Cognitive impairments in developmental dyslexia
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The nature of cognitive impairments in developmental dyslexia was investigated in two studies. The issue of heterogeneity was addressed and an attempt was made to identify cognitive processes which might feature in a dimensional model of reading ability. The first study examined the hypothesis that developmental dyslexics are delayed in their general perceptual development. Phonological, visual and tactile segmentation skills were assessed together with nonword naming ability. As a group, the dyslexics were only impaired for reading age at phoneme deletion and nonword naming. However, individual variation was present within the dyslexic group. Individuals exhibiting severe impairments were identified in tests of rhyme judgement, auditory organisation and visual segmentation. The perceptual delay hypothesis received only limited confirmation. The dyslexic group was impaired at the most analytical level of phonological segmentation, but not at more holistic levels. Visual and phonological segmentation skills showed some association, but were dissociated from performance in the tactile modality. The second study further explored these findings using a new sample. This dyslexic group also suffered nonword naming impairments for reading age. However, like their reading age controls, they showed a processing advantage for onset and rime units in a phonological deletion task but not in an orthographic lexical decision task. The efficacy of long-term memory representations was assessed. Individuals within the dyslexic group displayed a very deviant performance in a repetition memory task, and the group as a whole was impaired at recognising words to which they had been repeatedly exposed. It was concluded that the difficulties experienced by individual dyslexic children were varied, and that less frequent problems were likely to be overlooked by assessing impairment in developmental dyslexia in terms of group performance. Further investigation of how visual and memory processes relate to reading development would be a worthwhile addition to the extensive work linking phonological processing with reading achievement.
Thesis, PhD Doctor of Philosophy
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