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This article appeared in Volume 2 (2) of The Semiotic Review of Books.
From Reading to Neurons. Edited by Albert M. Galaburda. Cambridge, MA: MIT Press, 1989. Pp. 545 & xxii. ISBN 0-262-107115-0
Two major publications in the middle 1980's held out great promise for understanding the biological basis of reading disability. The first was the publication by the late Norman Geschwind and Albert Galaburda of an impressive speculative theory concerning the biology of cerebral lateralization (Geschwind & Galaburda, 1985 a, b, c), a series of papers that subsequently appeared as a book (Geschwind 8 Galaburda, 1987). Geschwind and Galaburda reviewed literature from a diversity of sources to argue that alterations in fetal testosterone affected both the development of the left cerebral hemisphere and the development of the immune system. The alterations of cerebral development resulted in deviations from what they termed the standard pattern of cerebral lateralization in which language is lateralized to the left hemisphere, visuospatial function to the right hemisphere, and the right hand is the preferred and more skilled hand. The immune system, on the other hand, was altered by the effects of testosterone on the thymus. These joint influences of fetal testosterone led to associations between handedness, reading disability, and disorders of the immune system and of neural crest development. Geschwind and Behan (1982), for example, reported that lefthanders were more likely to suffer from various autoimmune disorders, and also were more likely to have manifested some form of developmental language disability. Admittedly, some recent research has failed to verify certain of the associations claimed by Geschwind and Galaburda (1987)-such as the relation between handedness and myasthenia gravis (McManus, Naylor, & Booker, 1990). Nevertheless, there remain numerous studies suggesting that there are meaningful biological correlates of reading disability, such as that of Burke, Yeo, Vranes, Garry, & Goodwin (1988) relating immune disorders to language dysfunction and that of Dlugosz, Byers, Msall, Marshall, Lesswing, & Cooke (1988) showing a relation between language disability and congenital limb malformation.
During the same period, Albert Galaburda was studying the brains of dyslexic individuals. Whereas most people show anatomical asymmetries such that the left temporal planum is larger than the right, Galaburda found that the two plana were of approximately equal size in four consecutive dyslexic brains (Galaburda, Sherman, Rosen, Aboltiz, & Geschwind, 1985). Given a generous estimate that 20% of normal brains are symmetric, the chances of finding 4 consecutive symmetric brains are less than one in 500, strongly suggesting that there is something unique about the dyslexic brain.
From Reading to Neurons is a volume stimulated by these lines of research. The volume exemplifies the bouillabaisse approach: the selection of contributors is based on the premise that, if one collects together everything that might possibly be related to the topic, throws it all into one big pot, and stirs, something interesting might emerge. Given this starting point, Galaburda has brought together a collection of papers dealing very broadly with topics that might possibly be of relevance to an understanding of the biological experiential, and cognitive mechanisms underlying developmental dyslexia. Given our present state of knowledge, the connections of some of the chapters to reading disability are not immediately obvious, but the twenty contributions provide a stimulating and rich overview of a diverse set of topics that may very well provide a basis for understanding reading disability. Some very cogent interpolations by the editor serve to make the links to reading more readily apparent. The reader who pauses to think about the diverse contributions will almost certainly find material worth pondering.
Broadly speaking, the contributors take two different approaches to the problem. One group of papers concerns the development of language in general, reviewing current views on the ways in which language development can alter processes, especially in special populations such as the deaf or the illiterate. A second group of contributions is concerned primarily with biological development in the brain, and the way in which experience can alter neural pathways and neural functioning. Not all the chapters fall neatly into these categories, but the classification provides a general picture.
Given the initial premise that developmental dyslexia has its origins in abnormal brain function, it is logical to argue that one should look at other groups of individuals whose reading behaviour is decidedly abnormal. For instance, many people in this world never do learn to read, and therefore insights might be obtained from those who are illiterate. Similarly, the congenitally deaf rarely become highly skilled readers, and some people become dyslexic in adulthood through brain damage; we should look for clues about potential problems in reading from those who are born deaf and from those who are acquired dyslexics. One might also expect to discover important things about reading by investigating the development of spoken language, aural language comprehension, the relation of orthography to the development of reading skills, the impact of different teaching methods, and even the significance of such factors as the gender of the primary school teachers (cf. Chall, 1967; Tallal, 1980; Frith, 1985; Goswami & Bryant, 1990). The majority of these topics are touched on, either directly or indirectly, by various contributors in the early chapters of the book.
The volume begins with two chapters, by Bertelson and DeGelder and by Lecours, on the relevance of literacy to the understanding of reading. Bertelson and DeGelder point out that illiterates cannot carry out the same phonological manipulations of spoken material as can literates, and suggest that the study of speech perception in the illiterate can provide some interesting clues about the role of phonology in reading. Lecours, studying the effects of unilateral brain damage in illiterates, finds problems in naming following right hemisphere damage that are not seen in literate patients. He therefore concludes that "left cerebral dominance for language can be less exclusive among illiterates than among school educated individuals" (p. 36). Such a position reinforces the view that language lateralization is not entirely pre-wired, nor altered only by gross insult to the brain, but can be modified by what would be considered to be relatively benign experiences.
The second two chapters are, in effect, a debate between John Morton and John Marshall as to whether one can learn anything about development dyslexia from the study of acquired dyslexias, with Morton arguing that one must first understand the development of reading, and Marshall defending the view that the various forms of acquired dyslexia find their counterparts in developmental literature. Marshall takes the position that developmental stages are additive rather than interactive, and therefore that one might expect adult damage to some portion of the brain to mimic effects seen in development. Continuing with the behavioural perspective, there are treatments of initial language learning by Gleitman, Gleitman, Landau, and Wanner; of the relevance of acquiring sign language by Bellugi, Tzeng, Klima, and Fok; and of early sensory and linguistic capabilities by Krasnegor and by Mehier. Each of these chapters show how the development of language and reading might be influenced by the initial sensory and cognitive capacities of the child.
The currently popular parallel distributed processing (PDP) models of cognitive activity are defended in principle by Churchland and Sejnowski. At a more empirical level, Seidenberg and McClelland provide an overview of a parallel distributed processing model of word recognition, and attempt to show that it can serve as a model of both normal language acquisition and of developmental dyslexia. The Seidenberg and McClelland chapter, in particular, shows how the PDP approach can provide detailed predictions about the types of errors to be seen in dyslexics, if one is willing to accept the basic assumptions of the model. While many investigators are not enthusiastic about PDP models, arguing that they neither oversimplify very complex problems or build on the very assumptions they wish to test, recent attempts to apply PDP models to acquired dyslexia have been at least reasonably successful in predicting the errors made by different types of dyslexics (Hinton & Shallice, 1991).
If one can obtain insights about reading by examining groups of people who have special problems in reading, one might also expect to find clues by looking at the demography of developmental dyslexia and studying those individuals most at risk. Geschwind and Behan (1982), for example, have claimed that dyslexics are more likely than normal readers to be lefthanded, male, and to suffer from various autoimmune disorders. Hugdahl, Synnevag, and Satz (1990) have recently confirmed the relation between dyslexia and immune disorders, although they failed to find any evidence of an elevated incidence of left-handedness. Likewise, Burke, Yea, Vranes, Garry, and Goodwin (1988) have reported that the occurrence of developmental language disorders is significantly related to both the incidence of immune disorders and to handedness, although handedness and immune disorders are unrelated in their sample. Although the Geschwind and Galaburda (1987) model is undoubtedly not correct in its details, there are evidently interactions between the immune system and the developmental processes involved in reading that warrant further research.
In the present volume, Hanna, Damasio, Tranel, Spradling, and Alliger examine the evidence for differences between men and women in the incidence of various types of aphasia, concluding that men and women develop similar deficits in speech and language if lesions are in the same location, although women are more likely to have severe deficits in fluency and articulation, while men are more likely to show deficits in reading comprehension, aural comprehension, and sentence repetition. In a stimulating commentary on this chapter, Galaburda offers an innovative model of neural organization in which he suggest that language functions are less asymmetrical and more bilaterally interconnected in the male brain than in the female brain, but that the male brain is less efficient. Of course, the recent data of Shaywik, Shaywik, Fletcher and Escobar (1990), suggesting that sex differences in developmental dyslexia are the result of applying different criteria to boys and girls, may make it unnecessary to incorporate a sex-related factor into a model of dyslexia.
Given the Hugdahl et al. (1990) data, handedness may not turn out to be a critical variable in developmental dyslexia. Nevertheless, Bever, Carrithers, Cowart, and Townsend report differences in linguistic performance between right-handers with a familial history of sinistrality and those without. Like many others, Bever et al. have accepted the view that there is a genetic component to handedness and cerebral organization without thinking through the ramifications of such a position. Handedness data, for instance, indicate only a modest parent-child association (Annett, 1985), and most successful contemporary models see random handedness, not lefthandedness as the alternative to righthandedness. Thus, identifying a right-handed person as having a left-handed parent or sibling is not of much use in predicting anything about cerebral organization (Bishop 1990). It has been estimated that the presence of familial sinistrality in a right-hander increases the likelihood of anomalous language lateralization from about 4% to about 12%. Such an effect would require about 80 subjects in each of the familial and nonfamilial groups to detect; a far larger sample size than most researchers are wont to use.
A chapter by Antonio Damasio on visual agnosia and visual recognition focusses primarily on disorders of face recognition; while this contribution provides an interesting conceptualization of object and face recognition, tying in well with the chapters on PDP models, the application of the Damasio model to problems of reading remains vague.
In many ways, the core of the book is represented by two chapters by Galaburda and his coworkers. In one, Galaburda, Rosen and Sherman review the literature indicating greater anatomical symmetry and microdysgenesis in the brains of dyslexics. In the other, Sherman, Rosen, and Galaburda show how animal models of brain lateralization can be of value. They show that the anatomically asymmetric brain is one in which one hemisphere is abnormally small, while the symmetric (and presumably abnormal) brain is one in which both hemispheres are large. Thus, they argue, the symmetry seen in dyslexic brains is the consequence of abnormal growth of the right hemisphere, rather than an abnormal loss of cortical mass in the left hemisphere. Of course, to establish the case, we need stronger evidence for a direct relationship between asymmetries of cortical anatomy and behaviour.
The book concludes with several chapters showing ways in which abnormalities of development and experience can lead to modifications of brain activity at various levels. Thus, Caviness, Misson, and Gadisseux review disorders of neural development; Rakic deals with the interaction of neurons during development; McEwen discusses the role of hormones; Crossin and Edelman discuss the role of cell adhesion molecules in embryological development; and Dudai shows how an examination of learning and memory at the very molecular level can lead to an understanding of the basic neurobiologic mechanisms.
This is a genuinely multidisciplinary book -- contributors represent a broad array of different approaches to contemporary neuroscience. While the immediate application of many of the ideas to the understanding of dyslexia is not always apparent, the intercalated commentaries by Galaburda provide an excellent linking of the diverse chapters to the central theme of the book. At one level, a book such as this is of value not so much because it provides the expert on a particular topic with new information or a new theoretical view, but because it brings together a diversity of different approaches to a general problem. It thus serves to suggest new ways of looking at old problems, and to show how both molar and molecular approaches to a particular neuropsychological problem can be of value. As such, it is well worth a careful perusal by those interested in the acquisition of reading skills and how they can fail to develop. Furthermore, the book will serve to acquaint the interested generalist with the diversity of different approaches to reading disability.
In the final analysis, however, it is open to question as to whether such a multi-level approach is the best way to examine the problems of reading. While neuroanatomical studies such as those of Galaburda may very well help us to understand the neural substrate of reading, and careful analysis of the reading process in both normal and abnormal children is certainly necessary, it is less clear how one can or should move from the specific points made in the volume to a realization of how one can alleviate the problems of developmental dyslexia. The next few years will certainly see numerous experimental studies building on the ideas presented in this volume: whether or not they will provide significant advances in our understanding of developmental dyslexia remains to be seen.
Annett, M. (1985). Left, right, hand, brain. London: Lawrence Erlbaum.
Bishop, D.V.M. (1990). "On the futility of using familial sinistrality to subclassify handedness groups." Cortex, 26 , 153-155.
Burke, H.L., Yeo, R.A., Vranes, L, Garry, P.J., & Goodwin, J.S. (1988). "Handedness, developmental disorders, and in vivo and in vitro measurements of immune responses." Developmental Neuropsychology, 4,103-115.
Chall, J. (1987). Learning to read: The great debate. New York: McGraw-Hill.
Dlugosz, LJ., Byers, T.E, Msall, M.E., Marshall, J., Lesswing. A., & Cooke, R.E. (1988). "Relationships between laterality of congenltal upper limb reduction defects and school performance." Clinical Pediatrics, 27, 319-324.
Frith, U. (1985). "Beneath the surface of developmental dyslexia." In KE Patterson, J.C. Marshall, & M. Coltheart (Eds.), Surface dyslexia. London: Lawrence Erlbaum Associates.
Galaburda, A.M., Sherman, G.F., Rosen, G.D., Aboltlz, F., & Geschwind, N. (1985). "Developmental dyslexia: Four consesutive patients with cortical anomalies." Annals of Neurology, 18, 222-223.
Geschwind, N., & Behan, P. (1982). "Left-handedness: Association with immune disease. migraine, and developmental learning disorder." Proceedings of the National Academy of Sciences (USA), 79, 5097-5100.
Geschwind, N., & Galaburda, A.M. (1985b). "Cerebral lateralization: Biological mechanisms, association, and pathology. II." Archives of Neurology, 42, 521-556.
Geschwind, N., & Galaburda, A.M. (1985a)." Cerebral lateralization: Biological mechanisms, association, and pathology. II." Archives of Neurology, 42, 634-654.
Geschwind, N., & Galaburda, A.M. (1987). Cerebral lateralization. Cambridge, MA: MIT Press.
Goswami, U., & Bryant, P. (1990). Phonological skills and Learning to read. London: Lawrence Erlbaum Associates.
Hinton, G.E., & Shallice, T. (1991). "Lesioning an attractor network: investigations of acquired dyslexia." Psychological Review, 98, 74-95.
Hugdahl, K., Synnevag, B., & Satz, P. (1990). "Immune and autoimmune dyslexic children." Neuropsychologia, 28, 673- 679.
McManus, I.C., Naylor, J., & Booker B.L. (1990). "Lefthandedness and myasthenia gravis." Neuropsychologia, 28, 947-955.
Shaywitz, S.E., Shaywitz, B.A., Fletcher, J.M., & Escobar, M.D. (1990). "Prevalence of reading disability in boys and girls." Journal of the American Medical Association, 264, 998- 1002.
Tallal, P. (1980). "Auditory temporal perception, phonics, and reading disabilities in children." Brain and Language, 9, 182- 198.
M.P. Bryden is Professor of Psychology at the University of Waterloo. His interests are in the differing functions of the two cerebral hemispheres and how these differences relate to normal behaviour. He has written extensively on perceptual asymmetries and handedness, and their relation to cerebral function.