Trait and state EEG indices of information processing in developmental dyslexia

A possible key to understanding the nature and specificity (or otherwise) of the difficulties experienced by children with developmental dyslexia can be found by comparing event-related EEG changes in tasks directly related to their reading difficulties with those in tasks where their performance is normal. Alpha, theta and beta activity at 28 electrode sites was measured in 19 children with developmental dyslexia and 22 age-matched children with normal reading ability, allowing comparisons at right and left frontal, temporal and parieto-occipital sites. EEG responses during a phonological processing task in which the dyslexic group significantly under-performed was compared with EEG responses during a visual search task (WISC Picture Completion) where the dyslexic children showed no deficit. There were significant Task Group differences in task-related alpha desynchronisation, task-related beta left–right asymmetries and task-related frontal theta inhibition. In both tasks, EEG responses from the dyslexic group were characterised by a lack of task-related reduction from resting levels in the amplitude of alpha frequency responses. There was a marked parieto-occipital R>L asymmetry in beta activity in the dyslexic group, again in both tasks. Theta activity did discriminate between the two tasks in the dyslexic group. In the phonological task, task-related frontal theta in the dyslexic group was significantly different from the control group, with the former showing an increase in amplitude and the latter a decrease. In the visual task, there was no significant difference between the dyslexic and the control group, with both showing a task-related decrease in amplitude. The inter-task variations in EEG response in the dyslexic group paralleling variations in task performance are interpreted in terms of the varying engagement of a frontally-based attentional system. Inter-task consistencies of EEG response despite variations in performance are interpreted in terms of the continued application of a specific cognitive strategy.