On subtypes of developmental dyslexia: evidence from processing time and accuracy scores.
Phonological dyslexics (Ph-DYS) are characterized by a phonological deficit, while surface dyslexics (S-DYS) are characterized by an orthographic deficit. Four issues were addressed in this study. First, we determined the proportion of Ph-DYS and S-DYS in a population of French dyslexics by applying Castles and Coltheart's (1993) regression method to two previously unused diagnostic measures: pseudo-word and irregular-word processing time. Thirty-one dyslexics were matched to 19 average readers of the same age (10 years, CA controls) and to 19 younger children of the same reading level (8 years, RL controls). Compared to CA controls, there were more Ph-DYS than S-DYS. Compared to RL controls, there were still a high number of Ph-DYS; however, the S-DYS profile almost disappeared. Next, we examined the reliability of these subtypes across different measures of phonological and orthographic skills. Compared to RL controls, both groups of dyslexics were found to be impaired only in phonological skills, either in processing time (Ph-DYS) or in accuracy (S-DYS). Then we assessed the moment at which the two dissociated profiles emerged in the course of cognitive development. In order to do so, we examined earlier longitudinal data, collected when the children were 7 and 8 years old, and found that only the S-DYS's orthographic deficit increased with development. Last, we looked at whether the Ph-DYS and S-DYS profiles were associated with other specific cognitive deficits. Specific deficits in phonemic awareness and in phonological short-term memory were found for both Ph-DYS and S-DYS. These data suggest that developmental dyslexia could be largely accounted for by an underlying phonological impairment.