Book - Brain and behavioural development 6

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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