Developmental and acquired dyslexias.

Marshall (1984) highlighted potential parallels between children with developmental disorders of reading and adults who had acquired reading disorders. He advocated the use of a cognitive neuropsychological framework in the investigation of children with developmental abnormalities of cognition, including those with developmental dyslexias. Developmental phonological dyslexia has been extensively described and is a pervasive disorder. The relationship between reading difficulty and phonological difficulties evident in explicit oral phonological tasks continues to be a focus for debate. Clear cases of developmental deep dyslexia have now been described and the syndrome has also been described as characterising early reading development in Williams syndrome (WS), where there are also semantic errors in other domains, including naming and receptive vocabulary and there may be a generalised difficulty with the activation of fine grain semantic specifications. In the domain of number, highly selective reading disorders characterised by high rates of semantic errors have been documented, indicating that semantic reading errors can be domain-specific. They can occur to number words despite intact ability to read Arabic numbers and they can occur to Arabic numbers and number words despite intact ability to read words in other domains. Current models of reading written words do not allow for such material-specific dissociation. Developmental surface dyslexia has also been described in a range of countries, languages and orthographies. Descriptions of cases for whom there is no phonological impairment in reading have generated contrary evidence for theories suggesting that phonological impairment underlies all developmental dyslexia. As reading develops in Williams Syndrome, phonological reading skills may improve with over-reliance on these leading to surface dyslexia. Surface dyslexia has also been reported in cases of developmental amnesia in which there are semantic memory impairments. Hyperlexia can take several forms including broad hyperdevelopment with elevated phonological reading abilities, lexico-semantic reading abilities and reading comprehension as in Turner's syndrome (TS). This advantage has early onset in school-starters. These specific modular effects do not have pervasive impact across systems but demonstrate the limitations of functional plasticity in developmental and genetic disorders. The framework Marshall (1984) outlined has provided a foundation for the development of systematic investigation of developmental disorders.