Dysfunctional hemispheric asymmetry of theta and beta EEG activity during linguistic tasks in developmental dyslexia.

The phonological deficit hypothesis of dyslexia was studied by analyzing language-related lateralization of theta (4-8 Hz) and beta rhythms (13-30 Hz) during various phases of word processing in a sample of 14 dyslexics and 28 controls. Using a word-pair paradigm, the same words were contrasted in three different tasks: Phonological, Semantic and Orthographic. Compared with controls, dyslexic children showed a delay in behavioral responses which was paralleled by sustained theta EEG peak activity. In addition, controls showed greater theta and beta activation at left frontal sites specifically during the Phonological task, whereas dyslexics showed a dysfunctional pattern, as they were right-lateralized at these sites in all tasks. At posterior locations, and reversed with respect to controls' EEG responses, dyslexics showed greater left lateralization during both Phonological and Orthographic tasks--a result which, in these children, indicates an altered and difficult phonological transcoding process during verbal working memory phases of word processing. Results point to a deficit, in phonological dyslexia, in recruitment of left hemisphere structures for encoding and integrating the phonological components of words, and suggest that the fundamental hierarchy within the linguistic network is disrupted.