Individual Differences in the Acquisition of Written Language:an integrated model and its implications for dyslexia

Abstract

The acquisition of written language competence is examined in a context of developmental, neurological and educational psychology. It is hypothesised that certain orthographic systems contain elements which demand a matching set of features in the learner. When these features are not present, difficulties arise which are described by many specialists as Dyslexia. The clinical features of such specific written language difficulties are described, and psychometric assessment and diagnostic procedures are presented. Previous research and theories relating to Dyslexia are reviewed. Many of the differing approaches examined, however, do not appear mutually exclusive, and this thesis presents an integrated model encompassing these theories. Dyslexic difficulties are seen to arise from individual differences in perceptual, motor and cognitive development. It is further hypothesised that these individual differences result from the various probabilities of underlying cortical processing in the brain. Perceptual, motor and cognitive skills are examined in the context of written language itself, i.e. the task demands of an orthographic system. Research in each of the above areas, viz: Brain Function, Perceptual Skills, Cognitive Skills and Written Language is reviewed. Original research under each of these headings has been carried out, and the findings reported. The research into ‘Brain Function' explores the probability of cortical-hemisphere differences in the processing of verbal/symbolic material, and their relationship to the neuropsychology of language. Dichotic listening experiments and laterality studies are reported and provide further evidence for a cortical organisation not favouring the easy acquisition of symbolic and serial events. The ‘Cognitive Skills' section examines the factors underlying "general intelligence." The sub-test scores from a standard intelligence test have been examined, and profiles obtained for dyslexics and non-dyslexics. It emerged that the dyslexics had good spatial, as opposed to poor sequential abilities, providing evidence for a typical 'scatter' of subtest scores. The ‘Perceptual Skills' section reviews types of perceptual deficit amongst dyslexics. An analysis is made of the psycho-linguistic abilities needed for acquiring written language competence. Sub-tests have been devised to measure these abilities in children. Predictive and concurrent validity studies have been undertaken, providing evidence for poor performance in these abilities amongst dyslexics, e.g. difficulties in short-term memory for sound and symbol sequences, sound blending, etc. The written language itself is analysed in linguistic and psycholinguistic terms; attention being drawn to the arbitrary, sequential, directional and symbolic aspects of our alphabetic script. Experimental studies on reading errors, letter perception, etc. have been carried out and provide evidence for specific difficulties in dyslexics for the acquisition of many of the distinctive features of written language. The results from the studies provide support for a model of written language learning. It would appear that the intrinsic features of the dyslexic child-learner are not compatible with the task demands of an alphabetic, phonetic orthography. A learning style favouring spatial, as opposed to symbolic ordering and serial skills, is postulated. Dyslexia is thus regarded as an individual difference in the development of perceptual and cognitive skills. These in turn result from underlying differences in brain function. This individual difference then "interacts" with the task demands of written orthography, resulting in a situation unfavourable to the early acquisition of reading, spelling and writing fluency in the dyslexic. The theoretical and "applied" implications of such a model are discussed.