A Behavioural and Electrophysiological exploration of the Working Memory impairment in Developmental Dyslexia

The current thesis provides a behavioural and electrophysiological exploration of Working Memory (WM) processing in developmental dyslexia. This thesis identifies a debate in the literature regarding the extent to which individuals with dyslexia have a specific phonological WM impairment, or a domain general Central Executive (CE) impairment. Predictions from the latter account suggest that dyslexics should show an impairment in visual, and verbal domains of WM. However, findings in the visual domain have been inconsistent, and research has predominantly focused on children. The experimental work in this thesis examines CE processing in dyslexic adults by assessing the behavioural and ERP responses associated with WM, across 8 experiments. Experiments 1-5 present stimuli in the visual domain, while Experiments 6-8 are conducted in the auditory domain. The results indicate that dyslexics are impaired for verbal information specifically, however subtle RT differences emerge during visual-spatial WM, when participants are required to manipulate information. In order to assess why effects are more robust in the phonological domain, Experiment 8 examines the contribution of auditory perceptual problems and phonological WM processing in dyslexia. The Temporal Sampling Theory of Developmental dyslexia (TSTDD; Goswami, 2011) specifies that dyslexics have a difficulty processing tones with long rise-times. In Experiment 8, dyslexic participants show a WM impairment that is specific to tones with long rise-times. The theoretical implications of these findings are discussed, and a new hypothesis regarding the phonological WM impairment in dyslexia is proposed. The original contribution to knowledge of this thesis are threefold. 1) The ERP responses associated with WM processing in developmental dyslexia are examined across modality, using a range of stimuli. 2) A novel task is used to directly investigate CE processing in dyslexia (Experiment 5). 3) The TSTDD is applied in order to investigate phonological WM in dyslexia.