Exploring the nature of the phonological deficit in dyslexia: are phonological representations impaired?

Abstract

Developmental dyslexia is widely believed to be caused either mainly or in part by an impairment of phonological representations. Although this hypothesis predicts that individuals with dyslexia should show deficits in tasks which require the use of implicit phonological knowledge, this has not yet been directly tested, as the evidence cited in support of this hypothesis usually comes from metalinguistic tasks which demand explicit awareness of phonological units. Additionally, since the ability to perform metalinguistic tasks which involve phonological segments can be enhanced by an individual’s competence in alphabetic literacy, the possibility remains that phonological skills may have been inadequately isolated from the influences of literacy acquisition in many cases. The study reported in this thesis investigated both the representations and the metalinguistic skills of a group of adults with a history of developmental dyslexia, examining areas of phonology which do and do not have orthographic counterparts. To isolate phonological skills from orthographic skills, the representations of conventional segmental contrasts (e.g. /k/ vs /g/) were compared with the representation of suprasegmental contrasts (as seen in minimal pairs such as ′toy factory and toy ′factory), which have no orthographic counterpart. Basic metalinguistic skills were tested by means of a phonological awareness task targeting both segmental and suprasegmental units, and phonological manipulation skills were tested using a Pig Latin task and a Spoonerism task, where participants were required to manipulate both segmental and suprasegmental units (e.g. extracting the segment /b/ from consonant clusters and the main stress from SWW or WSW stress patterns). The results showed that although the performance of the dyslexic group was weaker than that of the control group when tasks required the manipulation of either the segmental or suprasegmental components of words, no evidence was found for a deficit in the tasks which drew on implicit representations or basic metalinguistic skills. These findings suggest that the phonological deficit in dyslexia may be restricted to the ability to manipulate phonological units rather than in the representation of them per se.