A systematic comparison of the developmental and acquired dyslexias

Traditional approaches to the investigation of the developmental dyslexias have involved cognitive or peripheral impairments which are correlated with the reading disorders. These studies have examined the average performance of groups of developmental dyslexics. By contrast, recent studies of the acquired dyslexias have analysed the pattern and nature of the reading deficits themselves. The variation between patients has necessitated the employment of a single case study methodology.
The objective of this thesis is to investigate the developmental dyslexias in the same manner as these recent studies of acquired dyslexia. That is, detailed case studies of the natures of the reading disorders of children with developmental dyslexia are presented. These case reports include investigation of the ability to read non-words; qualitative and quantitative analysis of reading errors; investigation of the effects of different linguistic dimensions upon reading performance; investigation of spelling in relation to the same variables. The experimental materials involved in these investigations include a number of newly constructed tasks. The error analysis, although based on the categories used with the acquired dyslexias, newly introduces the category of 'valid' errors.
Two cases of developmental surface dyslexia are described. One is analogous to classical cases of acquired surface dyslexia, the other to the 'pure' case recently reported by Bub et al (1984). The reading of both children is significantly influenced by spelling-to-sound regularity. Reading errors are predominantly neologistic and many are valid. There is no significant incidence of semantic, derivational, visuo-semantic, or visual+semantic errors. There is homophone confusion. Spelling is phonological. Performance on rhyme tasks is good. In the discussion of surface dyslexia, an expanded representation of the phonological route is incorporated into the reading model. The system proposed operates upon orthographic units of varying size and selects from a number of potential translations. Cases of surface dyslexia are interpreted in terms of the model.The origination of 'visual' errors in surface dyslexia is also discussed.
Four cases of developmental phonological dyslexia are described. The reading performance of these children differs significantly from surface dyslexia. Non-word reading is impaired in comparison to word reading. Reading errors are predominantly paralexias : visual, derivational, or visuo-semantic. Spelling-to-sound regularity does not significantly influence reading performance. Function word reading is impaired in isolation and/or in text. Reading of reversed typed words is impaired and performance on rhyming tasks is poor. The mode of reading acquisition for these children and the limitations of the resultant skills are discussed. Spelling patterns differ between cases and indicate that the phonological route for spelling may develop despite impairment of the phonological route for reading.
The existence of these two distinct subgroups of developmental dyslexia indicates that semantic and phonological routes to reading may develop in relative isolation from each other. The appropriate remedial strategies for the subgroups are discussed. Researchers that use a group study approach to the developmental dyslexias should accordingly be aware that averaging across subgroups may render the data uninterpretable.
Finally, two cases of children who read without phonology are presented. They are unable to pronounce non-words. Most words read correctly are of high frequency and imageability. Individual letter transcoding and function word reading are impaired. Visual, derivational and semantic errors are in evidence. The reading of these children resembles acquired deep dyslexia although the incidence of 'Semantic errors is very low. But, in one case the rate of semantic errors is significantly above chance and within the range spanned by acquired cases. The absence or rarity of pure developmental deep dyslexia is discussed in relation to strategies for the acquisition of reading.