INDIVIDUAL DIFFERENCES IN DYSLEXIA

INDIVIDUAL DIFFERENCES IN DYSLEXIAMargaret J. Snowling & Yvonne M. Griffiths1ABSTRACTWith the phonological deficit hypothesis of dyslexia as a back-drop, this reviewdiscusses the issue of how indivdual differences in its behaviouralmanifestation should be conceptualised. It begins by reviewing ways ofclassifying children with dyslexia from a clinical perspective and proceeds todescribe the cognitive neuropsychological approach to classification that hasfocused on the reading and spelling profiles of such children. It argues thatchildren’s reading difficulties should be couched within the framework of typicalreading development and that sub-typing systems that have not acknowledgeddevelopmental data have limitations. An interactive model of learning to read isused to propose that individual differences in dyslexia depend upon the severityof the phonological processing difficulties experienced by an individual child,the proficiency of other cognitive skills and the environment in which they learn.

Key wordsDyslexiasubtypesindividual differences phonological skillsphonological dyslexia surface dyslexia

1 AcknowledgementsThis chapter was prepared with support from Wellcome Trust grant 040195/Z/93/A toMargaret Snowling and a Dyslexia Institute/University of York studentship to Yvonne Griffiths.IntroductionAlthough the first case of unexpected reading difficulty in childhood wasdescribed over a century ago (Morgan, 1896), the definition of dyslexiacontinues to be debated (Lyon, 1995; Stanovich, 1994; Tonnessen, 1997).Dyslexia is commonly defined as a specific difficulty in learning to read, despitenormal IQ and adequate educational opportunity. It is a disorder ofdevelopment that primarily affects the acquisition of literacy and the most widelyaccepted view is that it lies on the continuum of language disorder. Thus,dyslexia is characterised by verbal processing deficits (Vellutino, 1979) and aspecific theory that will be considered here views impairments in phonologicalprocessing to be at its core (Brady & Shankweiler, 1991; Morton & Frith, 1995;Snowling, 1995; Stanovich & Siegel, 1994; see Brady, 1997, for a review).The phonological deficit theory of dyslexia is compelling. First, it is wellgrounded in theory. The strong developmental association betweenphonological skills and learning to read forms a back-drop to much dyslexiaresearch (Goswami & Bryant, 1990; Share, 1995). The theory also makessense of the different manifestations of dyslexia across the age-span from pre-school (Scarborough, 1990) to adulthood (Pennington, van Orden, Smith,Green & Haith, 1990; Paulesu, Snowling, Gallagher, Morton, Frackowiak, & Frith1996). Second, phonological sensitivity shares heritable variance with readingskills (Olson, Wise, Conners, Rack & Fulker, 1989) and importantly, there issubstantial evidence that interventions that enhance phonological skillsfacilitate reading development in dyslexia (Snowling, 1996).The traditional view concerning the mechanisms that account for therelationship between phonology and learning to read is that children who dowell on tests of phonological awareness are quick to understand howphonemes and graphemes relate in the orthography, and to use thisknowledge of letter-to-sound rules as a self-teaching device (Share, 1995).Hence, successful reading depends upon learning explicitly how to assignsounds to letters sequentially, and to blend them to synthesise apronunciation. However, an alternative view (Ehri, 1992; Rack et al, 1992;Laing & Hulme, 1999) argues instead for a direct mapping mechanismaccording to which children at the very earliest stages of learning to read areable to set up direct mappings between orthography and phonology. Accordingto this view, children learn to read by directly mapping sequences of letters totheir pronunciations and not by relying on decoding rules (Marsh. Friedman,Welch & Desberg, 1981).From the cognitive perspective, the phonological deficit theory proposesthat delayed phonological development in dyslexia affects the development ofphonological representations that are the foundation for orthographicdevelopment (Snowling & Hulme, 1994; Metsala, 1997). In turn, awareness ofthe phonological components of spoken words is not available at a time whenthis is critical for the acquisition of reading and spelling skills (Elbro, Borstrom& Petersen, 1998; Fowler, 1991). Typical consequences are a slow rate ofliteracy development, poor generalisation of word reading skills to nonwordreading and poor spelling development (Snowling, 2000 for a review).