Allophonic perception in developmental dyslexia: Origin, reliability and implications of the categorical perception deficit

Current theories on dyslexia refer either to phonological or perceptual factors. The phonological theory explains dyslexia by a deficit in phonological awareness which would affect the build-up of grapheme–phoneme correspondences. This is challenged by the magnocellular theory which ascribes dyslexia to a deficit in temporal processing of auditory and visual signals. However, the auditory deficit in dyslexia is not specifically temporal. Further, the perceptual deficit is not merely sensory but cognitive in nature as evidenced by both weaker discrimination of phonological contrasts and stronger discrimination of differences within phonological categories. This reflects a deficit in “Categorical Perception” which is also sometimes associated with a weaker precision of the perceptual boundary between phonemes (“Boundary Precision” deficit). Categorical deficits are more reliable than magnocellular ones and might be no less reliable than those in phonemic awareness. The categorical deficit suggests that dyslexic children perceive speech with allophonic rather than phonemic units, which has straightforward consequences for the acquisition of phoneme–grapheme correspondences and might also explain the other phonological troubles associated with dyslexia.