Dyslexia is most often attributed to phonological impairments, manifested in abnormal activation of the left temporal and temporoparietal cortex in response to auditorily presented language and possibly associated with anomalies in the cytoarchitecture and hemispheric symmetry of the plana temporale. The immediate cortical correlate of the severely impaired reading process has, however, remained obscure. Here we report on the distinct time courses of cortical activation in dyslexic and control subjects during passive viewing of single words, tracked with whole-head magnetoencephalography. A striking difference was found in the left inferior temporo-occipital region where intracranial recordings have recently identified word-specific responses within 200 msec after stimulus onset: controls showed a sharp activation at about 180 msec after word presentation, whereas dyslexics failed to activate this area entirely, or showed a slowly increasing late response. Perception of words as specific units thus seems to be impaired in dyslexics.

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A recent functional magnetic resonance imaging (fMRI) study concluded that the motion-specific visual area V5 is not activated in dyslexic subjects. We report here opposing evidence based on whole-scalp neuromagnetic recordings. Apparent-motion stimuli elicited similar activation of V5 in both dyslexic and control subjects, with a trend for longer latencies in dyslexics. Both high- and low-contrast stimuli activated the V5 region in dyslexics. The lack of significant blood flow changes despite modified neuronal synchrony would explain the absence of fMRI signals and the presence of neuromagnetic signals in dyslexic subjects.

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