Laterality and dyslexia: A critical view

Laterality and Dyslexia: A Critical View

Merrill Hiscock Marcel Kinsbourne

Departments of Psychology and Psychiatry University of Saskatchewan

Saskatoon, Saskatchewan, Canada

Eunice Kennedy Shriver Center Waltham, Massachusetts

Introduction

The study of learning disabilities has entered an era in which neural models play a primary role (Chall and Mirsky, 1978; Cru- ickshank, 1980; Gaddes, 1980; Kinsbourne and Caplan, 1979; Knights and Bakker, 1976). Factors such as family dynamics, emotional ad- justment, and motivation, which attracted considerable attention from learning-disability specialists in the past, have been de-emphasized in favor of neurological factors such as perceptual and attentional disorders, neurodevelopmental lag, and cerebral dominance. In this respect, Samuel Orton's (1937) general position with respect to the neural basis of learning disability--a minority viewpoint in the 1920's and 1930's---has become established in the mainstream of contemporary thinking about dyslexia and associated disorders.

Prominent among the neural models of dyslexia are those that attribute reading disorders to a defect in cerebral dominance. Thus, not

Presented at the 32nd Annual Conference of The Orton Dyslexia Society, Seattle, Washington, November, 1981.

Preparation of this article was supported by a grant to the first author from the Medical Research Council of Canada.

Annals of Dyslexia, Vol. 32, 1982. Copyright © 1982 by The Orton Dyslexia Society ISSN 0474-7534

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only Orton's general view regarding the importance of neurological factors, but also his more specific argument regarding cerebral dominance, is reaching the height of its popularity after several decades of less widespread acceptance. To what can we attribute this belated enthusiasm for cerebral dominance as a major explanatory factor in dyslexia? There are several possibilities, including new findings from the neurosciences about cerebral dominance, new behavioral methods for measuring laterality, and more sophisticated concepts of cerebral dominance. These developments may very well account for the re- newed interest in questions concerning cerebral dominance and dyslexia. Nonetheless, definitive answers to those questions continue to elude us. Evidence for deviant cerebral dominance in dyslexic children does not seem to be more convincing today than it was in Orton's time (see reviews by Benton, 1975; Critchley, 1970; Naylor, 1980; Satz, 1976; Vernon, 1957, 1971; Zangwill, 1962).

Our purpose is not to review once again the voluminous and frequently contradictory literature on laterality in dyslexic children. Instead we shall try to determine whether cerebral-dominance models of dyslexia are tenable in light of what we currently know about laterality and cerebral dominance in the general population. In other words, we shall summarize what is known and what remains to be learned about cerebral dominance, and we then shall show how this knowledge, and lack of knowledge, constrains the formulation and testing of hypotheses about cerebral dominance and learning. We shall discuss first (a) the current state of knowledge about normal hemispheric specialization; (b) differences among the meanings of terms such as dominance, laterality, and lateralization; and (c) models of abnormal cerebral organization in dyslexic children. Following that, we shall outline some of the major conceptual, empirical, and logical constraints on attempts to relate dyslexia to an abnormal state of cerebral dominance: (d) the heterogeneity of dyslexia; (e) the relationship between behavioral laterality and cerebral lateralization; (f) attentional biases that influence laterality; (g) the dubious effect of deviant lateralization on cognitive ability; and (h) the developmental invariance of lateralization.

The Zeitgeist

The implications of cerebral lateralization would seem to be ubi- quitous if one believes all that is claimed in the educational literature

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(e.g., Armstrong, 1977; Baty and McConnel, 1976; Beyer, 1977; Grady, 1976; Hunter, 1976; V.R. Johnson, 1977; Pines, 1975; Regelski, 1977; Rennels, 1976; Samples, 1975; Williams, 1977). Most of these claims are based on an assumed mismatch between educational practice and the characteristics of the nondominant (usually right) cerebral hemisphere. In other words, the educational system, with its traditional emphasis on "left-brain" techniques and content, fails to educate both halves of children's brains. If this state of affairs is far from ideal as far as normally-achieving children are concerned, it becomes more serious for children with learning difficulties. At least, the average child receives half of an education. The learning-disabled child cannot learn adequately through the traditional left-brain methods, and his or her right brain is neglected by the system. Thus, in the most extreme case, the learning-disabled child receives no education at all. Popularized claims such as these go so far beyond our knowledge of brain function that we can neither confirm nor refute them.

Preoccupation with the significance of left- and right-hemispheric specialization is not limited to the discipline of education. In anthro- pology, psychiatry, psychology, sociology, and even the world of commerce, we find an eagerness to ascribe various phenomena to putative differences between the left and right sides of the brain. Differential specialization of the cerebral hemispheres has been in- voked as an explanatory principle in connection with the psychoses (Gruzelier and Flor-Henry, 1979); criminal behavior (Gabrielli and Mednick, 1980); choice of university major (Bakan, 1969); preference for sitting on one side of the classroom or the other (Gur, Gur, and Marshalek, 1975); individual differences in personality, attitudes, and values (Bakan, 1969, 1971; Etaugh, 1972; Weiten and Etaugh, 1973); sex differences of various kinds (Buffery, 1976; Waber, 1977; Witelson, 1976); individual differences in hypnotizability (Bakan, 1969); consumer behavior (Hansen, 1981); and many other human characteristics. Clearly, the notion that hemispheric specialization is a major factor in human behavior is enjoying a wave of popularity. In this milieu, it is especially important to strive for objectivity.

The Facts

Compared to what one reads in the newspapers and in some of the education magazines, scientists' knowledge of hemispheric special-

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ization is meagre. One may conclude that, despite all the clinical and experimental studies and the methodological innovations, no major question about the specialization of the cerebral hemispheres has been answered definitively (Kinsbourne, 1978a). The scientific literature abounds with isolated findings, but there is little convergence among them and even a greater lack of explanatory principles. We have a little information about the functional characteristics of each hemisphere in isolation (e. g., Zaidel, 1978), but much less about the collaboration and competition between the two hemispheres of normal people (Kins- bourne, 1981; Kinsbourne and Hicks, 1978).

The main limitations to knowledge about brain function stem from limitations in the methods available for investigating such functions and from the limitations in the conceptual framework in which research results are interpreted.

We shall consider first the methods. The strongest, most direct methods can be applied only to people who already have some neurological abnormality. Thus, great caution must be used in generalizing findings obtained with these methods to other populations. Evidence from commissurotomized (split-brain) people illustrates this dilemma (Whitaker, 1980). In order to be a candidate for what constitutes a surgical last resort, a person must have an epileptogenic disorder of great severity. Since the purpose of the surgery is to prevent the spread of seizure activity from one hemisphere to the other, the seizure focus (and thus the brain damage) may be confined to one hemisphere. Nevertheless, if that hemisphere has been abnormal for several years, as is usually the case, the "good" hemisphere probably will be organized in an atypical fashion to accommodate functions that cannot be performed by the damaged hemisphere. The patient is hardly an ideal source of information about left- and right-hemispheric specialization if one hemisphere is damaged and the other is organized in an idiosyn- cratic manner. Similar problems limit the generalizability of findings based on other clinical methods such as surgical removal of a damaged hemisphere (hemispherectomy); temporary incapacitation of most of one hemisphere with an injected barbituate (Wada technique); electrical stimulation of cortical regions; extirpation of brain tissue; and unilateral electroconvulsive shock. The limitations inherent in these studies usually are exacerbated by the unavailability of large numbers of patients. However, even large collections of clinical data may be difficult to interpret. For instance, it has proven to be no easy matter to

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construct a valid model of left- and right-hemispheric language representation on the basis of hundreds of people who developed a language disorder (aphasia) following unilateral brain damage (e.g., Annett, 1975; Satz, 1979; Zangwill, 1960).

There is a trade-off inherent in the choice to study normal, healthy people in lieu of brain-damaged patients: the results may be more generalizable but the methods are much more restricted. The invasive methods used with patients cannot, of course, be applied to the population at large. However, if any information at all about brain organization in normal people can be obtained through alternative means, then it is worthwhile to gather that information; the perils of extrapolating clinical findings to the general population are so great as to justify the most heroic attempts to study normal people. Most of the means of studying laterality in normal children and adults are behavioral. They involve the measurement of perceptual and motor asymmetries such as eye preference, hand preference, listening asymmetry, etc. We shall discuss these techniques--with emphasis on the newer methods---in a subsequent section of this paper. A promising, more direct source of information about brain functions is the electroencephalogram (EEG). With the aid of computers, a wealth of information can be derived from the EEG. Most prominent today are the evoked-potential (EP) techniques that allow measurement of stimulus-dependent electrical activity from the scalp. Although these methods have been used repeatedly in attempts to answer questions about hemispheric specialization in normal children and adults, the results thus far have been inconsistent and frequently confusing. Much of the blame can be attributed to methodological problems with this new and complex technology (Donchin, Kutas, and McCarthy, 1977). Beyond the methodological difficulties, however, there are marked and stable individual differences in the evoked potential that militate against making general conclusions (Amochaev and Salamy, 1979; Ehrlichman and Wiener, 1979). If the perceptual, motor, and cognitive functions of the normal brain are organized in a standard fashion, the EP data fail to reflect this standardization.

The existing collection of facts about left- and right-hemispheric function lacks convergent validity. Insofar as the methods are often indirect and the subject populations often unrepresentative, it is im- perative that consistent findings emerge from different methods and different populations. Otherwise even a finding that is consistent for a

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given method or population may be artifactual, or valid only for that population. It is not clear, for example, that the right visual half-field advantage in studies of word perception represents left-hemispheric dominance for reading rather than a bias introduced by scanning habits (cf. McKeever and Huling, 1971; White, 1973). Smith (I972) has shown that a person with no left hemisphere may be very skilled verbally, but one cannot conclude from this evidence that the right hemisphere of normal people is capable of such high-level verbal functioning. Findings about hemisphere specialization become much more credible when results from different methods point to the same conclusion. An important example of convergent validity concerns the incidence of left-sided speech representation in right-handers. The traditional source of information about this matter is the aphasia literature, which suggests that fewer than 5 percent of right-handed adults are right-hemisphere dominant for speech (Annett, 1975; Zang- will, 1960). It is reassuring that use of the Wada technique has led to similar estimates (Rasmussen and Milner, 1975; Rossi and Rosadini, 1967). With respect to left-handers, however, even the conclusions about speech lateralization that are based on aphasia evidence are widely discrepant (Satz, 1979).

Despite the difficulties encountered in obtaining direct and gen- erally valid information about brain organization, the main impedi- ments to progress in this field of study are conceptual rather than technological (Kinsbourne, 1978b). Our theories are less adequate than our methods. Most of the research into hemisphere functions still is guided by a structural or "s~vitchboard" model of brain organization (Kimura, 1961; Geschwind, 1965), in which the brain is viewed in much the same way as we might view an electronic device. Signals flow along pathways from a receptor to a processing center or from a processing center to an effector organ. The efficiency of the operation depends on circuit characteristics such as the length and attenuation of a particular pathway. Great importance is attributed, for example, to the time required to cross the corpus callosum (which connects the two hemispheres) or to the signal degradation that results from an indirect route. Without denying that the brain in some respects may resemble a passive electronic circuit, we would assert that it is much more than such a circuit. A brain capable of directing human behavior must be an active and dynamic organ. It must actively search for information and orient selectively to certain regions of the environment. It must in-

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tegrate information from the environment with stored information (Hebb, 1972), and the nature of this integration must change as a function of the information, the situation, and the arousal state of the person. In order to better unders tand brain organization, one must consider not only the point-to-point transmission of information within the brain but also the spread of excitation and inhibition from one brain region to another (Sherrington, 1906). Many laterality phenomena that appear anomalous from a structural, connectionistic viewpoint can be unders tood in light of a model that emphasizes the reciprocal, dynamic balance in activation and inhibition between the hemispheres (Kinsbourne, 1970, 1973, 1974, 1975, 1978b).

What, then, can we conclude about cerebral dominance in the general population? First, there can be little doubt that the left hemisphere of most right-handers plays a special role in the production of speech. As we noted earlier, the prevalence of different forms of speech representation in left-handers remains to be determined, although it is clear that a substantial percentage of left-handers has either right-hemispheric or bilateral speech representation (Rasmussen and Milner, 1975; Satz, 1979). Probably the linguistic superiority of the dominant hemisphere is much greater for expressive than for receptive functions (Searleman, 1977), but the upper limit of receptive language ability in the normal nondominant hemisphere is not known. Hemi- spheric dominance for speech production may stem from a more general capacity of the dominant hemisphere, viz., control of complex, sequential movements in the absence of visual control (Kimura, 1977), but the evidence thus far is not conclusive.

As for the nondominant hemisphere, its specialties seem to include numerous perceptual skills, many of which are elemental in nature. For example, the ability to match two lines on the basis of their orientation in two-dimensional space requires an intact right hemisphere, or at least a right hemisphere whose posterior half is intact (Benton, Varney, and Hamsher, 1978). Dot detection (Davidoff, 1977) and color discrimination (Davidoff, 1976) also typify the kind of elemental perceptual skill for which the nondominant hemisphere is specialized. Skills of greater complexity--facial recognition (Benton, 1980), the perception of melodies (Kimura, 1964), visual memory (Kimura, 1963; Milner, 1968), and different forms of spatial and con- structional ability (McFie, Piercy, and Zangwill, 1950; Piercy and Smyth, 1962)~-~also depend to a greater degree on the right hemi-

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sphere than on the left. As the skill becomes more complex, however, the degree of right-hemispheric superiority seems to diminish, perhaps because verbal meditation plays an increasing role. In any event, the specialized capabilities of the right hemisphere frequently are misrepresented. The right hemisphere is not specialized for music processing but only for certain aspects of music processing. Some of its alleged superiority in visuospatial tasks seems to depend on the requirement for a manipulative response (LeDoux, Wilson, and Gaz° zaniga, 1977). The left hemisphere is specialized for at least one spatial task, viz. finger recognition (Kinsbourne and Warrington, 1962), and skill in drawing pictures depends as much on the left hemisphere as on the right. Thus, even relatively modest claims about right-hemispheric superiority for music and spatial functions must be qualified.

These findings imply that the importance of the right-left distinction has been over-emphasized (see Corballis, 1980). Clinical experience suggests that the anterior-posterior dimension of the cerebrum is at least as important, but this dimension tends to be overlooked. Moreov- er, exclusive attention to left and right obscures the importance of the vertical dimension of the brain, i.e., the Jacksonian levels of organization from brainstem to neocortex along which brain development and associated behavioral development may be plotted (Kinsbourne and Hiscock, in press). In short, there seems to be ample reason to question whether cerebral lateralization is as important a factor in learning disabilities as it often is claimed to be.

The Terminology of Left and Right

No discussion of laterality can be clear unless certain terms are used in a clear and consistent manner. There are several potentially confusable terms that we shall distinguish from one another. Any of these terms might be defined differently from the way in which we have chosen to define it, but we have found the following distinctions to be useful.

Laterality refers to "sidedness," or the degree to which a receptor or effector organ on one side of the body is superior to its counterpart on the other side or the degree to which it is used in preference to its counterpart. Laterality is measurable; it is manifested as handedness, footedness, eyedness, etc., and it may be defined as some combination of scores from two or more paired organs.

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In contrast, lateralization refers to a state of cerebral organization in which there are qualitative or quantitative differences in function between the two hemispheres. If a given function, e.g., speech, is represented exclusively in the left hemisphere, then there is cerebral lateralization of speech. Alternatively, if the left hemisphere plays a greater role in speech production than does the right, or if the left hemisphere makes certain unique contributions to speech production and the right hemisphere makes contributions of a different kind, then there is cerebral lateralization although in this event it would be more accurate to refer to the lateralization as lateralization of the differential components rather than lateratization of speech in toto. Cerebral lateralization cannot be measured; it must be inferred from patterns of electrical activity on the scalp, perceptual asymmetries, or other accessible sources of information. Even the interpretation of clinical evidence requires that inferences be drawn. Consider, for example, studies of people who have incurred focal brain lesions. Perhaps Function A characteristically is impaired following damage to Region X of the left but not the right hemisphere. This might appear to constitute direct evidence that Function A is lateralized, but it in fact does not. Function A may depend on both hemispheres of the intact brain, but the left hemisphere may be critical for Function A because, for instance, the necessary verbal output mechanism is accessible only to the left hemisphere. What we observe is lateralization of a deficit, i.e., a lesion produces different consequences depending on which hemisphere is involved. Lateralization of function remains an inference.

The term cerebral dominance has two meanings (Zangwill, 1962). Often it is used as a synonym for cerebral lateralization. Dominance in this sense refers to the superiority of one hemisphere for a particular function or to the tendency for one hemisphere to play the leading role in performing a function. Thus, we might say that the left hemisphere is dominant for speech. The term cerebral dominance also may be used to imply a general mastery of one hemisphere over the other. This usage is a vestige of an earlier era during which the contribution of the right hemisphere to higher mental functioning was a matter of dispute. Now that the right hemisphere's cognitive abilities are better appreci- ated, there is less justification for attributing general dominance, or executive control, to the left hemisphere. Besides, if the left hemisphere is claimed to be dominant in the general sense, one might question how and when this control is relinquished so that the right hemisphere can perform the tasks for which it is specialized.

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Double, or dual, dominance means that the left hemisphere is dominant for some functions and the right hemisphere is dominant for others. The exact meaning of this term depends, of course, on how dominance is construed. Double dominance could refer only to com- plementary specialization, or it could imply that at any given time one hemisphere establishes control over behavior and perhaps even suppresses the function of the opposite hemisphere.

The final term on our list is hemisphericity. This is a typological term that refers to the notion that some people tend to exhibit the putative characteristic of one hemisphere or the other when they interact with their environment. For example, a person who is analytic and rational might be called a left-hemisphere type whereas someone who is guided more by feelings might be called a right-hemisphere type (Ornstein, 1972). Although these labels are intriguing as meta- phors, they should not be taken too seriously. First, very little is known about the characteristics of each hemisphere with respect to global personality and cognitive-style variables (Corballis, 1980). Second, it remains to be shown that people consistently "use" one hemisphere in preference to the other.

Hemisphere-Related Models of Dyslexia

There are numerous ways in which the organization of higher mental functions within the cerebrum might vary from one person to another (Kinsbourne, 1975). Brains may differ not only in terms of which functions are mapped onto which regions of cortex (topographic spedalization), but also in the degree to which different regions share the same funchons (degree of specialization). In addition, these two facets of specialization--topography and degree--may be applied to either hemisphere in isolation or to both hemispheres together. If we could examine the functional organization of a sample of right hemispheres, for instance, we might find a variety of localization patterns and we might also find diversity in the degree to which different regions are specialized. In some right hemispheres, we might be able to pinpoint discrete functional regions; in others, the boundaries between regions may be so ill-defined that localization amounts to no more than slight quantitative differences.

The respective hemispheres may differ in topography and degree

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of specialization independently of the topography and degree of specialization within each. For example, Function A may be represented in the left hemisphere or in the right. This is a matter of topography as we have defined it. If A is represented in the left hemisphere, it may be represented only in the left hemisphere or it may be duplicated in the right hemisphere to the extent that the left hemisphere is only marginally superior to the right in the performance of Function A. This is a matter of degree of specialization.

The simple framework set out above yields six variables: topography and degree of specialization within the left hemisphere, within the right hemisphere, and between the hemispheres. Possibly, each of these variables may differ from one function to another. Given the impressive number of functions in the human repertoire, we could generate an enormous number of combinations of specialization patterns. In addition, we might speculate that specialization would be adaptive for some skills and maladaptive for others, or adaptive at one stage of skill development and maladaptive at another. The co- incidence of two functions in a particular cortical territory, if maladaptive, might impair one function, or the other, or both. Even if specialization of the two hemispheres for a function were completely normal, performance might be impaired by excessive or insufficient communication between hemispheres via the corpus callosum or by some defect of dominance in the sense of executive control.

We shall describe only a few of the possible hemisphere-related models of reading disability. We shall consider topography and degree of specialization between hemispheres and not within a hemisphere. Moreover, we shall invoke the simplifying assumptions that there are only two kinds of skills that are relevant to reading performance, viz., verbal skills, which normally are lateralized to the left hemisphere, and visuospatial skills, which normally are lateralized to the right hemisphere. Beginning with these assumptions, we still can generate a large number of possible models of dyslexia.

Translocafion Models Hemisphere-related models of dyslexia may be organized into

four classes. The first class, which we call translocation models, is illustrated in Table I. The first line of Table I describes the presumed normal state of affairs, in which verbal functions reside in the left hemisphere and visuospatial functions reside in the right. What if a

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Table I. Models of Cerebral Organization

Model Left Hemisphere Right Hemisphere

1. Normal Verbal Spatial 2. Mirror Image Spatial Verbal 3. Unspecialized Verbal and Spatial Verbal and Spatial 4. Bilateral Spatial Verbal and Spatial Spatial 5. Bilateral Verbal Verbal Verbal and Spatial

mirror-image arrangement exists, as shown on the second line? What if visuospatial functions are represented in the left hemisphere and verbal functions are represented in the right? One possibility, of course, is that such topographical reversal will have no deleterious consequences. It is also possible that reversed lateralization confers an advantage for verbal functioning, for visuospatial functioning, or for both. However, since we are generating models for dyslexia, we shall consider only outcomes that are disadvantageous in some way. Why should it matter whether a function is represented in one hemisphere or the other? It may be argued that the left hemisphere constitutes an ideal substrate for language and the right hemisphere constitutes an ideal substrate for visuospatial functions. To support this contention, one could point to claims that the hemispheres differ in basic neural organization (Semmes, 1968), capacity for programming sequential movements (Kimura, 1977), perfusion of blood (Carmon, Harishanu, Lowinger and Lavy, 1972), or size of various regions (Galaburda, LeMay, Kemper and Geschwind, 1978). For any or all of these reasons, it may be advantageous to have verbal functions on the left and visuospatial functions on the right. Reversal of this normal pattern of lateralization could have a deleterious effect on verbal functions, visuospatial functions, or both. Thus, three specific models of dyslexia could be generated from the hypothesis that verbal and visuospatial functions in dyslexics are lateralized in a reversed fashion.

Another translocation model is shown on the third line of Table I. This is the bilateral spatial model, in which verbal functions are lateralized in the usual fashion but visuospatial functions are distributed across both hemispheres. Any of the usual three deleterious outcomes may be postulated, viz., impaired verbal functioning, im-

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paired visuospatial functioning, or impaired functioning in both realms. Verbal functions might be disrupted because the finite capacity of the left hemisphere must be divided between verbal and visuospatial systems. In other words, the presence of visuospatial systems within the left hemisphere might interfere with that hemisphere's ability to subserve verbal functions. Witelson (1977) has in fact proposed a dyslexia model of this nature, and she has supported her model with evidence that dyslexic boys show normal laterality for verbal tasks, lack of laterality for visuospatial tasks, and impaired performance on verbal tasks. In theory at least, visuospatial functions might be impaired with bilateral visuospatial organization, either because it is inefficient to have a system spread across both hemispheres or because there is competition between hemispheres.

The fourth model shown in Table I is the bilateral verbal model. In this case, visuospatial functions are lateralized in the usual manner but verbal functions are represented in both hemispheres. The bilateral distribution of visuospatial functions hypothetically may impair verbal functioning, visuospatial functioning, or both. Levy (1969) first proposed a bilateral verbal model, although she used it to account for ability patterns in left-handers and women rather than for reading disability in dyslexic children. According to her hypothesis, verbal systems in the right hemisphere disrupt the neurological organization of that hemisphere, which otherwise is optimal for the holistic processing required by various nonverbal tasks.

The final translocation model, which is depicted on the bottom line of Table I, is a logical extension of the previous two models. In this case, verbal functions and visuospatial functions are represented in both hemispheres. Each function is represented in its usual place, but it is represented in the opposite hemisphere as well. This model also could be construed in terms of anomalous degree of specialization, but we shall consider it along with other topographic models. As in the models outlined previously, the overlap of verbal and visuospatial functions within the same cortical territory could impair verbal func- tionLng or visuospatial functioning or both. Although this model has not been described by anyone as an explicit model of dyslexia, it seems to underlie the various attempts to show reduced laterality in dyslexic children. Thus, this model, in which neither verbal functions nor visuospatial functions are lateralized, seems to be a popular implicit model of hemispheric specialization in dyslexia.

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Beginning with a simplified view of the brain, in which we considered only two functions and ways in which their topographic representation might vary between the hemispheres, we were able to generate four anomalous patterns of organization, each of which has three possible maladaptive outcomes. That provides twelve specific predictions that one might make regarding the effects of anomalous topographic arrangement of verbal and visuospatial functions. If we were to consider the topography of the functions within each hemisphere, as well as differences in degree of specialization between and within hemispheres, we would soon become overwhelmed with possibilities. Even then we would not have exhausted the possibilities, as the following three additional classes of models should be considered.

Unilateral Deficit Models The translocation models presuppose that an otherwise normal

processor may perform inadequately if it is located in the wrong hemisphere, if it must share its territory with another processor, or if it is spread too thinly across the cortex. One might speculate instead that inadequate performance implies either a defect in the processor itself or an inability to employ that processor appropriately. A child may be deficient in left- or right-hemisphere functions even if those functions are lateralized in the usual fashion. Even though verbal functions may be represented in the left hemisphere, for example, those functions may be impaired or under-utilized.

Bakker (1979, 1980, 1981) recently has proposed what he terms a balance model of dyslexia. He asserts that some children (P-type dyslexics) have a functionally overdeveloped right hemisphere (cf. Masland, 1975) whereas others (L-type dyslexics) have a functionally overdeveloped left hemisphere. P- and L-type dyslexics may be equally impaired in their ability to read, but they show qualitatively different patterns of disability. The Bakker model appears to em- phasize overdevelopment of one hemisphere rather than under- development of the other. The "weak" hemisphere may be under- developed only relative to the "strong" hemisphere and not necessarily in an absolute sense. Consequently, the model seems to exemplify unilateral deficit models in which the deficit stems from an inability to use one or the other hemisphere effectively. Another variant of unilateral deficit models might require that there be an actual impairment of the functioning of the "weak" hemisphere. In either

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kind of model whether hemispheric utilization or hemispheric impairment is emphasized--there would be different outcomes depending on whether the left or right hemisphere is the "weak" hemisphere. Thus, there are at least two kinds of unilateral deficit models, each of which implies two possible outcomes.

Callosal Models In the aftermath of the split-brain studies, there has been a special

fascination in neuropsychology with the corpus callosum. It is commonly assumed that this massive band of nerve fibers carries information from one hemisphere to the other (e.g., Gazzaniga and Sperry, 1967). One might speculate that certain disorders of learning are attributable to a defect of the corpus callosum. Specifically, the corpus callosum might conduct too little information or too much information; or it might somehow distort the information or perhaps transmit it to the wrong region of the opposite hemisphere.

If separation of verbal and visuospatial functions in the brain confers an advantage, then it follows that an excessive degree of interconnection between the hemispheres would be maladaptive. Having a highly efficient corpus callosum might then be tantamount to having verbal and visuospatial functions overlapping within the same cerebral territory. Alternatively, since verbal and visuospatial information must somehow be integrated in the act of reading, it seems reasonable to suggest that dyslexia is associated with inadequate communication between the hemispheres. This model is consistent with the possibility that callosal functions are relatively undeveloped in young children (Galin, Johnstone, Nakell, and Herron, 1979). If it can be established that the cerebral hemispheres of young children function independently, as in adult split-brain patients, then dyslexia might be attributed to a delay in the maturation of the corpus callosum. At this time, however, developmental changes in capacity for cross- callosal transfer remain a matter of conjecture. Moreover, a deficiency of interhemispheric communication need not be attributed to a defective corpus callosum; it could stem instead from a deficiency within one or the other hemisphere (Masland, 1975).

If we consider only the simplest callosal models of dyslexia, there are two possibilities, viz., that the callosum transfers too much information and that it transfers too little. Further elaboration of the models requires assumptions about the direction in which information is

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transferred in the process of reading. Is configural information transmitted from the right hemisphere to the left so that verbal labels can be applied? Or is phonemic and lexical information transmitted from the left hemisphere to the right so that visual configurations can be identified and labeled within the right hemisphere? Depending on the hypothesized direction of information flow and on the relative ability of the callosum to conduct verbal and visuospatial information, different outcomes of callosal deficiency would be predicted.

Output Competition Models Since Orton's (1937) model of strephosymbolia does not fit into

any of the three classes previously discussed, it will be treated as an exemplar of a fourth class of hemisphere-related dyslexia models---a class in which competition between the hemispheres is the central feature. Models of this class depend on three assumptions: (1) that the two hemispheres process information independently; (2) that there is some quantitative or qualitative difference between hemispheres in the performance of a particular task; and (3) that one hemisphere normally suppresses the activity of the other or denies the other hemisphere access to output mechanisms. In Orton's (1937) model, the hemispheres differed with respect to the left-right orientation of en- grams (internal representation of stimuli) and the left hemisphere normally exerted control over the right. When the dominance of the left over the right hemisphere broke down, the two hemispheres competed for access to the mechanisms that would generate either an oral or written response. If the right hemisphere prevailed at a time when the correctness of the response depended on the perceived orientation of the stimulus, a reversal error would result. Inconsistent suppression of the right hemisphere by the left would lead to inconsistent performance.

If one begins with the three assumptions stated above, it is not difficult to construct competition models that differ from Orton's model in specific details but nevertheless follow the same general scheme. For example, one could hypothesize that the left hemisphere reads via phonological processing whereas the right hemisphere reads via direct mapping of visual configurations onto lexical memory. The hemisphere with privileged access to output mechanisms might vary with word familiarity, reading skill, etc. Interhemispheric competition presumably would decrease reading efficiency under circumstances in which one or the other modes of reading was more appropriate.

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It is not our intent at this point to promote or criticize any hemisphere-related model of dyslexia. Rather, we want to make two points. First, there is no single interpretation for the statement that reading-disabled children suffer from a defect in cerebral lateralization, cerebral dominance, laterality, etc. Such a statement has a multitude of possible meanings. Second, it is apparent that the realm of hemisphere-related models of dyslexia is a theorist's paradise. Several models have been proposed but there are many more that could be proposed. If data concerning the laterality characteristics of a sample of dyslexic children do not fit one model, they very likely will fit another. If they fit no existing model, it probably will be possible to construct a new and plausible-sounding model that does account for the findings. We have outlined four classes of models that contain nine existing or potential exemplars and a large number of specific predictions. No doubt, our list is incomplete.

Until we know much more than we now know about the cerebral organization of various functions in the normal brain, we have an inadequate conceptual basis for evaluating claims of association between dyslexia and anomalies of hemispheric specialization, cerebral dominance, and interhemispheric transfer. Moreover, the large potential number of hemisphere-related models and the even larger number of variants of each model make it inefficient to put every possibility to empirical test even if definitive methods for empirical validation were available. However, it is possible to evaluate certain issues of relevance to these models in light of current findings from neuropsychological studies. These findings suggest problems, limitations, and constraints; some may affect all hemisphere-related models of dyslexia and other's may affect only certain models. We shall devote the remainder of this paper to a discussion of some fundamental issues that must be dealt with by any theorist who would propose a hemisphere-related model of dyslexia and by any experimentalist who would endeavor to test such a model.

The Question of Dyslexic Subtypes

Most investigations of laterality and dyslexia follow a common procedure. A group of dyslexic children is identified according to specified criteria, which usually include a score from a reading test and

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a score from an IQ test. Age- and IQ-matched control subjects are then found. The dyslexic and control groups are administered a laterality test or battery of laterality tests. In most cases, the investigator predicts that the dyslexic children will fail to show laterality or that they will show a lesser degree of laterality than that shown by the controls. This procedure is logical only if dyslexia is a unitary disorder with a universally standard brain basis. If there are different varieties of dyslexia, i.e., subtypes, then this procedure makes no sense at all. Consider, for instance, what one might expect to find if Bakker's (1979, 1980, 1981) balance model were correct. L-type dyslexics presumab!y would show the normal pattern of laterality but to an exaggerated degree; P-type dyslexics presumably would show the reverse asymmetry. If one adminsters a laterality test to an undifferentiated group of dyslexic children, the mean laterality score for that group would depend on the relative number of L- and P- type dyslexics in the sample. If the sample contains a disproportionate number of L-type dyslexics, the dyslexic children may show the same pattern of asymmetry as the control group. In fact the magnitude of asymmetry may be greater than that found in controls. If, however, the dyslexic sample contains a preponderance of P-type dyslexics, then the dyslexic group may show reversed laterality, no laterality, or at least significantly less marked laterality than that found for control subjects. A similar, if less dra- matic, scenario may be envisaged on the basis of other hemisphere- related models. For example, consider that Orton's (1937) model of incomplete left-hemisphere dominance is correct but that it only ap- plies to a minority of dyslexic children. The majority of dyslexic children would have a reading disability for reasons entirely unrelated to cerebral dominance. In that case, whether or not one finds a deviant pattern of laterality among dyslexic children again would depend on the proportion of children in the sample whose dyslexia stems from defective cerebral dominance.

Bakker's subtyping scheme is exceptional in that the subtypes are defined, at least in part, on the basis of laterality characteristics. Consequently, further laterality testing for his subtyped children would serve only to assess the reliability and concurrent validity of the classification test (dichotic listening). In the usual case, children are sorted into subtypes according to characteristics of their reading performance or according to their pattern of scores on other skill tests (e.g., Boder, 1970; Doehring and Hoshko, 1977; Kinsbourne and War-

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rington, 1963; Mattis, 1981; Myklebust, 1965; Rourke, in press). If hemisphere-related models of dyslexia are to be validated con- vincingly, it will be necessary to show that different subtypes, selected on an a priori basis, show predicted laterality patterns. In many cases it is not clear what one should predict; there is no obvious relationship between subtypes and particular hemisphere-related models of dyslexia. For example, one of us (Kinsbourne, 1977) has proposed that many learning-disabled children have a language-based disorder whereas others show signs of the developmental Gerstmann syndrome (Kinsbourne and Warrington, 1963). On the basis of characteristic IQ test profiles, we might expect that the two subtypes would show opposite laterality patterns. However, the neurological disorders that the subtypes resemble--aphasia and the Gerstmann syndrome, respectively--are both associated with lesions in the dominant hemisphere. It is doubtful that other classification schemes lend them- selves much more readily to predictions concerning laterality patterns.

Thus, we face a dilemma. Contemporary thinking about learning disabilities leads us to conceive of dyslexia not as a single entity but as a heterogenous collection of disorders. It is unlikely that these various manifestations of dyslexia would all stem from the same abnormality of cerebral organization. Yet, when we classify children into more homogenous subgroups, it often is unclear how the characteristics of any subgroup might be related to cerebral lateralization, cerebral dominance, interhemispheric transfer, etc.

Laterality and Cerebral Lateralization: Conceptual Issues

A restricted range of techniques is available to the investigator who wishes to study the cerebral organization of dyslexic children. Since the use of invasive clinical techniques for studying the brain cannot be justified, the investigator must work with less direct methods. In some settings, it may be possible to obtain computed tomograms (CT scans) of the brains of dyslexic children. This procedure provides information only about brain structure and there is no reason to expect that a substantial proportion of dyslexic children would have gross structural brain.abnormalities, particularly if the children are free of neurologic signs and have no history of delayed speech acquisition (Haslam, Dalby, Johns, and Rademaker, 1981). EEG and evoked-

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potential data can be collected without danger to the child. These techniques provide potentially relevant information insofar as they measure functional characteristics of the brain and can be used while the child is performing tasks such as reading (e.g., Galin and Yingling, 1981). Nevertheless, the methodology is demanding and the results often are difficult to interpret. Especially when questions of left-and right-hemisphere functioning are to be answered, electrophysiological methods have not led to any straightforward answers (Donchin, Kutas, and McCarthy, 1977). It is not surprising that most investigators have chosen to use behavioral measures of laterality when addressing questions of cerebral lateralization in children with reading disability.

In this section, we shall overlook difficulties in measuring laterality in order to focus attention on the theoretical relationship between behavioral laterality and underlying cerebral lateralization. In other words, we shall pose the general question: If a particular facet of human laterality (handedness, eyedness, etc.) could be measured with perfect reliability and validity, what would that measurement tell us about underlying asymmetry of the brain?

Handedness There are two separate issues involved in the attempt to relate

handedness and cerebral lateralization. The first concerns differences between right- and left-handers. The second concerns the association between strength of handedness and degree of cerebral lateralization.

One of the best-established facts of modem neuropsychology is the frequency of left- and right-hemispheric speech in right-handed adults. Numerous studies of acquired aphasia (e.g., Gloning, Gloning, Haub, and Quatember, 1969; Luria, 1970; Zangwill, 1960, 1967), as well as studies using the sodium amobarbitone (Wada) technique (Ras- mussen and Milner, 1975; Rossi and Rosadini, 1967) indicate that 95-99 perceni; of right-handed adults have speech control lateralized to the left cerebral hemisphere. Although the same sources of information yield more variable estimates of left lateralized speech in left-handers, most estimates fall within the 60-70 percent range (e.g., Annett, 1975; Rasmussen and Milner, 1975; Zangwill, 1967). It appears, as well, that left-handers are much more likely than right-handers to have bilateral speech representation (Rasmussen and Milner, 1975; Satz, 1979). The implication of these statistics is clear: a right-handed adult is almost certain to have speech represented in the left hemisphere and, al-

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though the odds favor a left-hander also having left hemispheric speech, there is greater likelihood that the left-hander will have right- hemispheric or bilateral speech representation. Ambidextrous people apparently cannot be distinguished from left-handers with respect to the incidence of different patterns of speech lateralization (Rasmussen and Milner, 1975).

The second issue--strength of handedness and degree of lateralization is a more difficult one because there is little relevant information. Results of testing with sodium amobarbitone (Rasmussen and Milner, 1975) suggest that speech is clearly lateralized to the left hemisphere of all right-handers irrespective of the degree to which they favor the right hand. There is some evidence that auditory asymmetry varies with strength of handedness, at least in left-handed males (Dee, 1971; Lake and Bryden, 1976), but the relationship does not appear to be strong. In any event, until the association between degree of ear asymmetry and degree of cerebral lateralization can be established, even a strong correlation between ear asymmetry and handedness would fail to establish an association between strength of handedness and degree of lateralization.

Possibly, the relationship between handedness and speech lab eralization is moderated by a third variable, viz., the presence or absence of familial left-handedness. There is some evidence from both clinical and non-clinical sources that language representation varies with family sinistrality (e.g., H6caen and Sauget, 1971; Hines and Satz, 1971; Luria, 1970; Subirana, 1958; Warrington and Pratt, 1973; Zurif and Bryden, 1969), but this evidence is so contradictory that no conclusions can be drawn.

Eyedness The relationship between handedness and speech lateralization is

neither strong nor straightforward; nevertheless, there is a special link between either cerebral hemisphere and the contralateral hand. There is no such special link between a cerebral hemisphere and the contralateral eye, and it is primarily for this reason that we would reject eyedness as an index of cerebral lateralization. The conceptual difference between hemisphere-hand relationships and hemisphere-eye relationships can be illustrated as follows. A lesion in the motor cortex or basal ganglia of one hemisphere is likely to disrupt the functioning of the contralateral limbs while sparing that of the ipsilateral limbs. In

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contrast, a lesion in the visual cortex or lateral geniculate nucleus of the same hemisphere will not impair the vision of the contralateral eye while sparing the vision of the ipsilateral eye. The visual defect will involve one visual half-field of both eyes. Thus, there is no physiological basis for inferring an association between lack of right-eye dominance and a deficiency of the left hemisphere.

Mixed Dominance The concept of mixed dominance implies that there is a general

characteristic of sidedness in normal people, such that a person's hand preference will be consistent with his or her eye, foot, and ear preference. Deviations from this consistent pattern frequently are considered to be associated with weak or incomplete cerebral dominance, and to entail risk of behavioral deficiency of one kind or another. Orton (1937), for example, stressed that clear-cut left-handedness was not associated with strephosymbolia; rather, strephosymbolia was thought to be related to slight lateral preference in either direction or inconsistency of preference among hand, eye, and foot.

What can we conclude, on the basis of contemporary knowledge, about mixed dominance and its relationship to hemispheric specialization? First, we might question the rationale for including eyedness as a component of a general pattern of sidedness. There are two reasons for doubting the appropriateness of eyedness. One of those reasons is the frequency with which deviant eye dominance occurs in the general population. About one-third of the population--adults and children alike---deviates from the norm of right-eyedness (Porac and Coren, 1976), but only 8-10 percent of the population departs from the norm of right-handedness (Hardyck and Petrinovich, 1977); there- fore, a substantial number of normal children and adults must show crossed hand-eye dominance (Benton, 1975). The second problem with eyedness as a measure of sidedness is more direct: eyedness and handedness appear to be largely independent of each other (Porac and Coren, 1976). Especially when handedness and eyedness are treated as continuously distributed variables rather than as dichotomies or tri- chotomies, there is little or no correlation between the two measures (Coren and Kaplan, 1973; Porac and Coren, 1975). Thus, apart from the lack of a physiological rationale for using eye dominance as an index of cerebral dominance, it appears that eye dominance differs from other aspects of sidedness.

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Elimination of eye dominance leaves foot and ear preference in addition to handedness as the major dimensions of laterality. Ear preference has been overlooked in most laterality studies, but some recent findings suggest deviations from right ear preference are even more common than deviation from right eyedness, and that the correlation of ear preference with hand and foot preference is modest (Porac and Coren, 1979; Porac, Coren and Duncan, 1980). The incidence of deviant foot preference, however, approximates that of deviant hand preference, and the correlation between footedness and handedness is at least moderate (Annett and Turner, 1974; Clark, 1957; Porac and Coren, 1979; Porac et al., 1980).

On the basis of sodium amobarbitone studies (Rasmussen and Milner, 1975), we can conclude that adults with right-hand preference almost invariably have left hemispheric control of speech irrespective of their eye, ear, and foot preference. Possibly sidedness measures other than handedness might be predictive of speech lateralization in left-handers. Nevertheless, on the basis of the evidence currently available, it seems that laterality measures other than handedness can contribute little to our understanding of cerebral lateralization. Foot preference might be related to speech lateralization but it is unlikely that the association between footedness and speech lateralization is stronger than that between handedness and speech lateralization.

Visual Half-Field Asymmetries In recent years, considerable attention in neuropsychology has

been focused on perceptual measures of hemispheric specialization. One of the two major techniques for assessing perceptual asymmetry entails brief presentation of visual stimuli in one or the other visual half-fields, i.e., to the left or to the right of fixation. In many studies, only one stimulus is presented during a trial; in other studies, two stimuli are presented simultaneously, one in the left half-field and one in the right half-field. The rationale for interpreting asymmetric performance in terms of hemispheric specialization is clear: stimulation from either side of fixation, if sufficiently brief so as to disappear before a fixating eye movement can be made, will be conducted more directly to the contralateral hemisphere than to the ipsilateral hemisphere (Kimura and Durnford, 1974). This asymmetric conduction is attributable to the semi-decussated (partially crossed) arrangement of the visual pathways, which is a well established property of the mam-

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malian visual system. However the functional significance of the asymmetric conduction, with respect to perceptual laterality, remains to be established. Many of the results of visual half-field studies might be attributable to other factors, such as directional scanning (White, 1969) or biased attention (Kinsbourne, 1970, 1973; Hellige and Cox, 1976).

Consider, for example, a task in which children try to identify words as they are flashed in the left or right visual half-field via a tachistoscope. If the majority of children shows a right visual half-field superiority for the recognition of the words, then that perceptual asymmetry might be attributable to left hemisphere specialization for reading. If a group of dyslexic children shows no asymmetry, that outcome might be attributable to a lack of left hemisphere specialization for reading. Alternatively, according to a directional scanning explanation, the right visual half-field advantage in normal readers might be attributable to a well developed rightward scanning habit for reading that is less developed in dyslexic children. The attentional explanation is that normal readers and dyslexics probably approach the word-recognition task with different strategies. Normal readers might respond to the task with a verbal strategy that is associated with left hemispheric activation and a consequent bias toward the right side of space. Dyslexic children--even though they may not differ from normal readers in cerebral lateralization--might approach the task with a strategy that does not entail selective activation of the left hemisphere. Consequently, dyslexic children would fail to show a right visual half-field advantage.

In short, the putative link between hemispheric specialization and asymmetries of visual perception has a plausible basis. More precisely, it has two plausible bases and it is not clear that perceptual asymmetries stem from structural properties of the nervous system rather than from attentional biases generated by unequal activation of the two hemispheres. In some cases, the effect of hemispheric specialization-- whatever its mechanism may be--probably is less important than directional scanning habits.

Dichotic Listening Asymmetries In dichotic listening, pairs of aural stimuli are presented simul-

taneously so that one stimulus enters one ear and the other stimulus enters the opposite ear. When the stimuli are linguistic, the stimulus entering the right ear tends to be reported in preference to the one

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entering the left ear; this right-ear advantage (REA) is taken as a reflection of left hemispheric superiority for the processing of linguistic material. Auditory asymmetry, much as visual asymmetry, has been related to hemispheric specialization via both structural (Kimura, 1961, 1967) and attentional (Kinsbourne, 1973) mechanisms, and there is evidence to support both views (Studdert-Kennedy, 1975).

The rationale for the structural model is weaker than in the case of visual perception. There is no conclusive evidence that the crossed ear-to-auditory cortex pathways in humans are functionally prepotent over the uncrossed pathways, and occlusion of the ipsilateral pathway, as hypothesized by Kimura (1967), has been shown to be un- necessary (e.g., Bakker, 1968; Simon, 1967). Some studies suggest that the REA is not an ear advantage at all, but rather an advantage for signals originating from the right side of space (Morais, 1975; Morais and Bertelson, 1973, 1975). Although these findings cast doubt on the existence of structural determinants of the REA, they are consistent with the hypothesis that activation of the left hemisphere tends to direct attention toward the right side of space (Kinsbourne, 1970, 1973).

It should be noted that the dichotic ear advantage has been validated with patients as an indicator of speech lateralization. When Kimura (1961) administered a dichotic digits task to adults with known side of cerebral speech representation, she found that patients with left hemispheric speech representation reported more digits from the right ear than from the left and that patients with right hemispheric speech representation reported more digits from the left ear than from the right. Whether the smaller ear differences found in normal children and adults also reflect speech lateralization is more an article of faith than a demonstrable fact. Nevertheless, an average REA for linguistic material is a remarkably consistent finding across samples of normal people.

Tactile Asymmetries Clinical studies suggest that the right hemisphere is more pro-

ficient than the left in the perception of shape, number, pattern, and direction when the information is obtained by touch (Carmon and Benton, 1969; Fontenot and Benton, 1971; Milner and Taylor, 1972; Nebes, 1971). Since the somatosensory system is largely a crossed system, one might expect that the left hand of normal people would

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outperform the right in performing various tactile recognition tasks. Although the literature on tactile asymmetry is not extensive, investigators have used two tasks with normal children and adults, viz., unimanual reading of Braille or Braille-like material (e.g., Rudel, Denckla,.and Spalten, 1974; Wagner and Harris, 1979), and simultaneous (dichhaptic) left- and right-hand recognition of two shapes (Witelson, 1974).

As a rule, tactile recognition tasks do yield left-hand advantages in normal children and adults. As in the instances of the other two perceptual modalities, the asymmetric performance probably is related to hemispheric specialization but the mechanisms are not fully understood. In the tactile modality, as in the visual and auditory modalities, the asymmetries may reflect attentional biases rather than structural factors. In addition, the active information-seeking inherent in tactile tasks makes them particularly susceptible to strategy effects.

Dual-Task Asymmetries As a special case of a more general "functional distance" hypothe-

sis (Kinsbourne, 1981; Kinsbourne and Hicks, 1978), one might expect to find greater interference between two concurrent activities if they are both programmed within the same hemisphere than if they are programmed in different hemispheres. Consistent with this expecta- tion is Kinsbourne and Cook's (1971) finding that speaking interferes to a greater degree with right-hand performance than with left-hand performance of an attention-demanding task. This finding has been confirmed in children (e.g., Hiscock and Kinsbourne, 1980b) as well as in adults (e.g., Hicks, 1975).

The validity of the dual-task technique as a measure of functional lateralization stems from the crossed nature of the primate pyramidal (motor) system (e.g., Brinkman and Kuypers, 1972) and from electrophysiological and behavioral evidence regarding the degree to which different limb control regions in the brain are interconnected (Kinsbourne and Hicks, 1978). The dual-task technique is especially promising for the study of dyslexia because reading may be used as one of the two concurrent tasks (Hiscock, Antoniuk, and Prisciak, 1982). Children may engage in a naturalistic reading task while they are performing a concurrent motor task with either the right or the left hand. In addition, there are two important advantages of measuring motor (output) laterality rather than perceptual (input) laterality. One

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of these is the more overt nature of strategy, attentional shifts, and other variables that are difficult to detect in perceptual task~. The other advantage is related to the likelihood that cerebral lateralization for output processes is more complete than lateralization for input processes (Searleman, 1977).

The Validity of Laterality Measures as Predictors of Speech Lateraliza- tion: General Conclusions

We should stress that we have overlooked methodological issues in attempting to evaluate the conceptual validity of various measures of laterality. Techniques with a reasonably sound theoretical relationship to cerebral lateralization---e.g., visual half-field techniques--may entail the most difficult methodological problems. Apart from methodological concerns, however, it is clear that some laterality measures are more promising than others. A person's handedness provides some information, albeit not much, about his or her speech lateralization. Foot preference and ear preference probably provide less information, and eye dominance quite likely provides none at all.

From strictly theoretical considerations, the various perceptual measures and the dual-task technique would seem to offer the greatest insight into a person's cerebral lateralization. In practice, even these measures tell us very little about an individual's pattern of cerebral lateralization. Satz (1977) made that point forcefully with a Bayesian analysis of a hypothetical set of circumstances. He first constructed a model that seems to approximate the state of affairs, as we now understand it, with respect to cerebral lateralization and perceptual laterality: 95 percent of a population has left lateralized speech but only 70 percent of the people sampled from that population shows a REA on a dichotic listening test. If an investigator were to infer left- hemispheric speech representation for each of the people with a REA, he or she would be correct in 97 percent of the cases. If, however, the investigator were to infer anomalous speech lateralization in each of the people who failed to show a REA, he or she would be correct in only 10 percent of the cases. Irrespective of their failure to show a REA, 90 percent of these people would have the usual left hemispheric speech representation.

Paradoxically, the more we know about laterality in a given sample of normal people, the less likely are we to make correct inferences regarding the lateralization of their speech. Assume, for

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instance, that the incidence rate for left hemispheric speech is actually 98 percent in right-handers and 70 percent in left-handers. Assume further that 10 percent of the population is left-handed. Over repeated sampling of 90 right-handers and 10 left-handers, an average of 95 out of every 100 people would have left-lateralized speech. Thus, if we knew nothing about their laterality and were to guess that every person had left-lateralized speech, we would be correct 95 percent of the time. If we were to measure handedness and to assume that left-handers have deviant speech representation, we might predict that every right-hander and no left-handers have left-lateralized speech. (Predicting that all left-handers have normally lateralized speech would improve our accuracy, but it would also eliminate the need to assess handedness). With handedness taken into account, our predictive accuracy drops to 91 percent, i.e., we will correctly classify about 88 of the 90 right-handers and 3 of the 10 left-handers. If we disregard handedness and make our prediction on the basis of presence or absence of a REA in dichotic listening, the accuracy of our prediction depends on the proportion of people who show a REA. If 70 percent shows a REA, and if we predict speech lateralization according to ear advantage, we would face precisely the situation that Satz (1977) used in his Bayesian analysis. From his results, we calculate that our predictive accuracy now has declined to approximately 71 percent: we will correctly infer speech lateralization in 68 of the 70 people who show a REA and in 3 of the 30 people who fail to show a REA.

We conclude that knowledge of a person's handedness will not enhance our ability to predict his or her speech lateralization until such time as we are able to distinguish between those left-handers who have left-lateralized speech and those who do not. Levy and Reid (1978) proposed that speech representation in left-handers is reflected in writing posture, but subsequent research has failed to substantiate their claim (see Weber and Bradshaw, 1981). Perceptual measures of laterality may be related in theory to cerebral lateralization, but the utility of those measures in predicting the speech lateralization of individuals is reflected in the number of people who show the expected pattern. If only 70-80 percent of normal people show the expected asymmetry, then predicting speech lateralization on this basis will always be less accurate than simply assuming that all people have left-lateralized speech. If the true incidence of left-hemispheric speech representation in the general population is 95 percent, then a

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perceptual measure of laterality must yield the expected asymmetry in at least 90 percent of randomly sampled people before the perceptual measure will be a useful predictor at the level of the individual person.

Attentional Biases in the Measurement of Laterality

Numerous methodological, experiential, and contextual factors contribute to the variability of results from one subject to another and from one study to another. As we have reviewed these factors elsewhere (Kinsbourne and Hiscock, in press), we shall describe here only one kind of factor--an attentional biasing factor--that may alter laterality quite dramatically. Our brief discussion of attentional factors will serve not only to illustrate one reason why normal people may fail to show the expected asymmetry in a test of perceptual laterality; it also will illustrate a major reason for questioning whether perceptual laterality tells us anything about the structure of the nervous system.

Attentional Biases in Visual Perception Perceptual laterality, irrespective of modality, may be influenced

by (a) lateral biases that the subject carries into the experimental setting and (b) biases that arise within the experimental setting. As mentioned previously, literate subjects bring with them into a tachistoscopic experiment certain directional scanning habits that depend on the language in which they are literate. People who can read two languages may have acquired two opposing tendencies: Mishkin and Forgays (1952) found a right visual half-field superiority for English words but not for Yiddish words among English-Yiddish bilinguals. Orbach (1953) reported a similar finding, although the laterality effect for Yiddish words depended on the order in which subjects had learned the two languages. Only those subjects who had learned Yiddish before learning English showed a left visual half-field superiority for Yiddish words. Arranging words in a vertical orientation may eliminate scanning biases (Barton, Goodglass, and Shai, 1965), but it may also eliminate the right visual half-field advantage (Ethier, 1980).

Attentional biases also may stem from a previous task within the same experimental session; from some characteristic of the laterality task itself; or from a concurrent activity. Kimura and Durnford (1974) reported a carry-over effect from one visual laterality task to another.

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Their adult subjects performed a geometric form-identification task either immediately before or immediately after a letter-identification task. There was a right visual half-field advantage for letters irrespective of the order in which the tasks were administered, but the laterality for form identification depended on whether the form- identification task preceded or followed the letter-identification task. If the forms were presented first, there was no significant laterality for form identification; if the forms followed the letters, there was a significant right half-field superiority for the forms. Similar priming effects have been reported by other investigators (Klein, Moscovitch, and Vigna, 1976; Paivio and Ernest, 1971). Heron (1957) and Terrace (1959) raised the issue of pre-exposural sets being established within an experiment, i.e., perceptual laterality may vary depending on whether the subject expects a verbal or nonverbal stimulus on any given trial (Hellige, 1978). Moreover, in a direct attempt to bias visual asymmetry, investigators have required subjects to perform a concurrent task that is either verbal or nonverbal in nature. A secondary task that is verbal in nature frequently will enhance performance in the right visual half-field, presumably because activation of the left hemisphere biases attention to the right, and a nonverbal secondary task may enhance performance in the left half-field (Gardner, Eagan, and Branski, 1973; Goodglass, Shai, Rosen, and Berman, 1971; Hellige and Cox, 1976; Hellige, Cox, and Litvac, 1979; Kinsbourne, 1970, 1973, 1975). Perhaps even the subject's private thoughts during the experiment will bias his or her perceptual asymmetry.

The introduction of a safeguard to prevent fixational shifts may itself generate a lateral bias. It has been customary, during the past several years, to ensure midline fixation by requiring the subject to identify a fixation stimulus that is presented on the midline, usually at the same time that the target stimulus is flashed in the left or right visual half-field. In two independent studies of children's laterality, the nature of the fixation stimulus altered the outcome (Carter and Kinsbourne, 1979; Kershner, Thomae, and Callaway, 1977). In both studies, either a digit or a geometric shape was presented centrally at the same time that digits were presented bilaterally. Children were instructed to report the fixation digit or shape prior to reporting the digits in the lateral fields. The results from both studies were similar: there was a right visual half-field advantage when digits served as fixation stimuli and a left half-field advantage when shapes served as fixation stimuli.

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Attentional Biases in Auditory Perception The free-report methodology commonly used in dichotic listening

may be criticized because it allows subjects great latitude in choice of strategy (Bryden, 1978). Especially when multiple stimuli are presented on each trial, the subject may attend mainly to one ear or the other and may alter the manner in which the stimuli are processed and reported. If the subject notices that more stimuli are being reported from the right ear than from the left, he or she may attempt to compensate for this inequality by focussing more attention on the left ear.

Even in young children, the REA for strings of digits can be overcome simply by asking the children to monitor the left ear (Hiscock and Kinsbourne, 1980a; Geffen, 1978; Geffen and Wale, 1979). Never- theless, children are more adept at monitoring the right ear than at monitoring the left. In fact, the REA tends to be stronger when children are asked to focus attention on one ear at a time than when they are asked to divide their attention between the two ears (Hiscock and Kinsbourne, 1977, 1980a; Hiscock, Kinsbourne, Caplan, and Swanson, 1979; Kinsbourne and Hiscock, 1977). This is not surprising, insofar as previously uncontrolled shifting of attention from one ear to the other is now at least partially controlled by the experimenter. We find, however, that children have difficulty in shifting attention from one ear to the other. Table II illustrates a "priming effect" that we obtained in three different studies of selective listening in children (Hiscock and Bergstrom, 1982; Hiscock et al, 1979; Hiscock and Kinsbourne, 1980a). In each of these studies, children were asked to listen selectively to one ear as they heard dichotic digit names. Half of the children listened first to the left ear and half listened first to the right ear. The children subsequently switched their attention to the previously unattended ear. Even when the interval between monitoring one ear and monitoring the other ear was as long as one week, the children had limited success in monitoring the previously unattended ear. They continued to commit intrusion errors, i.e., to report signals from the ear previously monitored. This priming effect exerts a powerful effect on the average strength of the REA and on the percentage of children who show a REA. In most instances, children who monitor the right ear first show a marked REA and those who monitor the left ear first fail to show a significant degree of laterality. In the most recent study (His- cock and Bergstrom, 1982), 97 percent of the children who listened first to the right ear showed more intrusion errors from the right ear than

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Table II Left- and Right-Ear Performance in Selective Listening as

a Function of the Order in which the Ears were Monitored

Subjects Order in Which Ears were Monitored

Mean Percentage of Correct Responses

Left Right

Hyperactive children, 6-16 years old (Hiscock et al., 1979)

Left ear first (N=11) Right ear first (N= 11)

86.8 84.4 62.6 83.9

Normal children, 3-12 years old (Hiscock & Kinsbourne, 1980a)

Left ear first (N=77) Right ear first (N = 78)

73.6 75.8 64.5 79.7

Normal children, 6-10 years (Hiscock & Bergstrom, 1982)

Left ear first (N=36) Right ear first (N = 36)

66.6 73.1 61.1 86.2

from the left; only 67 percent of the children who listened first to the left ear showed the same pattern.

If persistent lateral biases can be generated so readily in the laboratory, one must wonder what adventitious biases the subject is bringing with him or her into the laboratory. Perhaps the child who fails to show a REA has had a recent experience of listening to someone who was positioned on the child's left side (cf. Bakker and Van Rijnsoever, 1977).

Attentional Biases in Tactile Perception Tests of tactile laterality also are susceptible to carry-over and

order effects. Witelson (1974) reported a carry-over effect similar to those found in tachistoscopic studies. In her study, normal right- handed boys attempted to identify by touch objects that were pre-

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sented simultaneously to both hands. One group of boys felt non- descript shapes prior to feeling letters, and another group accom- plished the two tasks in the reverse order. The expected left-hand superiority for shape recognition was found only when the shape- identification task was administered first; there was no significant asymmetry in shape perception if the shape-perception task was preceded by the letter-recognition task. Perception of letters was symmetric irrespective of the order in which the two tasks were administered.

In those tactile studies in which each hand is tested separately (Rudel and Denckla, 1979; Rudel, Denckla, and Spalten, 1974; Wagner and Harris, 1979), the order of left- and right-hand testing is important. Right-hand performance tends to remain constant irrespective of testing order but left-hand performance may be enhanced quite markedly by the previous experience of the right hand. Without this inequality between hands in the ability to benefit from the experience of the other hand, these Braille-learning tasks probably would reveal no asymmetry at all. In other words, the left hand emerges as the superior hand only after the hands are switched.

The Significance of Attentional Biases Attentional biases may be viewed either as troublesome extra-

neous factors that confound studies of perceptual laterality, or as variables that are intrinsic to the phenomena under study. For example, if the formation of a verbal mental set biases attention to the right, that bias might be construed either as a factor to be controlled in laterality studies or as the very basis of asymmetrical perception. From either perspective, attentional biases must be better understood if the relationship between perceptual laterality and underlying functional asymmetry of the brain is to be understood. At present, it is clear that attentional biases may exert an effect on laterality that is sufficiently strong under some circumstances as to overwhelm other factors and to amplify, nullify, or reverse the expected pattern of laterality.

Deviant Lateralization and Cognitive Ability: Nature's Experiment

As we pointed out previously, the translocation models of dyslexia presuppose that a deviant geographic arrangement of processors

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within an otherwise normal brain may lead to cognitive deficiency of some kind. This claim could be put to empirical test if there were some noninvasive means of ascertaining speech lateralization in normal children and adults. Unfortunately, as we have seen, none of the available measures of laterality is useful for predicting the locus of speech representation in normal people. Even though we may be able to specify the speech lateralization of patients, the study of these people invariably will cause us to confound the effects of deviant lateralization per se with the effects of brain damage. We need for our study a population of people whose brains are lateralized in a deviant fashion but who are free of central nervous system pathology. In other words, we must study people who are normal in every way except for the topographical organization of functions within the cerebrum.

A substantial minority of left-handers meets the criteria of deviant cerebral lateralization and absence of known brain pathology. Al- though it is not possible to distinguish these left-handers from other left-handers, we nevertheless may exploit this experiment of nature. If we accept the clinical evidence that deviant speech lateralization is very rare in right-handers, then we simply can contrast randomly sampled left- and right-handers with respect to whatever cognitive performance we wish to measure. If deviant laterality per se has deleterious effects on performance, then the sample of left-handers should show Ca) lower averag e performance and (b) greater variability of performance than do right-handers. In a few studies, investigators have found evidence that left-handers are deficient in certain nonverbal skills (Levy, 1969; Miller, 1971; Nebes, 1971). Other studies, however - -many of which are based on large and representative samples fail to find any deficiency among left-handers in either general intelligence or in nonverbal ability (Fagan-Dubin, 1974; Hard- yck, Petrinovich, and Goldman, 1976; Keller, Croake, and Riesenman, 1973; Newcombe and Ratcliff, 1973; Orme, 1970; Roberts and Engle, 1974; Wilson and Dolan, t931). In a massive study described in a United States Government National Health Survey report (Roberts and Engle, 1974), 762 left-handed children could not be differentiated from more than 6,000 right-handed children on the basis of their scores on either the Block Design or Vocabulary subtests from the Wechsler Intelligence Scale for Children. In another study, Newcombe and Ratcliff (1973) reported that pure left-handers were notable only for the low variability of their scores on IQ measures.

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Nature's experiment seems to have yielded negative results. The preponderance of negative findings, especially those based upon large and representative samples, suggest that left-handers in the general population are as intelligent as their right-handed counterparts. Con- sequently, we conclude that, when deviant lateralization of speech is found in the absence of brain pathology, there are no detrimental effects with respect to cognitive ability. This conclusion not only challenges the rationale underlying the translocation models of dyslexia that we outlined previously; it also runs counter to the assump- tion that an inadequate degree of lateralization is maladaptive. Since many left-handers seem to have a lesser degree of speech lateralization than do right-handers (e.g., H6caen and Sauget, 1971; and Luria, 1970; Rasmussen and Milner, 1975; Subirana, 1958), a low degree of cerebral lateralization apparently does not imply cognitive deficit.

The argument stated above does not necessarily conflict with numerous reports that left-handers are over-represented among dyslexic children and among children with more general cognitive defi- ciencies (e.g., Gordon, 1920; Zangwill, 1960). What may appear to be a conflict can be resolved by positing that many of the left-handers seen in clinical or institutional settings are pathological left-handers, i.e., that they are left-handed because of lateralized brain insult (Satz, 1972, 1973; Satz, Baymur, and Van der Vlugt, in press). In these cases, it likely is the brain pathology, rather than the deviant cerebral lateralization associated with left-handedness, that contributes to the cognitive deficiency (see Kinsbourne and Hiscock, 1981).

The Developmental Invariance of Cerebral Lateralization

The term dyslexia as we have used it is an abbreviated form of the term developmental dyslexia. Despite possible resemblence between reading difficulties in children and reading difficulties observed in adults following brain damage (Orton, 1937), the two kinds of disorder are distinct from each other. Developmental dyslexia differs from acquired dyslexia not only in presumed etiology (Orton, 1937, 1943), but also in that developmental dyslexia is an impairment of an emer- gent skill in an immature human. In other words, dyslexia in children must be studied against a background of maturational and cognitive growth.

Proper emphasis on the developmental aspect of reading dis-

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ability leads to confrontation with two issues concerning cerebral lateralization, viz., developmental changes in cerebral lateralization itself and changes in the nature of the reading process as the child progresses from one stage of reading development to the next. If the establishment of cerebral dominance for language is thought to occur gradually during childhood (Lenneberg, 1967), it follows that dyslexic children may lag behind their peers in attaining a particular level of cerebral lateralization. Moreover, the relative demand for left- and right-hemispheric skills may change with developing proficiency in reading (e.g., Bakker, 1973; Bakker and Reitsma, 1973; Sparrow and Satz, 1970). If the critical skills at an early stage of reading are nonverbal (right-hemispheric), then lack of language lateralization might be irrelevant or even adaptive, depending on the assumptions one is willing to entertain about the costs and benefits of different topographical arrangements and different degrees of lateralization. If the critical skills at a later stage of reading are phonetic (left-hemispheric), then lack of language lateralization at that stage might prove to be deleterious even though it had no adverse effect at an earlier stage.

Although it is plausible that reading, at different stages of proficiency, draws differentially upon the specialized skills of the left and right hemispheres, there is no satisfactory empirical basis for such a claim. We do not know the respective contributions of the two hemispheres to beginning reading, nor do we know the manner in which those contributions might change during the protracted process of learning to read fluently. One might speculate that the left hemisphere is critically important throughout the learning-to-read process (His- cock, Antoniuk, and Prisciak, 1982). It is clear, with respect to hemisphere-related models of dyslexia, that the significance of any lateral shifts in the brain basis of reading depends on whether or not cerebral lateralization changes during the course of maturation. If cerebral lateralization is established long before children begin to read and if it remains constant during the period over which reading skill is being developed, then difficulties in the development of reading skill cannot logically be attributed to delayed cerebral lateralization. We shall now review briefly the evidence that suggests that cerebral lateralization does, in fact, originate early in life and remain invariant throughout the period in which reading skill is acquired.

Clinical Evidence Until a few years ago, the doctrine of progressive lateralization of

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language in the normal child was accepted universally. Lenneberg (1967) was particularly influential as a proponent of the progressive lateralization hypothesis. From his own observations as well as the published cases of Basser (1962) and others, Lenneberg noted that children with unilateral cerebral damage differ from similarly damaged adults in two respects. First, when children's language is impaired, the aphasia tends to be less severe and more transitory than in adults. Second, according to Lenneberg, aphasia in children is associated with right-hemispheric damage more often than in adults. Lenneberg reasoned that language initially is represented bilaterally in the young child, and that, as the child develops, the left hemisphere assumes a gradually increasing role in language processing until, at puberty, language is represented exclusively in the left hemisphere.

Lenneberg's interpretation of the clinical data, as well as the adequacy of the data base itself, have been challenged in recent years (Kinsbourne and Hiscock, 1977; Krashen, 1973; Woods and Teuber, 1978). Krashen (1973) pointed out that Lenneberg misinterpreted the data with respect to the age at which lateralization appears to be complete; the data suggest that full lateralization of language is achieved by five years of age rather than thirteen. Kinsbourne and Hiscock (1977) argued that data cited by Lenneberg are subject to distortion from several factors, which include: selective reporting of exceptional cases by clinical investigators; inclusion of children whose brain organization has been altered by previous pathology; inadequate criteria for localizing the lesion; and inadequate criteria for inferring the presence of aphasia. It was shown in an independent clinical sample that, as the criteria for aphasia and for laterality of the lesion are made more stringent, the number of cases with right-sided lesions decreases markedly (Kinsbourne and Hiscock, 1977). Woods and Teuber (1978) reported a series of well-documented cases of childhood aphasia in which there was a striking preponderance of left- hemispheric damage. After a comprehensive review of the literature dealing with childhood aphasia, Woods and Teuber (1978) concluded that the incidence of aphasia after right-hemispheric injury, even in the youngest children, is similar to that reported for adults.

Other Evidence for Developmental Invariance of Lateralization Other data favoring the developmental invariance hypothesis can

be divided into two categories, i.e., those data that suggest early hemisphere specialization and those that show no change in degree of

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asymmetry with increasing age. As we have discussed both categories of evidence in considerable detail elsewhere (Hiscock and Kinsbourne, 1980c; Kinsbourne and Hiscock, 1977, 1978, 1981), we shall describe here only some representative findings.

Anatomical asymmetries in the infant brain resemble those in the adult brain. Of particular interest is the finding that the temporal speech region of the infant usually is larger on the left side than on the right (Wada, Clark, and Hamm, 1975; Witelson and Pallie, 1973).

In infants, as in older children and adults, there are stimulus- dependent asymmetries of electrical activity recorded from the scalp. Scalp potentials evoked by speech stimuli tend to be greater in ampli- tude over the left hemisphere than over the right, but the potentials evoked by nonverbal stimuli such as music and noise tend to be greater over the right hemisphere than over the left (e.g., Molfese, 1973). Spontaneous EEG also may show stimulus-related asymmetries (Gar- diner and Walter, 1977). Whereas the meaning of electrophysiological asymmetries is not at all clear (cf., Davis and Wada, 1977), there can be little doubt that the left and right sides of the neonatal brain, under some circumstances, show differential levels of response.

Infants show asymmetries of posture and head turning that suggest an early prepotency of the left brain (e.g., GeseU, 1938; Liederman and Kinsbourne, 1980; Turkewitz, Gordon, and Birch, 1965). The rightward turning tendency of infants may constitute a precursor of left lateralized speech (Kinsbourne and Lempert, 1979).

Contrary to common belief, a preference for the right hand as well as superior right-hand skill can be shown very early in life (Caplan and Kinsbourne, 1976; Hawn and Harris, 1979; Petrie and Peters, 1980; Ramsay, 1979). A hand difference in grasp duration has been demonstrated in children only seventeen days old (Petrie and Peters, 1980).

Infants show perceptual asymmetries that resemble those observed in older children and adults. Using measures of response habituation, investigators have demonstrated a right-ear advantage in infants for detection of transitions from one speech sound to another and left-ear advantage for detection of transitions in musical stimuli (Entus, 1977; Glanville, Best, and Levenson, 1977). In a study of premature infants, it was shown that exposure to speech, but not exposure to music, has a disproportionate effect on tremors of the right limbs (Segalowitz and Chapman, 1980).

In view of the evidence suggesting early functional differentiation of the left and right brain, there is little justification for retaining

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Lenneberg's (1967) progressive lateralization hypothesis in its entirety. One must at least question the claimed bilaterality of language representation at the beginning of language development. One might argue, nevertheless, that lateralization in the infant is quantitatively less than that in the older child or in the adult. Some of our own work has addressed this question, and we shall summarize the results.

Investigators have used dichotic listening data to argue that the degree of language lateralizafion increases during childhood (Bryden and Allard, 1978; Satz, Bakker, Teunissen, Goebel, and Van der Vlugt, 1975). In our own dichotic studies, however, we find no sign of a developmental increase between the ages of three and twelve years in the magnitude of the REA for digit names (Hiscock and Kinsbourne, 1977, 1980a; Kinsbourne and Hiscock, 1977). On the contrary, we repeatedly have found an unambiguous REA in three- and four-year- old children, the magnitude of which equals or exceeds that found in older children. In one study of three year-olds (Kinsbourne, Hotch, and Sessions, described in Kinsbourne and Hiscock, 1977), the average right-ear score on a selective listening task was 67 percent, as compared to the left-ear average of 26 percent. In another selective listening study (Hiscock and Kinsbourne, 1977), three- and four-year-olds reported 50 percent of right-ear signals but only 32 percent of left-ear signals. Presumably, these large asymmetries are attributable to our efforts to adjust the difficulty of the task for the limited capacity of preschool children. When preschool children were administered standard dichotic listening tasks along with older children (Hiscock and Kinsbourne, 1980a; Kinsbourne and Hiscock, 1977), the REA of the preschoolers was comparable to that of the other children. Thus, whereas we would not argue that the magnitude of the REA universally decreases with increasing age, we would cite our findings as strong evidence against claims that the REA increases developmentally. Several other studies--including one longitudinal study (Bakker, Hoefkens, and Van der Vlugt, 1979)---support our findings of a constant degree of listening asymmetry across the childhood years (see Witelson, 1977). If the REA increased with increasing age, it would not be clear whether that developmental change reflected a cor- responding change in the degree of language lateralization or some other factor. However, since the REA does not increase, dichotic listening data provide no basis for arguing that cerebral lateralization increases developmentally.

The dual-task procedure, as described previously, also has

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yielded a constant degree of asymmetry across the age range of three to twelve years (Hiscock and Kinsbourne, 1978, 1980b; White and Kins- bourne, 1980). The overall effect of speaking upon concurrent finger tapping decreases with increasing age, but we usually find no age- related changes in the degree to which interference is lateralized.

Even though dichotic and dual-task asymmetries are measurable in young children and do not grow more marked with increasing age, developmental patterns for other laterality tasks are less clear-cut. For instance, some tachistoscopic studies have demonstrated a right visual half-field advantage for words in children as young as seven years (e.g., Marcel and Rajan, 1975; Marcel, Katz, and Smith, 1974; Olson, 1973) but others have failed to show such an effect in children younger than ten to twelve years (e.g., Forgays, 1953; Miller and Turner, 1973; Reitsma, 1975; Tomlinson-Keasey, Kelly, and Burton, 1978). With tests of face recognition (Carey, 1979) and Braille reading (Rudel et al., 1974; Rudel, Denckla, and Hirsch, 1977; Wagner and Harris, 1979), no significant degree of asymmetry is found in young children. Asymmetries appear with increasing age and then level off. A similar pattern is found for females in a test of dichhaptic shape recognition (Witelson, 1976). In contrast, it has been reported that the degree of asymmetry on a mirror-image face task decreased between the age of five years and adulthood (Roszkowski, Snelbecker, and Sacks, 1979). Until we gain a more complete understanding of the parameters that influence laterality in these various tasks, differing developmental patterns will remain difficult to interpret. Nevertheless, one would not expect that, in a brain lateralized from the start for processing speech sounds, other functions would become lateralized only after several years of further maturation. It seems more likely that certain complex tasks are performed in different ways as children grow older. If the performance of the younger child is symmetric, that symmetry need not be attributed to bilateral representation of component skills that later will become unilaterally represented. The lateralized components necessary for adult-like performance may not yet have become functional or they may not yet have become intergrated into the organization of the performance. In other words, the younger child may rely on non- lateralized component skills as a temporary means of performing a task that subsequently will be performed by lateralized components when the lateralized components develop. The brain basis for the components does not shift with maturation, but skilled performance in the

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older child is based on a different set of component skills than is beginning performance in the younger child. This re-organization of the skill appears similar to the re-organization that seems to differ- entiate skilled adult performers from unskilled adult performers of a task (e.g., Bever and Chiarello, 1974; P. R. Johnson, 1977; Papcun, Krashen, Terbeek, Remington, and Harshman, 1974; Van Lancker and Fromkin, 1973). A shift from symmetric to asymmetric performance thus may mark increasing cognitive development without implying any lateral shifting of processors within the brain. In any event, such an argument is academic with respect to developmental changes in laterality until such a time as clear and consistent developmental changes can be documented.

Summary

When hemisphere-related concepts of dyslexia are viewed in light of current knowledge about the lateralization of higher mental functions in the human brain, some major problems and constraints become evident.

At the outset, the dyslexia theorist and dyslexia researcher con- front two discouraging facts. First, there is a remarkable paucity of confirmed findings from scientific research. The data base is inadequate, not only with respect to laterality and its significance in dyslexic children, but also with respect to the more fundamental issues of laterality and its relation to hemispheric specialization and cognitive function in children and adults who read normally. Second, there is almost no limit to the number of hemisphere-related models of dyslexia that may be generated. The traditional assertion that dyslexic children have a defect of cerebral dominance is ambiguous. Yet, the potential number of more specific models is so great as to preclude exhaustive hypothesis-testing.

Early attempts to link dyslexia and defective cerebral dominance were based on the concept of dyslexia as a unitary disorder. Now that dyslexia is regarded as a general label for a variety of manifestations of reading disability, it is not clear which subtype or subtypes of dyslexic child should show a particular abnormality of cerebral lateralization.

Additional problems are encountered when investigators attempt to use individual differences in laterality (sidedness) as indices of

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cerebral lateralization for speech. Even if methodological problems are disregarded, and different laterality dimensions are assessed on conceptual grounds alone, it is clear that some dimensions of laterality bear little or no relationship to speech lateralization in the brain. Other measures of laterality, such as perceptual asymmetries, are related theoretically to cerebral lateralization but the mechanism through which the relationship is mediated remains a matter of dispute. In many instances, it can be shown that perceptual asymmetries reflect attentional biases rather than a fixed, structural attribute of the nervous system. In any event, measures of behavioral laterality mis- classify such a high proportion of normal people with respect to language representation as to render them almost useless for predicting the manner in which functions are lateralized in an individual child.

Finally, recent evidence of two kinds has major implications for hemisphere-related concepts of dyslexia. First, it appears that left- handers in the general population are as intelligent as right-handers. Since a substantial minority of left-handers has speech represented within the brain in an anomalous fashion, the absence of cognitive deficit among these left-handers suggests that topographical variations in cerebral lateralization have no functional significance. Second, numerous sources of evidence suggest that cerebral lateralization for language does not develop gradually during childhood as was previously believed. On the contrary, the left and right brains of infants seem to be specialized differentially very early in life. If cerebral lateralization is invariant throughout childhood, the putative lack of lateralization in dyslexic children cannot be attributed to a developmental lag. Rather, the dyslexic child presumably would be born without normal lateralization of functions, and the deviant pattern of lateralization presumably would persist throughout his or her life.

It must be concluded that, despite the current popularity of hemisphere-related explanations for dyslexia, there are strong logical and empirical arguments contrary to those explanations. Several decades have passed since it was first proposed that some forms of dyslexia stem from irregularities of cerebral lateralization, but the thesis remains unproven.

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Laterality and dyslexia: A critical view