Specific disorders and broader phenotypes: the case of dyslexia.
Two studies investigating the cognitive phenotype of dyslexia are described. Study 1 compared three groups of English and Italian children on speed of processing tasks: (a) children with dyslexia, (b) generally delayed poor readers and (c) CA-controls. In tests of simple and choice reaction time and two visual scanning tasks, children with dyslexia performed like controls and significantly faster than generally delayed poor readers. A second prospective longitudinal investigation of children at family risk of dyslexia showed that problems of literacy development were less circumscribed, with affected children showing phonological deficits in the context of more general oral language difficulties. An important finding was that the risk of dyslexia was continuous in this sample; among at-risk children with normal literacy development, mild impairments of phonological skills were apparent early in development, and subtle difficulties with reading fluency and spelling emerged in early adolescence. A case series extended these findings to show that phonological deficits alone are insufficient to explain literacy difficulties, and it is children with multiple deficits (including language problems) that are more likely to succumb to reading failure.