Early cognitive neuropsychological profiles and development of reading skills
The present thesis sought to investigate the precise relationship between the cognitive and psychophysiological profiles of developing readers, of established readers and of failed readers. Phonological processing tasks and visuospatial tasks were used to assess relevant auditory and visual cognitive skills; handedness and EEG measures were used to provide indices of cortical organisation and activation. A 21/2 year longitudinal investigation of some 150 pre-readers provided evidence of mutually facilitative relationships between and within specific types of phonological skill and phonological memory. Early significance of visual skills was subsequently superseded by the importance of these phonological skills. The acquisition of early reading skills was associated with a shift towards increased dextrality as measured by hand skill and hand preference; this relationship was not evident in subsequent stages. Cross-sectional studies comparing dyslexic children with chronological- and reading-age matched controls extended these findings. The dyslexic readers displayed impaired phonological processing and phonological memory skills relative to chronological-age matched competent readers; similarities were observed between dyslexics and reading-age matched controls. Visual perceptual skills failed to differentiate between the chronological-age matched competent and impaired readers, although both out-performed younger control readers. ERP measures consistently demonstrated diffuse patterns of bilateral activation in dyslexic readers as opposed to asymmetric activity lateralised to the left hemisphere in control readers. Between group comparisons of inter-hemispheric activity revealed greater levels of right-hemisphere involvement in the dyslexic samples; between group comparisons of intra-hemispheric activity revealed evidence of greater involvement of fronto-central regions in the dyslexic samples. It is proposed that these data provide supportive evidence for the central involvement of phonological processing skills in the development of reading, underpinned by the normal development of asymmetric patterns of cortical lateralisation. Children where this development is delayed or deficient will display the reading difficulties characteristic of developmental dyslexia.