EEG correlates of phonological processing in dyslexic children

Previous studies using indirect measures of hemispheric specialization have indicated a link between lack of phonological awareness and reduced cerebral lateralisation. Using topographical mapping techniques this study aimed to investigate patterns of cortical activity in dyslexic children characterized by phonological processing difficulties. The test group comprised 31 dyslexic children recruited from a county-wide Reading Advisory Service and 29 control children recruited from local schools. Functional lateralisation in the children was assessed using both indirect (handedness) and direct (EEG) measures. Handedness was assessed using a peg-moving task, giving a relative hand skill measure of L - R/L + R. 28 channels of EEG data were collected using Brain Electrical Activity Mapping techniques; electrode pairs were compared using a similar measure. Results show that children with reading difficulties are significantly worse on a test of phonological processing and that this population is significantly less lateralised than the control population, both in terms of handedness and task-related cortical lateralisation. EEG anomalies in the dyslexic population were revealed in theta, alpha and beta wavebands, with increases in frontal theta, lock of alpha blocking, and little or no event-related desynchronisation in beta activity during the phonological processing task. The asymmetrical pattern of beta activity shown by the controls during the phonological processing task was not shown by the dyslexics. The differences were most marked over the parieto-occipital areas.