Medical approaches to dyslexia, such as that exemplified by the work of pioneers such as Orton and Macdonald Critchley (Critchley, 1970) fell from favor mostly
*A more detailed version of the argument in this chapter can be found in Snowling (2000). Margaret J. Snowling, Department of Psychology, University of York, Heslington, YorkY01 5DD, UK. The Study of Dyslexia, edited by Turner and Rack.
Kluwer Academic Publishers, New York, 2004. 77
because they were difficult to put into practice. The dyslexic child was described as having severe difficulty in learning to read and write, despite adequate IQ, normal sensory function, and adequate opportunity both at home and at school. It is perhaps worth taking time to consider these criteria. What is adequate IQ? What is adequate opportunity? Why should a sensory impairment rule out the possibility of dyslexia? It will be clear that many of the terms in this kind of a definition are difficult to operationalize. It is a "definition by exclusion" and does not list any of the positive signs of dyslexia.
In fairness, the medical model of dyslexia also carried with it a list of behavioral symptoms, such as the tendency to reverse letters and numbers, directional confusion, clumsiness, and delays and difficulties with language. However, it was never made clear which were the critical symptoms, how many of them needed to be observed or at what degree of severity. While checklists for dyslexia have their place, such instruments do not fare well in discriminating dyslexia from other types of learning disorder. In fact, they can be most misleading to parents whose knowledge of normal development may be poor.
A more scientific approach to dyslexia emerged in the late 1960s when one of the main issues of debate was whether "dyslexia" was different from plain poor reading. Studies of whole child populations, notably the epidemiological studies of Rutter and his colleagues, provided data about what differentiated children with specific reading problems (dyslexia) from those who were slow in reading but for whom reading was in line with general cognitive ability (Rutter & Yule, 1975). The results of these studies were salutary for proponents of dyslexia. In fact, there were relatively few differences in etiology between children with specific reading difficulty and the group they described as backward readers (often referred to in the US literature as "garden-varietypoor readers"). The group differences that were found included a higher preponderance of males among children with specific reading difficulties, and more specific delays and difficulties with speech and language development. On the other side of the coin, the generally backward group showed more hard signs of brain damage, for example, cerebral palsy and epilepsy.
Importantly, the two groups differed in the progress they had made at a 2-year follow-up. Contrary to what might have been expected on the basis of their IQ, the children with specific reading difficulties (who had higher IQ) made less progress in reading than the generally backward readers. This finding suggested that their problems were intransigent, perhaps because of some rather specific cognitive deficit, at the time unspecified. It is perhaps worth noting here, that this differential progress rate has not been replicated in more recent studies (Shaywitz et al., 1992), perhaps because advances in knowledge have led to better focused remedial approaches.
Following on from these large-scale studies, the use of the term "dyslexia" became something of a taboo in UK schools. Rather, children were described as having specific reading difficulties (SRD) or specific learning disability (SpLD) if there was a discrepancy between their expected attainment in reading, as predicted by their age and IQ, and their actual reading attainment. The use of IQ as part of the definition has attracted a lot of criticism. First, IQ is not strongly related to reading. Indeed, many children with low IQ can read perfectly well even though they may encounter reading comprehension difficulties. Second, and perhaps most important, there is suggestive evidence that verbal IQ may decline over time among poor readers. To some extent, this decline might be as a consequence of limited access to knowledge in books. However, there are other reasons too. Difficulties in retrieving verbal information and problems of verbal short-term memory can influence test performance, as can low self-esteem. Whatever the reason, the discrepancy definition of dyslexia may disadvantage those children with the most severe problems whose apparently low verbal IQ may obscure the "specificity" of the reading problem.
Another problem with the discrepancy definition of dyslexia is that it does not work well for younger children, who may as yet not have failed sufficiently to fulfill its criteria! Indeed there is a considerable lack of stability at the individual level in children who obtain the classification during their early school years. Moreover, the definition is silent with regard to at-risk signs of dyslexia, and to how to diagnose dyslexia in young people who may have overcome basic literacy difficulties. This limitation relates to the same problem that was mentioned above, the lack of positive diagnostic criteria for dyslexia.
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