Executive functioning in adults with dyslexia

The aim of this thesis was to identify the differences in executive functioning between adults with dyslexia and adults without dyslexia. Executive function was examined at a finer-grained level than previous research has done by using multiple measures of executive function across a number of different processing modalities, such as visuo-spatial, phonological, and semantic. In so doing, the rationale was to minimise task impurity, and ultimately, to develop a deeper understanding of the extent of a possible causal link between executive control problems and developmental dyslexia (on top of the well-recognised problems in phonological processing). Study 1a, which compared random number generation performance in participants with dyslexia to control participants found that performance amongst the group with dyslexia gradually improved over the duration of the experiment, whilst performance in the control group remained stable. These results indicate that the group with dyslexia initially struggled with task novelty and the ability to quickly instantiate the appropriate schemas, which is suggestive of Supervisory Attentional System (SAS) dysfunction. Study 1b showed that the presence of dyslexia was not a significant predictor of performance in design fluency, but the presence of dyslexia was a significant predictor of weaker performance in phonemic and semantic fluency. These findings provide support to the phonological processing deficit account of dyslexia in that a dyslexia-related impairment was only observed for the verbal task modality. Nevertheless, it is still possible that at the same time, dyslexia-related executive control problems may have contributed to the poorer performance in the phonemic and semantic fluency tasks. Study 2a examined set shifting performance, however, no significant group differences were found. Study 2b investigated updating; a dyslexia-related deficit was observed on the listening span, which is thought to place a large load on central executive resources. Furthermore, the adults with dyslexia also had shorter visual working memory spans, which challenges the phonological processing deficit account of dyslexia. In Study 3, which examined inhibition; no significant group differences were found across the inhibition tasks. Small sample size may explain the non-significant results of Study 2a and Study 3. Collectively, these studies lend some support to the notion of executive control problems and SAS dysfunction underlying developmental dyslexia. However, further research is needed so that deficient phonological skill can be excluded from the explanation of impaired performance amongst adults with dyslexia.