Comparing the phonological and double deficit hypotheses for developmental dyslexia

This study tested the predictions of the phonological and double deficit hypotheses by experimentally examining speech perception, phoneme awareness, lexical retrieval (serial and discrete), articulatory speed, and verbal STM in school age child (N = 35) and adolescent (N = 36) dyslexics, and both chronological age (CA) and reading age (RA) controls. The results confirmed the findings of previous studies of a deficit in phoneme awareness in developmental dyslexia. At both age levels, dyslexics performed significantly more poorly than both their CA and RA controls. Although deficits in the other processes investigated, particularly in rapid serial naming, were also apparent, they were not as clear-cut as the deficit in phoneme awareness. In general, definite evidence of a deficit in rapid serial naming was limited to the more severely impaired dyslexics. Furthermore, although rapid serial naming contributed independent variation to various literacy skills, its contribution was modest relative to the contribution of phoneme awareness, regardless of whether the literacy skill relied more or less heavily on phonological or orthographic coding skills. Further analyses suggested that variation in rapid serial skill is particularly important for fluent reading of text, whereas phoneme awareness is particularly important for the development of the ability to read by phonologically recoding letters or groups of letters in words into their phonological codes. This explains the relatively strong contribution of phoneme awareness to reading and spelling ability in general. In sum, the phonological hypothesis offers a more parsimonious account of the present results than the double deficit hypothesis.