ON THE BASES OF 2 SUBTYPES OF DEVELOPMENT DYSLEXIA

This study examined whether there are different subtypes of developmen tal dyslexia. The subjects were 51 dyslexic children (reading below th e 30th percentile in isolated word recognition), 51 age-matched normal readers, and 27 younger normal readers who scored in the same range a s the dyslexics on word recognition. Using methods developed by Castle s and Coltheart (1993), we identified two subgroups who fit the profil es commonly termed ''surface'' and ''phonological'' dyslexia. Surface subjects were relatively poorer in reading exception words compared to nonwords; phonological dyslexics showed the opposite pattern. However , most dyslexics were impaired on reading both exception words and non words compared to same-aged normal readers. Whereas the surface dyslex ics' performance was very similar to that of younger normal readers, t he phonological dyslexics' was not. The two dyslexic groups also exhib ited a double dissociation on two validation tasks: surface subjects w ere impaired on a task involving orthographic knowledge but not one in volving phonology; phonological dyslexics showed the opposite pattern. The data support the conclusion that there are at least two subtypes of developmental dyslexia. Although these patterns have been taken as evidence for the dual-route model, we provide an alternative account o f them within the Seidenberg and McClelland (1989) connectionist model . The connectionist model accounts for why dyslexics tend to be impair ed on both exception words and nonwords; it also suggests that the sub types may arise from multiple underlying deficits. We conclude that pe rformance on exception words and nonwords is not sufficient to identif y the basis of dyslexic behavior; rather, information about children's performance on other tasks, their remediation experiences, and the co mputational mechanisms that give rise to impairments must be taken int o account as well.