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This review appeared in Volume 4 (3) of The Semiotic Review of Books.

A Theory Of Dyslexia

George E MacKinnon

Neuropsychological Treatment of Dyslexia. By D.J. Bakker. 1990, Translated from Dutchby G. Spyer. pp. xiL 94. New York: Oxford University Press. ISBN 0195061322.

For well over a century now researchers have sought for an explanation of developmental dyslexia in terms of a dysfunction in neurodevelopment. Following the easy speculations of Morgan (1896) and others, in the most popular view childhood dyslexia has been attributed to developmental anomalies in the functional organization of cerebral hemispheric specialization. Several specific hypotheses about anomalous neurodevelopment that might lead to dyslexia have been proposed: for example, incomplete unilateral cerebral dominance (Orton, 1937 lag in the development of hemispheric specialization (Satz & Sparrow, 1970); failure to lateralize language to the left hemisphere (Pirozzolo & Rayner, 1979); a slowing of left hemispheric maturation due to excess prenatal testosterone (Geschwind & Galaburda, 1985a; 1985b); functional disorders in either the left or right hemisphere (Njiokiktjien, 1993); maladaptive hemispheric attentional activation (Obrzut, 1991); vulnerability to hemispheric specific attentional interference (Kershner & Stringer, 1991). While Bryden (1982, 1988) has pointed out the many pitfalls, methodological and theoretical, that await (haunt?) the unwary researcher in this field, there has been no lack of theory in what is arguably one of the most vigorously pursued topics in neuropsychology. One account, Dirk Bakker's Balance Model, provides the basis for some provocative proposals for treating developmental dyslexia. These proposals, the out growth of more than two and a half decades of research by Bakker and his colleagues at the Paedological Institute in Amsterdam-Duivendrecht, The Netherlands, are summarized in the little book (94 pages including bibliography and index) entitled Neuropsychological Treatment of Dyslexia.

While the Balance Model is essentially a neuropsychological analysis of learning to read, the model is used by Bakker for conceptualizing what happens neurologically in developmental dyslexia and for prescribing neuropsychological therapy. According to the model, normal skilled reading involves processes in both cerebral hemispheres. Each hemisphere, however, is thought to participate uniquely at successive stages in learning to read with initial learning mediated more by the fight and more advanced learning, more by the left hemisphere. Early reading activities such as perceptual learning of letter shapes and letter word discrimination are hypothesized to be primarily under right hemispheric control. Later, as perceptual analysis becomes automatic and reading involves the processing of syntactic and semantic aspects of text, control is said to shift to the "faster" linguitic left hemisphere. Thus as Bakker puts it here is a moment in the learning-to-read process at which the balance in the brain tips from right to left" (ll). Further, deviations in this normal shift in hemispheric control, Bakker contends, result in dyslexia.

Bakker distinguishes between two types of dyslexics. L-dyslexics rely on left hemispheric processes prematurely (i.e., even before perceptual analyses are automatic): as a result they are fast but inaccurate readers who make predominantly substantive errors (e.g. omissions, additions, substitutions). In contrast, P-dyslexics fail to switch from right to left hemispheric control: P-dyslexics, therefore, remain slow but accurate readers who make time consuming errors (e.g. letter and word repetitions). In each case the normal course of the normal balance in hemispheric participation (activation) has become upset. In pursuing these notions, Bakker has undertaken an impressive program of research. He has studied the responses of the brain by recording event-related-potentials (ERPs) during normal development to word stimuli, sought external validation for his dyslexia subtypes using ERPs, and developed and evaluated the efficacy of therapeutic regimens which are tailored to the L and P dyslexics' different needs. In keeping with his neuropsychological model, appropriate therapy is deemed to involve the presentation of stimulation to the purported underactivated hemisphere to reset the balance. It is these latter pursuits which occupy the bulk of Neuropsychological Treatment of Dyslexia.

The book begins with a short summary of the Balance Model, the characteristics of L and P dyslexics, and the techniques of recording of ERPs. There follows a more extended description and discussion of the results of two already published studies (Bakker et al., 1981; Bakker & Vinke, 1985) and brief summaries of several unpublished theses and reports in which Hemisphere Specific Stimulation (HSS) and Hemisphere Alluding Stimulation (HAS) were employed to treat L and P dyslexics. Hemisphere Specific Stimulation for L-dyslexics involved stimulating the right hemisphere by flashing in the left visual field concrete words (for oral reading) printed in "perceptually demanding" fonts and by transmitting to the left ear (via earphones) the trainer's instructions and the subjects' oral responses. For P-dyslexics, the left hemisphere was stimulated by flashing in the right visual field abstract words printed in a "perceptually neutral" font and transmitting words to the right ear. For Hemisphere Alluding Stimulation classroom materials and exercises were modified in an attempt to "engage" the underactivated hemisphere: for example, to activate the right hemisphere in L-dyslexics, reading materials were printed in mixed-up letter fonts so as to increase their perceptual complexity; to activate the left hemisphere in P-dyslexics, materials printed in simple typeface with no illustrations were used in rhyming exercises. After both HSS and HAS, L-dyslexics read slower and more accurately although the effects after HAS were weaker. The results with P-dyslexics were not so clear: P-dyslexics showed more reliable improvement after HAS than after HSS. With both groups, however, HSS lead to changes in ERPs in directions predicted by the model, leading Bakker to conclude that hemisphere specific stimulation indeed influenced the activity of the stimulated hemisphere. Bakker, however, provides no independent verification that hemisphere alluding stimulation in fact "engages" the alluded hemisphere.

Included in the book are three very useful chapters outlining the methods used in HSS, HAS and a more experimental tactile form of hemispheric specific stimulation involving palpation of three-dimensional letters. These chapters include material on aspects of the procedures that are not included in the original articles and thus are helpful for readers already familiar with Bakker's work. The book also includes an excellent extended bibliography, including sources (theses and unpublished reports) not cited elsewhere as well as a searchable index. Given that most of the material in the book is available elsewhere, one might well wonder why the book was published. Bakker himself states in the Preface, that the book is not to be taken as a primary source. Instead, one purpose of the book was to summarize the research undertaken at the Paedological Institute for a broad audience - interested professionals, neuropsychologists, psychologists, neurologists, psychiatrists, educators, teachers, etc. The book was originally produced in Dutch for market in Holland (as much of the research from the Institute is published only in English and typically in technical jargon) and subsequently translated into English for Oxford. While not entirely free of Technicalese, the book is clearly suitable for its intended audience, highly readable, with very good, indeed delightful, illustrations.

Bakker also intends the book to allow interested professionals to "work with (the research) therapeutically and educationally" (vii) and anticipates that "the interested clinician will be able to apply these techniques solely on the basis of the information provided here" (83). And therein lies the rub. Throughout the book Bakker quite rightly uses caution in the interpretation of the data, emphasizes the importance of appropriate controls in evaluating the treatments, poses alternative explanations for the effects of HSS and HAS, underscores the limitations of the research, and stresses the need for more refined measures to identify L and P dyslexics. Nevertheless, based on the data accumulated so far, he makes rather authoritative recommendations on how to treat L and P dyslexics, and considers the book sufficient for the interested clinician to employ the techniques. But as Hynd (1992) has cogently remarked, "A word of caution is clearly in order... These experiments should lead to studies aimed at empirically validating the theory and treatment strategies articulated by Bakker and (...) should not at present lead to direct critical applications, as gains in these studies have not been contrasted with the impact of other, more traditional, educational intervention procedures" (123). Hynd also decries the lack of validated procedures for identifying Bakker's subtypes of dyslexia.

To date, the effectiveness of hemispheric stimulation has been evaluated in terms of changes in the number and kind of oral reading miscues and overall reading speed. Such measures are appropriate given the predictions of the theory. Nevertheless, using these measures alone, it is difficult to assess the impact of hemispheric stimulation on overall level of reading skill. Additionally, much more needs to be known about what factors are at work when HSS results in gains and when it does not (see, for example, Grace and Spreen, 1990). As presently conceptualized and implemented HSS has much more in common with a medical treatment than with an educational intervention.

Indeed, studies by Bakker and others on its effectiveness have much in common with early clinical trials of a new drug, with study directed primarily to demonstrating positive pre-post treatment effects. Little emphasis has been placed on the behaviour of the child during treatment. The objective has been to activate a hemisphere, not teach a skill (e.g., word recognition, or comprehension). In this respect Bakker's work differs from much of the current more mainstream research on intervention for dyslexia (see, for example, Lovett (1993), Lovett et al. (1990)). But children undergoing HSS are exposed to words and other materials for reading (albeit at each individual's level of skill) and do receive some corrective feedback when they make errors. Thus, the responses of the children during treatment may provide a valuable source of data in evaluating treatment effectiveness. Finally, apart from the characteristics that define their dyslexic group status, we know very little about the subjects in these studies, either in terms of their ability in sound-symbol processing, or in speech and language generally. In the absence of measures of these and other individual difference processing variables, it is not possible to fully evaluate the effects of hemispheric stimulation. In sum, widespread application of the techniques at this time is clearly premature.

Apart from the material noted earlier describing the treatment techniques, for those familiar with Bakker's work there is little new here, and while there is much to admire, there is also much to argue about. For those unfamiliar with Bakker's research, the book does serve as an exceedingly well written introduction. Such readers, however, should heed Bakker's advice (83) to consult the sources he lists in the Bibliography. This reviewer would add, that to put Bakker's work in the larger context, they might do well to also read the chapters by Newby and Lyon, Harter, and Hiscock and Hiscock in Obzrut and Hynd's (1991) outstanding handbook, Neuropsychological Foundations of Learning Disabilities.


Bakker D.J, Moerland, R., & Goekoop-Hefkens, M. (1981) "Effects of hemisphere-specific stimulation on the reading performance of dyslexic boys: A pilot study." Journal of Clinical Neuropsychology, 3, 155-159.

Bakker, D.J. & Vinke, J. (1985). "Effects of hemisphere-specific stimulation on brain activity and reading in dyslexics." Journal of Clincal and Experimental Neuropsychology, 7, 505-525.

Bryden, M.P. (1982) Laterality: Functional asymmetry in the intact brain. New York Academic Press Inc.

---. (1988). "Does laterality make any difference? Thoughts on the relation between cerebral asymmetry and reading." In D.L. Molfese & S.J. Segalowitz (Eds.), Brain lateralization in children: Development implications (509-525). New York: Guilford Press.

Geshwind, N., & Galaburda, A.M. (1985a) "Cerebral lateralization: Biological mechanisms, associations, and pathology. I. A hypothesis and a program for research." Archives of Nuerology, 42, 428-459.

---. (1985b). "Cerebral lateralization: Biological mechanisms, associations, and pathology. Il. A hypothesis and a program for research." Archives of Neurology, 42, 521-552.

Grace, G.M., & Spreen, O. (1990). "Effects of hemisphere-specific stimulation (HSS) on scholastic performance and event-related potentials (ERPs) in dyslexic children." Journal of Clinical and Experimental Neuropsychology, 12(1), 101.

Hynd, G. W. (1992). "Neurological aspects of dyslexia: comment on the Balance Model." Journal of Learning Disabilities, 25(2),110-112.

Kershner, J.R., & Stringer, R.W. (1991). "Effects of reading and writing on cerebral laterality in good readers and children with dyslexia." Journal of Learning Disabilities, 24(9), 560-567.

Lovett, M. (1993). "Remedial outcome data: windows on the reading acquisition processes of dyslexic children." Paper presented to the third annual meeting of the Canadian Society For Brain, Behavior, and Cognitive Science.

Lovett, M., Warren-Chaplin, P.M., Ransby, M.J., & Borden, S.L. (1990). "Training the word recognition skills of reading disabled children: treatment and transfer effects." Journal of Educational Psychology, 82(4), 769-780.

Morgan, W.P. (1896). "A case of congenital wordblindness." British Medical Journal, 2, 1378.

Njiokiktjien, C. (1993). "Neurological arguments for a joint developmental dysphasia-dyslexia syndrome." In A.M. Galaburda (Eds.), Dyslexia and Development: Neurobiological aspects of extra-ordinary brains (205-236). Cambridge, Mass: Harvard University Press.

Obrzut, J.E. (1991). "Hemisphere activation and arousal asymmetry in learning-disabled children." In J.E. Obrzut & G.W. Hynd (Eds.), Neuropsychological foundtions of learning disabilities: A handbook of issues, methods, and practice (179-198). New York: Academic Press, Inc.

Obrzut, J.E., & Hynd, G.W. (Eds.) (1991). Neuropsychologicl foundations of learning disabilities: A handbook of issues, methods, and practice. New York: Academic Press, lnc.

Orton, S.T. (1937). Reading, writing and speech problems in children. New York: Norton.

Pirozzolo, F.J ., and Rayner, K. (1979). "Cerebral organization and reading disability." Neuropsychologia, 17, 485-491.

Satz, P., & Sparrow, S. (1970). "Specific developmental dyslexia: A theoretical formulation." In D. Bakker & P. Satz (Eds.), Specific reading disability: Advances in theory and method. Rotterdam: Rotterdam University Press.

G.E. MacKinnon is an Associate Professor in Psychology and Chairman of the Applied Psychology Programme at the University of Waterloo. His current research focuses on the cognitive and linguistic determinants of early reading and spelling, and on the implications of individual differences for models of reading skill development. A former Associate Editor of Reflections on Canadian Literacy, he co-edited the series Metacognition, Cognition and Human Performance and the series Reading Research: Advances in Theory and Practice both published by Academic Press. He is currently a contributor to the forthcoming Encyclopedia of Language and Linguistics published by Pergamon Press.

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