Working Memory and Dyslexia

Recent research has demonstrated that dyslexic children suffer impairments in both phonological skills and in working memory performance. In principle either a phonological deficit or a working memory deficit could underlie both sets of symptoms. This issue was addressed via a series of experiments designed to explore the relationships between dyslexia, age, phonological performance and working memory. Two groups of dyslexic children participated, with mean ages 15 and 11 years respectively, together with three groups of non-dyslexic children (mean ages 15, 11 and 8 years) matched for IQ with the dyslexic children and selected so as to provide both chronological and reading age controls for the two dyslexic groups. The performance of the older dyslexics across the range of tasks was slightly but not significantly worse than their chronological age controls and indistinguishable from that of their reading age controls. By contrast, the performance of the younger dyslexics was significantly worse than that of their chronological age controls on phonological discrimination, articulation rate, and nonword repetition, and was significantly worse even that of their reading age controls on repetition of longer nonwords. On tests of memory span, all five groups showed the normal phonological similarity effect and the normal word length effect. When memory span was regressed as a function of articulation rate, there was no evidence of impaired slope or intercept for the dyslexic children. It is concluded that the 11 year old dyslexic children show residual problems on phonological processing, especially for tasks involving unfamiliar stimuli, but that by their mid-teens dyslexic children have overcome these problems. The major remaining problem for the dyslexic children appears to be a continuing lack of fluency in articulation, a factor which is sufficient to account for the slight deficits on memory span. Furthermore, it is argued that the articulation rate deficit provides a parsimonious explanation of the range of deficits shown by the younger dyslexic children. It is concluded that neither impaired phonological skills nor impaired working memory is sufficient in itself to explain the deficits, but that some deeper explanation must be sought.