Alpha and beta band power changes in normal and dyslexic children

Objective: Previous research with healthy subjects suggests that the lower alpha band reflects attentional whereas the upper alpha band semantic proce sses. The aim of the present study was to investigate whether dyslexics sho w deficits in attentional control and/or semantic encoding. Method: The EEG was recorded while subjects were reading numbers, words and pseudowords and analyzed in a lower and upper alpha and two beta bands (sp anning a range of about 8-16 Hz). A phasic response is measured in terms of a decrease in event related band power during reading with respect to a re ference interval. Tonic power is measured in terms of (log) band power duri ng a reference interval. Results: In the lower alpha band dyslexics show an increased phasic respons e to words and pseudowords at right hemispheric sites but a lack to respond to words at O1. The upper alpha band exhibits a highly selective phasic re sponse to words at left frontal sites but fur controls only, whereas dyslex ics show a general increase in tonic upper alpha power. Whereas the low fre quency beta band (beta-1a) exhibits a rather diffuse pattern, a highly sele ctive finding was obtained for the beta-lb band. Conclusions: Dyslexics have a lack of attentional control during the encodi ng of words at left occipital sites and a lack of a selective topographic a ctivation pattern during the semantic encoding of words. Because only in co ntrols reading of words is associated with a strong beta-lb desynchronizati on at those recording sites which correspond to Broca's area (FC5) and the angular gyrus (CP5, P3), we may conclude that this frequency band reflects the graphemicphonetic encoding of words. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.