Categorical perception of speech sounds and dyslexia

Several studies indicate that dyslexics have a deficit in the categorical perception (CP) of phonemes when listening to speech. After reviewing data supporting the reliability of the CP deficit, we examine its possible site in the framework of a three-stage model of speech perception (auditory, phonetic and phonological). Different sources of evidence suggest that the deficit has a speech specific component. Further, the speech perception deficit appears to be phonological in nature insofar as dyslexics appear to be better than control subjects in discriminating allophonic distinctions, i.e. phonetic distinctions irrelevant for lexical processing in their language. Such sensitivity to allophonic variations in dyslexics might arise from a weakness in phonological couplings between phonetic features during perceptual development, which, in turn, can have specific implications for the build up of grapheme-phoneme correspondences. It may explain why dyslexics encounter significant problems in phonologically-demanding tasks like reading without experiencing similar difficulties in speech perception