Phonological representations in dyslexia : nature, influences and development
Developmental dyslexia is a specific difficulty in acquiring literacy skills that manifests despite normal IQ, adequate educational opportunity and in the absence of any obvious sensory or neurological damage. According to the Phonological Representations Hypothesis a core deficit for individuals with dyslexia across languages is a brain-based difficulty in accurately storing the sound sequences that make up words, or 'phonological' representations. In this thesis the Phonological Representation Hypothesis (PRH) of dyslexia was tested and elaborated. Twenty-four dyslexic children alongside chronological age and reading age matched groups were assessed over a three-year period. Consistent with the PRH, associations were found between the quality of the dyslexic children's phonological representations, as indexed by picture naming, and their performance on related input and output phonological processing tasks based on the same lexical items. Possible reasons for the underspecificity of dyslexic phonological representations were also investigated at cognitive and perceptual levels. The sensitivity of dyslexic individuals to the presence of similar-sounding words within their mental lexicon, 'phonological neighbourhood density', was assessed. Across a range of phonological awareness tasks the dyslexic group were found to be as sensitive to this lexical factor as their age peers. Perception of amplitude envelope onsets (AEOs) was also investigated. AEOs are an auditory parameter associated with speech rhythm and were hypothesised here to be important for the establishment of well-specified phonological representations. Dyslexic insensitivity to AEO variation was seen longitudinally through both behavioural and neurophysiological assessment. These findings suggest that for some dyslexic children perception of basic rhythmic speech cues may play a role in their phonological representation deficit.