain. Following Witelson’s (1977a) terminology, these will be referred to as noninvasive methods.

It is possible, for instance, to study asymmetries of motor control of parts of the body, and lateral preferences. Ontogenetic studies of lateral preference have been carried out for a long time. More recently attention has also been given to asymmetries following auditory, visual or tactile stimulus presentations, and these procedures have been adapted for use with children and, in some cases, infants.

In examining these noninvasive methods, studies of asymmetries in children for processing auditory, visual and tactile stimuli will each be considered in turn. Studies involving the auditory or visual presentation of stimuli to infants will then be discussed, and finally studies of asymmetries of motor control and lateral preferences.

Auditory presentation. The principal auditory nerve connections are contralateral, so that material presented to the right ear is directed to the left cerebral hemisphere and material presented to the left ear is directed to the right cerebral hemisphere. However, substantial ipsilateral auditory nerve connections between the left ear and left hemisphere and between the right ear and right hemisphere also exist.

When different linguistic stimuli (such as spoken digits) are presented simultaneously, one to each ear, the stimulus presented to the right ear tends to be reported more accurately than that presented to the left ear (Kimura, 1961, 1967). This general method has come to be known as dichotic stimulation, and is readily adapted for use with children. The finding of right ear superiority for linguistic material would seem to reflect its more efficient transmission to the specialized language areas of the left cerebral hemisphere, but it has also been thought that the ascendancy of the contralateral over the ipsilateral auditory nerve connections is particularly marked when both ears are simultaneously stimulated (Kimura, 1967; Cohen, 1977). When material is presented to one ear at a time the differences between ears are small and their demonstration requires the use of sensitive measures (Studdert-Kennedy, 1972; Fry, 1974; Morais and Darwin, 1974) or difficult tasks (Bakker, 1969, 1970; Frankfurter and Honeck, 1973; Van Duyne et al , 1977).

As well as its use in investigating the language specializations of the left hemisphere, the dichotic stimulation technique can also be used to study right hemisphere (and hence left ear) superiorities for the processing of some nonlinguistic auditory stimuli (Gordon, 1970, 1974; Knox and Kimura, 1970). For clarity and convenience the use of dichotic stimulation techniques to study the development of left and right hemispheric abilities will be discussed separately.

The studies of the ontogeny of right hemisphere superiorities for processing nonlinguistic sounds can be quickly dealt with, as few have been carried out. The principal studies are those of Knox and Kimura (1970) and Piazza (1977). Neither of these studies, nor the two unpublished studies referred to by Witelson (1977a), found any change in the left ear advantage across age in the range three years to adult.

The overwhelming majority of dichotic stimulation studies involving children have been addressed to the development of left hemisphere specializations. Witelson (1977a) gives a detailed summary of the methods and findings of over 30 published and unpublished studies carried out up to 1976. These studies differ on many points of methodology. Considering only the published studies reviewed by Witelson, the stimuli used included isolated speech sounds and nonsense syllables (Berlin et a/., 1973; Dorman and Geffner, 1974; Geffner and Dorman, 1976), spoken digits (Kimura, 1963, 1967; Inglis and Sykes, 1967; Bryden, 1970; Knox and Kimura, 1970; Geffner and Hochberg, 1971; Satz et al , 1971; Sommers and Taylor, 1972; Satz et al, 1975; Witelson, 1976a, 19766; Kinsbourne and Hiscock, 1977; Bryden and Allard, 1978), words (Knox and Kimura, 1970; Nagafuchi, 1970; Sommers and Taylor, 1972; Goodglass, 1973; Ingram, 1975a) and animal names (Bever, 1971). In some studies a report was required after each pair of stimuli, whilst in others two, three or even four pairs were presented before report of as many stimuli as possible was required. Both vocal and non vocal (such as pointing to a picture of a word’s referent) methods of reporting were used. There were also differences as to whether only right- handed children were used as subjects, and the criteria for establishing handedness when this was done. In addition, a point not taken up by Witelson (1911a) is that various different methods of aligning the left and right ear stimuli for ‘simultaneous’ presentation have been tried (Morton et a/., 1976).

Given that there have been such marked methodological differences between studies, the findings are surprisingly consistent. Almost all of the studies found right ear superiorities for the processing of linguistic stimuli, and almost all found right ear superiorities in the youngest groups of children studied. This has also been true of reports published since Witelson’s review was written (e.g. Mirabile et al, 1975; Borowy and Goebel, 1976; Geffen, 1976; Hynd and Obrzut, 1977; Hiscock and Kinsbourne, 1977; Piazza, 1977; Geffen, 1978; Geffen and Sexton, 1978; Geffen and Wale, 1979; Sextcn and Geffen, 1979). In a number of the published reports (Nagafuchi, 1970; Bever, 1971; Ingram, 1975a; Hiscock and Kinsbourne, 1977; Kinsbourne and Hiscock, 1977; Piazza, 1977) right ear superiorities have been demonstrated in children as young as three years old. Moreover, none of the studies that have investigated such young children has failed to find right ear superiorities.

It is clear, then, that insofar as right ear advantages for reporting dichotic linguistic stimuli are dependent on cerebral asymmetry for language functions, such asymmetries are present from at least three years of age. Supporters of the concept of progressive lateralization have, however, tended to see the most important question as being not so much the ages at which ear asymmetries can be demonstrated, but rather whether the degree of right ear superiority increases across age (Satz et al ., 1975). This is based on the contention that as the degree of cerebral hemispheric functional asymmetry increases, the size of ear advantages for dichotic stimulation should also increase. In other words, dichotic stimulation is regarded as a parametric measure of cerebral asymmetry. This view requires more careful consideration.

The first point that must be made is that even when dichotic stimulation scores are analysed by parametric statistical procedures most studies have not found the degree of right ear superiority for linguistic material to vary across age. In a small minority of studies, however, ear asymmetry was found to increase with increasing age (Bryden, 1970; Satz et al, 1975; Bryden and Allard, 1978). This raises the difficult question of the proper interpretation of findings of this type.

If it were the case that it is appropriate to regard dichotic stimulation as a parametric index of cerebral asymmetry, then the findings of Bryden and of Satz might substantiate the idea that the degree of cerebral asymmetry increases with increasing age. However, there are serious difficulties to be overcome before such a conclusion could be reached. No one has been able to demonstrate satisfactorily that the sizes of ear asymmetries are sufficiently closely or uniquely related to the degree of asymmetry of cerebral hemispheric function to serve as an index (Berlin and Cullen, 1977; Witelson, 1977a; Colbourn, 1978). Although it is probable that some modest relationship exists, there are many factors besides cerebral asymmetry which may influence the magnitude of ear advantages. A list of these factors would include the difficulty level of the task to particular subjects (and hence ‘ceiling’ or ‘floor’ effects), individual differences in the relative functional predominance of contralateral and ipsilateral auditory nerve fibres, different strategies for organizing reports of left and right ear stimuli, and attentional biases toward a particular ear. Moreover, the level or levels of stimulus processing at which ear asymmetries due to functional cerebral asymmetries can arise are not properly understood, and few investigations have explicitly controlled for the possibility that the contribution to observed asymmetries arising from different levels of information processing may vary between subjects. All of these potential influences on the size of obtained ear asymmetries are, of course, particularly likely to influence the outcomes of developmental studies which must necessarily sample across wide ranges of ages.

When these several factors are considered it is even more remarkable that the results of the majority of dichotic stimulation studies have been so consistent. The consistency is probably caused by most of the results happening to arise from the same general source of asymmetry, namely the left hemisphere’s superiority for speech decoding, and the few results that do not fit the main pattern of absence of developmental trends in ear asymmetry are best discounted until methods that allow more control over the factors involved are available. This conclusion is strengthened by the failure of Bakker, Hoefkens and Van Der Vlugt (1979) to confirm the developmental trend of Satz et al (1975) using a longitudinal instead of a cross-sectional research design.

From this discussion it is apparent that studies of ear asymmetry to dichotic linguistic stimulation in children must develop better methods for controlling unwanted sources of variance and for identifying the levels of stimulus processing at which cerebral asymmetries arise. Some researchers are beginning to do this. Most notably, an elegant series of studies by Geffen and her colleagues (Geffen, 1976, 1978; Geffen and Sexton, 1978; Geffen and Wale, 1979; Sexton and Geffen, 1979) has demonstrated that when attentional strategies are properly controlled there is no variation across age in the degree of right ear advantage for speech perception. Conversely, Geffen also found that the ability to deploy attentional strategies did vary across age, and that the use of attentional strategies can affect the size of obtained ear asymmetries, so that this factor does need to be controlled.

The identification of the levels of stimulus processing at which cerebral asymmetries arise is more difficult to achieve than the control of attentional strategies, but some progress is also being made. Consider, for instance, what aspects of cerebral asymmetry might contribute to the right ear advantage for linguistic stimuli. Two broad classes of effect can be readily distinguished. These are effects attributable to the left hemisphere’s superior abilities for the analysis and temporary storage of speech sounds, which will be called speech decoding asymmetries, and effects attributable to the different types of word that can be recognized by the left and right hemispheres, which will be called lexical asymmetries . Within the class of speech decoding asymmetries a further distinction might be drawn as to whether the asymmetries arise at the level of immediate perceptual analysis, or whether some short-term memory component is involved (as when multiple pairs of stimuli are presented before a report is required).

It is quite clear that a major contribution to obtained ear advantages is made by the general class of speech decoding asymmetries, which are sufficient to account for most of the observed results. This is evident from the fact that many of the ear asymmetries in the studies cited did not depend on the presentation of complete words, but could be obtained when isolated speech sounds or nonsense syllables were used as stimuli. It seems, too, that these speech decoding asymmetries can arise at the level of immediate perceptual analysis, but are heightened when a short-term memory component is introduced into the experimental task (Oscar-Berman et al , 1974; Yeni-Komshian and Gordon, 1974). This has important implications for developmental studies, which have been very free in varying the short-term memory requirements of the tasks used, as Porter and Berlin (1975) point out. It is likely that tasks with a large shortterm memory component will produce developmental trends in ear asymmetry not because cerebral asymmetry changes across age but because of age differences in short-term memory abilities and hence task sensitivity.

Ear asymmetries belonging to the lexical class obviously cannot arise when isolated speech sounds or nonsense syllables are used as stimuli. Although findings of lexical class ear asymmetries have been made in studies of adults using words as stimuli (McFarland et al, 1978; Kelly and Orton, 1979) they are by no means always found (e.g. McFarland et al, 1978; Kelly and Orton, 1979; Young and Ellis, 1980) and probably only arise under conditions that are not typical of most dichotic stimulation studies. The only study to date that has separately considered the possible implications for developmental findings of the distinction between the classes of auditory asymmetries described here as due to speech decoding and lexical factors has been that of Eling et al (1979).

In summary, then, studies of ear asymmetries to dichotic stimulation in children indicate that left hemisphere specializations for speech decoding and right hemisphere superiorities for the analysis of some nonlinguistic sounds are present down to at least three years of age. In most of the studies carried out the magnitude of ear asymmetries did not vary across age. In the few cases where the degree of ear asymmetry did increase with increasing age there is no reason to believe that this was a consequence of any process of increasing lateralization of cerebral hemispheric function.

Visual presentation. The optic nerve pathways are organized in such a way that information about visual stimuli falling to the left of the point at which a person is looking (in the left visual hemifield) is projected initially to the right cerebral hemisphere, whilst information about stimuli falling to the right of the point at which a person is looking (in the right visual hemifield) is projected initially to the left cerebral hemisphere. It is not established with certainty whether or not there is some degree of ipsilateral optic projection for stimuli falling close to the visual midline in the foveal and parafoveal regions of the retina, but outside this disputed area the projections are known to be exclusively contralateral (Cohen, 1977; Haun, 1978). This should not be taken as meaning that the left eye sends projections only to the right hemisphere. The fields of vision of each of the eyes overlap to a considerable extent, so that most left or right visual hemifield stimuli are seen by both eyes, and the contralateral optic projections consequently arise from a grouping together at the optic chiasm of the nerve fibres from the corresponding side of the retina of each eye. Because of this anatomical arrangement, the phenomena of eye dominance bear no clear relation to cerebral asymmetry (Porac and Coren, 1976), and will not be discussed.

If we know where a person is looking, then, it is possible to present visual stimuli in such a way that information is initially projected to whichever cerebral hemisphere we choose. Unfortunately, the presentation of a visual stimulus usually leads to an involuntary movement of the eyes to bring it into central vision. It is thus necessary to restrict the presentation time of stimuli to a time less than that needed to make such an eye movement. Estimates of this time vary, but it is usual to regard presentation times of 200 milliseconds (one-fifth of a second) or less as acceptable (Cohen, 1977).

The need to use briefly presented stimuli falling outside central vision obviously places a serious constraint on what can be studied using this technique, but a surprising amount has been achieved despite the limitations. It must be made clear, however, that the method can only permit the initial projection of stimulus information to one cerebral hemisphere or the other. What happens after that is not well understood, though it is probable that information is coordinated by means of the neocortical commissures, and the anterior commissure in particular (Risse et al ., 1978). Most investigators have been sufficiently reassured by the contralateral nature of the optic pathways to use unilateral stimulus presentations (in which stimuli appear only in one visual hemifield), but a case that bilateral presentation (in which different stimuli appear simultaneously in each of the visual hemifields) is a rather better procedure can be made out (McKeever and Huling, 1971; Hines, 1975). Although methods that can allow continuous lateralized input have been developed (e.g. Zaidel, 1975) these have not been adapted for use with children.

When words are presented briefly in the left or the right visual hemifield and right-handed adults are asked to name them it is usual to find a right visual hemifield (RYF) superiority (Mishkin and Forgays, 1952; McKeever and Huling, 1971). This RVF superiority is principally due to information about words falling in the RVF being directly projected to the left cerebral hemisphere. However, it has also been claimed to relate to the fact that English is read from left to right. The argument in this case is that the memory trace of the stimulus is initially ‘examined’ by the subject with a left to right scan starting from the point of fixation (Heron, 1957; White, 1969, 1972, 1973). Hence, the RVF superiority would arise from a ‘post-exposural trace-scanning’ mechanism deriving from experience in reading.

This trace-scanning notion no longer needs to be taken very seriously. It can be varied so freely as to become almost unfalsifiable, and even if true it could only be making a minor contribution to the patterns of results found in studies that have used words as stimuli rather than arrays of unrelated letters (McKeever, 1974; Pirozzolo, 1977). It is known, for instance, that the RVF advantage holds for vertically as well as horizontally arranged words and for words in the Hebrew language, which is read from right to left (Barton et al , 1965; Carmon et al , 1976). Furthermore, the size of the RVF superiority is not constant for all types of word, but has been shown to be larger for abstract than concrete words (Ellis and Shepherd, 1974; Hines, 1976, 1977). This pattern of results is most readily interpreted by postulating that both cerebral hemispheres of the adult brain possess at least rudimentary abilities to decode print stimuli, so that the word-class effect derives from the different types of word that can be recognized by the left and right hemispheres.

Another convincing reason for interpreting the results of studies using brief lateral presentations of visual stimuli in terms of functional cerebral asymmetry is that in several studies using nonlinguistic visual stimuli left visual hemifield (LVF), and hence presumably right hemisphere, superiorities have been demonstrated (Kimura and Durnford, 1974). Face recognition has proved to be a particularly useful task in this respect, with many subsequent reports confirming the findings of LVF superiorities by Rizzolatti et al (1971) and Hilliard (1973).

The use of the visual modality of stimulus presentation in studies of asymmetry of cerebral hemispheric function in children is potentially of great interest because of the wide range of skills that can be examined and the considerable range of ages at which the differing skills are learnt. The ability to identify visually represented words, for instance, is achieved at a much older age than is the ability to recognize faces. Unfortunately, a large rumber of theoretical and methodological difficulties are encountered in the case of visual presentation, and progress has been slow in comparison with that made by studies using dichotic stimulation. There have not been nearly as many studies carried out, and several of those that have been attempted are seriously flawed.

The most pressing methodological requirement in studies of visual hemifield asymmetries in children is to control fixation. Studies of adults usually rely on a central fixation spot, which subjects are asked to fixate before each stimulus is presented. This procedure is obviously of dubious validity in a developmental investigation. Children may fail to fixate when instructed to do so for a number of reasons. The consequence of a failure by children to fixate when instructed is that stimuli will not fall in the positions in the visual field intended by the experimenter, and will probably be distributed randomly, thus reducing or eliminating ‘visual hemifield’ differences. Since younger children will be more likely to fail to fixate than older children, a bias will be introduced making it probable that findings of differences in asymmetries across age will arise as an artifact of lack of fixation. Moreover, the temptation not to fixate when instructed may be itself related in a complex manner to the difficulty of particular experimental tasks to particular subjects. For these reasons, some form of fixation control is necessary in developmental studies of visual hemifield asymmetries, and all developmental trends found in studies without adequate fixation control (such as Jeeves, 1972; Miller and Turner, 1973; Barosso, 1976; Reynolds and Jeeves, 1978a, 1978b; Tomlinson-Keasey et al ., 1978) must be discounted as irrelevant to any considerations of asymmetry of cerebral hemispheric function.

A difficulty which is partly methodological and partly theoretical is that of ensuring that subjects of different ages are relying on the same cognitive processes or strategies when faced with a given task. It is often assumed that the use of linguistic stimuli will automatically engage specialized left hemisphere mechanisms and lead to a RVF advantage, whilst the use of nonlinguistic stimuli will produce no visual hemifield difference or a small LVF advantage. Cases are known in the adult literature, however, where this generalization breaks down. Matching tasks provide a simple example. Suppose that a pair of words or a pair of letters is presented in one visual hemifield, and subjects are asked to say whether they are the same or different. This can be determined either by comparing the physical appearances of the stimuli (physical match) or by naming them and comparing the names (name match). Studies by Cohen (1972) and Gibson, Dimond and Gazzaniga (1972) have demonstrated that whereas name matches yield RYF advantages, physical matches may be more effectively carried out for LVF stimuli. The implication for developmental studies is that if matching tasks are used they must be arranged in such a way that subjects are forced to adopt only one of the possible strategies. If this is not done, differences across age may simply be attributable to strategy differences. Witelson (19776) first noticed this potential artifact in one of her own studies, but the criticism applies equally to the differences across age found by Tomlinson-Keasey et al. (1978) and in Broman’s (1978) experiment involving matching pairs of letters. The general point that it is important to know how subjects actually approach experimental tasks applies, of course, to a lot more than just matching tasks.

Having made these methodological cautions and eliminated some of the more poorly designed studies, the principal studies of visual hemifield asymmetries in children will now be considered, starting with studies of right hemisphere (LVF) superiorities.

The most common task used to investigate LVF superiorities, as in the adult literature, has been face recognition. Young and Ellis (1976) found LVF superiorities for face recognition in five-, seven- and eleven-year-old children, with no differences across age in the degree of visual hemifield asymmetry. Broman (1978) found no developmental differences in LVF superiority for face recognition in the age range seven years to adult. Marcel and Rajan’s (1975) study of seven-year-old children also showed a LVF superiority for face recognition. In contrast, failures to find LVF superiority in seven- and eight-year-old children have been reported by Leehey (1976) and Reynolds and Jeeves (1978b). Reynolds and Jeeves’ study, however, lacks adequate fixation control. Leehey (1976) reports three developmental experiments on visual hemifield asymmetries for face recognition by subjects aged eight to adult that are in most respects carefully designed. When she used bilaterally presented photographs of the faces of people known to her subjects a LVF superiority was found at all ages, but with bilaterally presented unfamiliar faces the eight-year-old children gave no visual hemifield difference in two experiments. Unfortunately, Young and Bion (1980a) were unable to replicate this result, and have suggested that it was probably due to an age difference in directional reporting strategies arising from Leehey’s use of bilateral stimuli without controlled order of report. Studies of visual hemifield asymmetries thus give no grounds at present for claiming any developmental change in the extent of the right hemisphere’s superiority for face recognition.

Studies of right hemisphere superiorities using visual presentation and tasks other than face recognition have also failed to reveal developmental trends. Witelson (1977b) found a LVF superiority for matching pictures of human figures (a task which can only be done by means of a physical match) in boys aged six to thirteen years. Witelson (1977a) described an unpublished experiment finding a tendency to greater LVF accuracy for dot enumeration (p < 0.1) in six- to thirteen-year-old boys. Young and Bion (1979) found greater LVF accuracy for dot enumeration in boys aged five, seven and eleven years, but no visual hemifield accuracy difference in girls. A similar sex difference was found in adult subjects by McGlone and Davidson (1973). The absence of any developmental trend in LVF superiority for dot enumeration is interesting in view of the fact that it is a skill that is present in only a rudimentary form before age three, and even after three years is learnt quite gradually (Klahr and Wallace, 1973; Young and McPherson, 1976), whereas recognition of many faces is possible in the first year of life (Schaffer, 1971; Ellis, 1975). It is thus clear that the absence of reliable developmental trends in asymmetry of face recognition by children aged five and above is not simply due to the early age at which the skill is acquired.

Two findings of LVF superiority in children allegedly induced by means of a spatial mental set must also be noted (Kershner et al , 1977; Carter and Kinsbourne, 1979). Only Carter and Kinsbourne tested more than one age group of children, and found no developmental differences in the tendency of spatial priming to produce a LVF superiority for digit naming.

Most studies of visual hemifield asymmetries for linguistic stimuli in children have used printed words. In several cases the principal focus of interest was not so much whether there were differences across age as the possibility of differences between normal and poor readers (Beaumont and Rugg, 1978). These studies will only be referred to when they provide data relating to normal readers under ten years of age.

The general finding has been one of RVF superiorities in normal readers down to as young as six years of age (Olson, 1973; Marcel, Katz and Smith, 1974; Marcel and Rajan, 1975; Carmon, Nachshon and Starinsky, 1976; and one of the experiments of Turner and Miller, 1975). Forgays (1953), however, did find an increase in visual hemifield asymmetry with increasing age. Turner and Miller (1975) and Butler and Miller (1979) reported larger asymmetries when using five- rather than three-letter words. Turner and Miller (1975) also found changes across age when using five-letter words, but not when using three-letter words, though Butler and Miller’s (1979) results did not confirm this observation. It is probable that these somewhat confusing results derive from a failure properly to control the characteristics of the words used. Longer words are more likely than short words to be abstract, and hence to produce larger visual hemifield asymmetries for reasons already mentioned. Conversely, the words recognized by young children under conditions of brief lateral presentation are likely to be mainly concrete, with older children recognizing a more even mixture of concrete and abstract words. Since smaller visual hemifield asymmetries derive from concrete than from abstract words a change in the size of the obtained visual hemifield asymmetry across age will ensue if scores from abstract and concrete words are pooled, but it has nothing to do with any possible difference across age in the organization of cerebral hemispheric functions. Studies which exercise proper control over the characteristics of stimulus words used are clearly needed.

Some developmental studies of left hemisphere specialization have tried using letters instead of words as stimuli. Of these, only the one reported by Carmon et al (1976) meets the minimal methodological requirements specified here. Carmon et al found traces of a developmental trend in visual hemifield asymmetry when using letters as stimuli, but not when using words. The absence of a developmental trend in asymmetry with words clearly implies that left hemisphere specialization for at least some of the skills involved in the recognition of visually presented linguistic stimuli was present at all ages. Beyond this, all that can be said is that letter recognition is not a very meaningful task for developmental comparisons, since Bryden and Allard (1976) have shown that even with adults the results obtained are easily affected by the difficulty experienced by subjects in reading the typeface employed. The more difficult typefaces tend to give LVF superiorities, and it is obviously the case that the difficulty of particular typefaces will vary across age.

Taken together, then, the findings of studies of children using visual hemifield stimulus presentations do not support the idea that the degree of asymmetry of organization of cerebral hemispheric functions varies across age. It has only proved possible to date to work with children down to age five, but this disadvantage is offset by the fact that some of the skills that can be studied are being learned at these ages, allowing the possibility of the investigation of initial stages of organization. What is now needed is a more precise analysis of the particular skills used at different ages for word recognition and other tasks, so that these skills can be examined separately. This might throw some interesting light on the role (or absence of any role) played by the right hemisphere in the early stages of learning to read. A related question which has not received the attention it deserves concerns the way in which the right hemisphere acquires the ability to recognize those words it can identify in adulthood. The results of studies by Ellis and Young (1977) and Young and Bion (1980b) suggest that the nature of the difference between the ‘visual vocabularies’ of the left and right hemispheres is semantic, and unrelated to the ages at which different words are first learnt.

Tactile presentation. Although both ipsilateral and contralateral somatosensory nerve connections exist, they are organized into discrete systems that probably serve different purposes (Wall, 1975). It is thought that ‘active’ touch and proprioception (Gibson, 1962) are mediated primarily through the contra- laterally organized pathway passing through the dorsal column and medial lemniscus, whilst passive touch, pain and temperature depend on the spinothalamic system, which has both ipsilateral and contralateral projections (Gazzaniga and Le Doux, 1978).

A useful review of the evidence relating to the role of different cerebral structures in tactile perception is given by Corkin (1978), who points out that there is no evidence for any asymmetry in elementary tactual functions. Tactile asymmetries only occur when the task used engages some higher-order function for which one cerebral hemisphere is superior (Corkin, 1978; Young and A. Ellis, 1979). Most of the findings of tactile asymmetries have derived from studies in which active tactile exploration of stimuli was required.


It is often the case that left hand superiorities are found for complex tactile perception, but in some cases right hand superiorities are observed. Cioffi and Kandel (1979) found a right hand superiority for identifying two-letter abstract words by touch, which was present down to age six. A right hand superiority for the report of sequentially touched fingers was found down to age seven by Bakker and Van der Kleij (1978).

Left hand superiorities for the identification of tactually perceived nonsense shapes have been found down to age six by Witelson (1974,1976a) and by Cioffi and Kandel (1979). These left hand superiorities did not increase with increasing age, but inconsistent sex differences were observed. Using an accuracy measure, Flanery and Balling (1979) also found a left hand superiority for this type of task which did not vary across age down to age seven. However, when they computed laterality coefficients’, differences across age were observed by Flanery and Balling. Since the computation of such coefficients involves several unjustified theoretical assumptions (Colbourn, 1978), and many different coefficients are available that may all lead to differing outcomes, it is not possible to satisfactorily interpret this particular result.

In some studies, Braille patterns of raised dots have been used as stimuli. Hermelin and O’Connor (1971a, 1971b) found that blind adults and children aged eight to ten years were better at reading Braille with the left than right hand. Rudel et al (1974), however, found that sighted children did not learn Braille letters more accurately using the left hand until they were over ten years of age. Witelson (1977a) objected that the use of a naming task with raised dot stimulus patterns confounds the linguistic and spatial components of the task, but Rudel et al (1977) repeated the finding in a study that required that raised dot configurations only be discriminated, not named.

One curious aspect of Rudel et a/.’s (1977) findings was that not only did children aged over ten years show left hand superiority, but children below ten showed a tendency toward right hand superiority. Attention has been drawn to this because it illustrates the danger inherent in regarding the results of studies of this type as direct measures of asymmetry of cerebral organization. Surely no- one would want to maintain that spatial functions moved from the left to the right hemisphere at age ten? What is evidently happening is that the type of task used by Rudel and her colleagues can be approached using more than one solution strategy, and the younger children rely on a method that involves the left hemisphere to some extent. Their conclusion should thus have been not that cerebral asymmetry varies across age but that more needs to be known about the possible ways in which subjects can approach this type of task. A similar point has been made by Bertelson (1978).

It should by now be clear that the minimum requirement for demonstrating that the extent to which particular cerebral hemispheric functions are asymmetrically organized changes across age is to show that younger children do not give a lateral superiority when using the same method of dealing with the given task that produces the lateral superiority observed in older children. This requirement applies generally to studies using auditory, visual or tactile presentation, and it has never been met by any of the studies claiming to find developmental differences in asymmetric cerebral organization. Consequently, the only valid conclusion at present with regard to tactile asymmetries is that left or right hand superiorities for tactile perception can be demonstrated in children down to at least age six under appropriate conditions.

Studies of asymmetries in infants for processing auditorily or visually presented stimuli. The failure of studies of asymmetries during childhood for processing laterally presented stimuli to provide any convincing evidence of changes across age in the asymmetric organization of cerebral hemispheric functions, and the existence of neuroanatomical asymmetries in infants, has led researchers to explore the possibility that functional cerebral asymmetries are present in infancy. A number of techniques have been devised, mostly using electrophysio- logical measures.

Electrophysiological studies have shown cerebral hemisphere differences in early infancy in terms of auditory and visual evoked potentials (Molfese et al, 1975; Molfese et al , 1976; Davis and Wada, 1977; Molfese, 1977; Molfese and Molfese, 1979), EEG power distributions (Davis and Wada, 1977; Gardiner and Walter, 1977), and photic driving (Crowell et al , 1973).

The dichotic stimulation technique has been adapted in order to demonstrate cerebral asymmetries in infants by Glanville et al. (1977), who used a response measure based on heart rate habituation. Entus (1977) also used dichotic stimulation with a sucking response, but a subsequent study (Vargha-Khadem and Corballis, 1979) has not been able to replicate her results.

The findings of these several studies of hemisphere function in infancy convincingly demonstrate that asymmetric organization of function is present, which is incompatible with Lenneberg’s (1967) views. Most investigators have, however, been satisfied to establish the basic point that functional asymmetries can be shown in infancy. Whilst the similarity of the asymmetries found in infants to those found in adults is usually obvious, this tactic avoids questions as to the precise mechanisms involved, and leaves open the possibility that some changes across age might occur. However, since studies of asymmetries for processing perceptually presented stimuli in infants and children have so consistently failed to produce any satisfactory supporting evidence for the notion of progressive lateralization of abilities, it is unreasonable to believe that such changes do occur unless strong supporting evidence can be found elsewhere.

Asymmetries of motor control and lateral preferences. It was mentioned in the introduction to this chapter that the principal innervation of movements of the body is mediated through contralateral nerve tracts. Although ipsilateral nerve fibres also exist, their role is not fully understood, but it is thought that their influence is confined to relatively gross movements. An example might be moving a hand by moving the whole arm. Fine motor movements, and especially movements from the level of the wrist of the hands and fingers, are seen as normally involving a relatively high degree of contralateral control (Brinkman and Kuypers, 1972, 1973; Trevarthen, 1974,1978). For this reason, in examining asymmetries of motor control and lateral preferences, particular attention will be paid to the fine control of movements of the hands and fingers.

An important distinction which needs to be made when considering motor asymmetries concerns the difference between lateral preference and relative skill (Annett, 1970; Ingram, 19757?; G. Young, 1977). This is perhaps best illustrated by means of an example. Most right-handed people will always write with their right hand, and nearly always pick up a pen with their right hand. The degree of preference for use of the right hand is similar for both activities. If, however, a right-handed person is asked to carry out these activities using his left hand, he is not likely to experience any difficulty in picking up the pen, but left-handed writing will prove to be much more tricky. The degree of relative skill of the hands for both activities is quite different. It is evident that relative manual skill and hand preference are not the same thing, though they are related (Annett, 1976). Their relation is probably most close for the more difficult and skilled tasks, as Brown (1962) found. Even with difficult tasks, however, the relation of preference and relative skill is not exact, and it is possible to find motor tasks that right-handed people can better execute with the left hand (Kimura and Vanderwolf, 1970). In addition to the contribution of asymmetry of cerebral hemispheric motor functions, hand preference can involve an element of choice, with one hand often being preferred regardless of whether the activities might cause considerable or little difficulty to the other. This means that studies of relative skill of the hands at different ages are of more direct relevance to asymmetry of cerebral hemispheric function than are studies of hand preferences (Denckla, 1974; G. Young, 1977).

The distinction of studies of lateral preference from studies of relative skill makes the results of an otherwise confusing body of studies of motor asymmetries during development fall into a neat pattern. Put simply, studies of relative skill have not found increases in asymmetry across age (one or two have actually found decreases), whereas studies of lateral preference have generated a mixture of results seen as indicating changes in lateral preference and results indicating absence of change in lateral preference across age.

A favourite type of task in studies of relative skill has involved comparisons between the hands for the highest speed or greatest accuracy with which repetitive movements can be carried out. Examples would be moving pegs on a pegboard, or tapping rhythms, and studies of this type which have used children down to age five or below include those of Knights and Moule (1967), Annett (1970), Denckla (1973, 1974), Ingram (19755), Finlayson (1976), and Wolff and Hurwitz (1976). The total range of ages covered by these studies is from three to sixteen years. All found right hand superiorities, and none produced evidence of an increase in the degree of right hand superiority with increasing age. In some cases, however, asymmetries were found to decrease in magnitude with increasing age (Denckla, 1974; Wolff and Hurwitz, 1976). These results may be attributable to a decrease in sensitivity of particular tasks across the considerable ranges of ages used in the studies concerned. They do, however, also raise the interesting possibility that there may be changes across age in the extent of asymmetric organization of some skills which do not take the form specified by the concept of lateralization.

Other tests of relative skill which have led to right hand superiorities include hand strength (Ingram, 19755; Finlayson, 1976), speed of writing (Reitan, 1971), and duration of grasp of a rattle (Caplan and Kinsbourne, 1976). Caplan and Kinsbourne’s finding, from a study of two- to four-month-old babies, remains the earliest demonstration of a manual asymmetry.

In two tasks used in Ingram’s (1975 5) study of three- to five-year-old children, which involved imitating hand postures or finger spacings, left hand superiorities were obtained, presumably reflecting the right hemisphere’s superiority for the complex spatial component of the tasks. This finding can thus be seen as both confirming the presence of superior right hemisphere spatial functions at age three and illustrating the importance of distinguishing questions of relative manual skill for different tasks from those of hand preference.

An interesting variation on the basic studies of relative skill on single tasks involves dual-task performance. Studies of adults have demonstrated that requiring them to talk whilst carrying out an independent manual task interferes more with right than with left hand performance (e.g. Kinsbourne and Cook, 1971; Hicks, 1975). Such interference probably occurs when speech and right hand movements demand the use of common left hemisphere functions (Lomas and Kimura, 1976). Studies of interference in dual-task performance in children down to age three have shown that the same types of effect occur (Kinsbourne and McMurray, 1975; Piazza, 1977; Hiscock and Kinsbourne, 1978). The only hint of any change across age arises in the report of McFarland and Ashton (1975), but since their groups contained as few as four subjects, sampling bias cannot be ruled out.

Early studies of motor asymmetries in infants and children were almost exclusively addressed to questions of lateral preference (e.g. Wile, 1934; Giesecke, 1936; Gesell and Ames, 1947; Hildreth, 1949). Although most of these studies would now receive low marks for adequacy of methodology and clarity in reporting what was actually done they were in general agreement that lateral preferences, and especially hand preferences, are established gradually throughout childhood, with periods of absence of preference or preferences opposite to those finally adopted. Some more recent studies have also reported results of this type both for infants (Cohen, 1966; Cernacek and Podivinsky, 1971; Seth, 1973; Ramsay, 1979) and for older children (Belmont and Birch, 1963), though there are also studies that have not found changes in hand preference across age in infancy (Ramsay, Campos and Fenson, 1979) or childhood (Annett, 1970).

The explanation of these discrepant findings from studies of hand preference lies in the measures used. Annett (1970) and Ramsay et al (1979) both studied hand preference for quite difficult skills. Hand preference for difficult skills is, for reasons explained previously, likely to be relatively closely related to differential skill, and it is studies of differential skill which do not tend to find changes across age. Most of the studies which did find developmental trends in hand preference used measures based on preference for picking up objects. There is no reason to assume that preference for the same actions is being examined at different ages, since there are a number of different ways of manipulating and picking up objects (Elliott and Connolly, 1973; Kopp, 1974; Bresson et al, 1977).

A type of investigation involving motor asymmetries which does not really fit into the scheme of studies of relative skill or studies of lateral preference also deserves mention. In several studies Turkewitz and his colleagues have shown that very young infants turn their heads more often to the right than to the left (e.g. Turkewitz et al ., 1965; Turkewitz et al, 1969; Turkewitz and Creighton, 1975). Although the demonstration of any motor asymmetry at early ages is of interest, no really satisfactory explanation as to the cause of the bias in head turning has been offered.

Studies of the consequences of cerebral injury at different ages

The findings of studies of infants and children using noninvasive methods have failed to provide evidence indicating that the extent to which particular cerebral hemispheric functions are symmetrically or asymmetrically organized changes across age in the manner implied by the concept of progressive lateralization of abilities. Moreover, they have shown that asymmetry of hemispheric function is present in some form in infancy. Both outcomes are clearly at variance with Lenneberg’s (1967) theoretical position. In fairness, however, it must be pointed out that most of this evidence was not available to Lenneberg, and that his theory was mainly derived from studies of hand preference and from studies of the consequences of cerebral injury at different ages.

It is notoriously difficult to draw valid inferences concerning the organization of cerebral functions from the effects of cerebral injuries, and this difficulty is compounded when it is necessary to draw conclusions about possible organizational differences across age. Several of the problems of methodology and interpretation that can arise have detailed in the reviews of Kinsbourne (1976) and Witelson (1977a), which seriously criticized many of the interpretations that have been offered. This is not, of course, to deny the great importance of studies of the developmental sequelae of cerebral injuries, but their relevance to understanding asymmetry of cerebral hemispheric function during development has often been overestimated and misunderstood.

The four main aspects of studies of the consequences of cerebral injury that have received attention will be examined in turn. These are the differences across age in the extent of recovery from unilateral cerebral injuries, the claim of the equipotentiality of the cerebral hemispheres for language acquisition, differences across age in the nature of aphasic symptoms, and the possible involvement of the right hemisphere in the early stages of language acquisition. As it will become clear that many of the studies carried out add little or nothing to our understanding of cerebral asymmetry, a systematic review of all the studies will not be attempted.

Age and the extent of recovery from unilateral cerebral injuries. Many studies of language disturbances in children following left hemisphere injury have shown that the younger the child the more rapid and complete is the recovery. Reviews are provided by Basser (1962), Lenneberg (1967) and Witelson (1977a); see also Parker, this volume. This recovery may be in part due to intrahemispheric reorganization of functions within the damaged left hemisphere (Hecaen, 1976), but it is known from cases where the extent of the injury eventually led to left hemispherectomy that considerable acquisition of language functions by the right hemisphere is possible in the first years of life.

These findings have often been taken to indicate that the lateralization of language abilities proceeds gradually throughout childhood, with the right hemisphere being involved in language functions in the early years. In fact, the findings do not indicate this at all. They simply attest to the remarkable ability of the young brain to recover and reorganize functions in response to injury. This is a complex phenomenon in its own right, widespread throughout the animal kingdom, to which there are a number of different contributory processes (Hecaen and Albert, 1978; Lund, 1978). None the less, the finding that recovery can take place tells us nothing about the organization of function before injury. There is no reason to connect loss of‘plasticity’ with an increase in lateralization.

The claim of the equipotentiality of the cerebral hemispheres for language acquisition. The extent of the recovery of language abilities following early left hemisphere injury is so marked that Lenneberg (1967) was led to the conclusion that the cerebral hemispheres are initially perfectly equipotential for language acquisition. This conclusion was supported by reports of the observations of clinicians, but more systematic and quantitative studies have shown that perfect equipotentiality does not obtain (Dennis and Kohn, 1975; Dennis and Whitaker, 1976; Dennis and Whitaker, 1977). Although extensive language functioning can be achieved by the right hemisphere following early injury to the left hemisphere, the left hemisphere is better able than the right to subserve language acquisition even in infancy.

Differences across age in the nature of aphasic symptoms. It has long been known that cerebral lesions causing disturbances of language in children (acquired aphasias) do not produce the same pattern of symptoms as found in adults (Guttman, 1942; Alajouanine and Lhermitte, 1965; Hecaen, 1976). The most common form of acquired aphasia in children involves difficulty with or absence of spontaneous expression (mutism), whilst jargonaphasia and logorrhea occur only in adults. Brown and Jaffe (1975) and Brown (1977) have extended these observations, arguing that the different types of aphasia are systematically related to age not only in childhood but throughout the human lifespan.

Such differences across age in the nature of acquired aphasias are of undoubted intrinsic importance and interest, but what do they tell us about asymmetry of cerebral hemispheric function? They might indicate that, at the 'psychological’ level, the organization of language functions and the relative contribution made by different linguistic skills changes during the lifespan, with some skills being developed to the level of practised fluency at which jargon- aphasia and logorrhea can occur. These changes could, however, be associated with intrahemispheric development and organization of processes principally located in the left hemisphere, and the concept of lateralization is not needed.

The possible involvement of the right hemisphere in the early stages of language acquisition. Although it has been customary to include them in discussions of asymmetry of cerebral hemispheric function during development, it is apparent that the lines of evidence concerning the consequences of cerebral injury at different ages described thus far are not really of central importance to the topic. There is one claim, however, which is potentially crucial, and which has been held apart from the others to show its special role in making the other lines of evidence appear to contribute more to our understanding of the problem than they actually do. The claim falls into two parts, which require separate consideration. Firstly, it is held that childhood aphasias are more likely than adult aphasias to occur as a consequence of injury to the right cerebral hemisphere, and secondly it is held that this implies that the right hemisphere is involved as well as the left hemisphere in the early stages of the acquisition of language functions.

The evidence concerning the first part of the claim is not completely convincing. It is clear that the proportion of children over five years of age experiencing aphasic difficulties following left as opposed to right hemisphere injury is comparable to the proportion found for adults (Krashen, 1973; Hecaen, 1976). For children aged two to five years, however, aphasia following right hemisphere injury would seem to be relatively frequent from the cases reported in the literature. Witelson (1977 a) gives a rough figure of 30%, but there are several difficulties in taking such a figure at its face value, as Kinsbourne (1976) and Witelson (1977a) have stressed. These difficulties include the possibility that many of the right hemisphere injuries were so extensive as to also involve parts of the left hemisphere, the danger of bias toward referral to specialists and reporting of the more unusual cases (i.e. those where aphasia apparently followed right hemisphere injury) and the poverty of the assessments typically given as to the nature, severity and duration of the aphasic symptoms. These methodological problems are not caused by any lack of competence of investigators, and it is difficult to see how they could all be fully overcome. Kinsbourne (1976) concluded that the existence of such difficulties is sufficient to invalidate the reports indicating greater frequency of aphasias following right hemisphere injuries in young children than in adults; Witelson also advocated that such reports should be treated with caution.

The attention paid to the methodological problems inherent in attempts to calculate the relative frequency of aphasias following right hemisphere injury in young children and adults has tended to draw attention away from the question of what the finding of a greater frequency in young children, if valid, should be taken to mean (a notable exception is the discussion by Moscovitch, 1977). Witelson (1977a) felt that it means that the right hemisphere may participate in the execution of language functions in the early stages of language acquisition, but that its contribution is always less than that of the left hemisphere. What needs to be clarified, though, is whether the right hemisphere’s contribution is of the same type as that made by the left hemisphere, as the concept of progressive lateralization of language abilities would imply, or whether it is important because of functions it can execute which would not normally be viewed as linguistic yet are integral to the early stages of language acquisition. Evidence from psychological studies of language acquisition, for instance, indicates that much of the initial organization involved is closely related to understanding of and interactions with the world of objects, events and other people (R. Brown, 1973; Lock, 1978). It is unfortunate that the level of analytic sophistication attained by psychologists has not been applied to neuropsychological studies of childhood aphasia. If this were done, differences between the types of aphasia following left and right hemisphere injuries sustained in childhood might be found. With mutism being the most common symptom this would obviously be difficult, but detectable differences could arise in the patterns of recovery.

At present, then, firm answers to the important questions that have arisen concerning the possible involvement of the right hemisphere in the early stages of language acquisition have not been provided by studies of childhood aphasias following right hemisphere injuries.

Overview and conclusions

Having examined the available evidence concerning asymmetry of cerebral hemispheric function during development, it is now possible to consider what general conclusions can be drawn. This will necessarily involve discussion of what type of conceptual and theoretical framework is most useful in describing the existing findings and generating new lines of investigation.

The results of the numerous studies that have been carried out show that asymmetric organization of at least some cerebral hemispheric functions is characteristic of the human brain at all ages during postnatal development. Although considerable recovery and reorganization of function can take place following unilateral cerebral injury sustained early in life, the cerebral hemispheres are not equipotential for language acquisition. Thus the claims of absence of functional asymmetry in infancy and perfect hemispheric equi- potentiality for language put forward by Lenneberg (1967) are simply incorrect.

The question as to whether the degree to which functions are asymmetrically organized increases across age cannot be given such a straightforward answer, and requires some clarification. The total number of asymmetrically organized functions may well increase during the first years of life for the simple reason that many are acquired during this period. In this trivial sense, ‘laterality’ quite probably does increase across age. The concept of lateralization, however, is only of real interest as applied to particular functions, for which it implies that unilateral organization develops progressively from an initial organization that is at least to some extent bilateral. It is this sense that was clearly intended by Lenneberg (1967), Krashen (1973) and Brown and Jaffe (1975).

This hypothesis of progressive lateralization of abilities has not found adequate support in the studies that have been carried out, irrespective of whether it is regarded as valid or as invalid to use parametric statistical analyses. When findings have been claimed to demonstrate progressive lateralization of abilities, it has been shown that enthusiasm for the concept of lateralization has led to lack of attention to more prosaic alternative explanations. Of course, as has been pointed out, the available methods of investigation have not always been adapted for work with all ages of children, so that all of the conceivable lines of enquiry have by no means been exhausted. It thus remains possible for people to believe that substantial positive evidence of genuine progressive changes in lateralization will one day be found. However, this is more a statement of faith than a scientific inference, and a more realistic theoretical framework for research findings needs to be built up.

The research approach dictated by the concept of lateralization has been to look for progressive changes in childhood in the extent of the asymmetric organization of certain functions. This means that studies have often been directed toward the possibility of change in functions that are already adequately established. The typical investigative tactic has involved the use of one or two tasks and a wide range of ages of subjects. Such studies have been worthwhile insofar as they have led to the conclusion that progressive lateralization of already acquired functions does not take place. Further studies of this type can still be of value in filling in the many missing details. It may now be more interesting, however, to look for changes in organization whilst functions are actually being acquired. For this purpose, the concept of lateralization should be abandoned, since it arbitrarily predetermines what form such changes would be conceptualized as taking, and they may turn out to be more varied. There is, for instance, no reason to discount the possibility that for some skills the extent of asymmetric organization may actually decrease as they become firmly established and integrated into a child’s repertoire.

A useful approach, then, may be to define the basic problem as one of understanding how newly learned skills are integrated with existing functions that are already symmetrically or asymmetrically organized. This shifts emphasis on to the possibility of relatively rapid changes occurring whilst functions are being acquired rather than long-term changes in already acquired functions, and does not prescribe the form such changes might take. It would require careful studies directed toward quite specific skills at the ages at which they are learned. A few studies of this type have been achieved, and suggestions have already been offered where others are obviously necessary, but they demand precise methods of investigation which have only recently begun to be available. As such methods are developed the studies of isolated tasks across wide ranges of ages deriving from the conceptual framework dictated by the concept of lateralization will probably become of less interest than very detailed studies carried out whilst functions such as prehension, enumeration or reading are being acquired.


Acknowledgements

The assistance provided by SSRC grants HR 5078, HR 6398, and HR 6876 is gratefully acknowledged. I am very grateful to Andrew Ellis for helpful discussion of several points of interpretation.


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Dickerson JWT. and McGurk H. Brain And Behavioural Development. (1982) Blackie & Son Ltd., Glasgow.

Brain and Behavioural Development - 1982: 1 Neural Development | 2 Comparative Neural | 3 Malnutrition | 4 Hormones and Growth Factors | 5 Cortical Activity | 6 Functional Asymmetry | 7 Plasticity | 8 Sex Differences


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