STIMULATING READING: A BEHAVIOURAL AND ELECTROPHYSIOLOGICAL INVESTIGATION OF THE IMPACT OF BRAIN STIMULATION IN DEVELOPMENTAL DYSLEXIA.

Abstract

Developmental dyslexia is a disorder of reading which is associated with reduced left lateralisation of the temporoparietal cortex (TPC), an area associated with phonology and subword processing. This thesis aimed to explore differences between dyslexics and non-impaired controls using behavioural measures of reading aloud and electrophysiological measures of brain activation. It also investigated the impact of left lateralising transcranial direct current stimulation (tDCS) over TPC on both of these populations’ behaviour and brain activity. While recent studies have found that left lateralising tDCS to the TPC can improve reading in dyslexics, no study to date has compared the relative effects of this tDCS montage on dyslexics and controls, or on different word types. Therefore, in Chapter 2 we first sought to examine the effects of left lateralising tDCS on reading aloud regular, exception, and nonwords in both dyslexics and controls. Results showed a significant increase in accuracy after active stimulation for nonwords in both dyslexics and controls. This increase was larger in dyslexics. Dyslexics also showed increased accuracy in exception words. These results demonstrate that left lateralisation tDCS can significantly enhance reading performance, and that it may be redressing underlying deficits Based on these findings, Chapter 3 and 4 sought to investigate underlying neural differences, using electroencephalography (EEG), between dyslexics and controls during reading aloud, and the associated neural changes which occur with reading improvement after stimulation. This revealed significant abnormalities in an early P1 component associated with visual attention. Stimulation reduced the heightened P1 and increased left lateralisation of N170 in response to nonwords in dyslexics. These results show a normalisation of attentional focus when processing visually presented nonwords after active stimulation, and a normalisation of lateralisation of the N170, a component associated with orthographic processing, and known to show reduced left lateralisation in dyslexic readers. In order to further explore the differences between dyslexics and controls, and the effects of tDCS, in Chapter 5, 6, and 7 we used an artificial orthographic learning task. This task enabled us to mimic the early stages of learning. Behavioural results revealed that dyslexics had a reduced ability to pick up on subword consistency manipulations which was improved with active stimulation. Similarly, to results for familiar items in Chapters 3 and 4, EEG responses to newly learnt artificial items revealed laterality abnormalities in P1 and N170 components associated with visual attention and orthographic processing. This shows that abnormalities in attentional focus and orthography to phonology processing are present in dyslexics in early stages of learning. Taken together, the results of this thesis demonstrate the potential of tDCS to improve reading ability in dyslexics, but also to explore the underlying neural underpinnings of reading and other higher cognitive processes.

Date of Award	31 Dec 2017
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Wael El-Deredy (Supervisor) & Anna Woollams (Supervisor)