Impaired visual word processing in dyslexia revealed with magnetoencephalography

TLDR

Dyslexia is largely attributed to phonological deficits and abnormal left temporal cortex activation, yet the immediate cortical correlate of impaired reading remains unclear. The study reports on distinct time courses of cortical activation in dyslexic versus control subjects during passive viewing of single words using whole‑head magnetoencephalography. Whole‑head magnetoencephalography was used to track cortical activation time courses during passive word viewing. Dyslexics showed a lack of early (~180 ms) activation in the left inferior temporo‑occipital region and a delayed or absent response, indicating impaired perception of words as discrete units.

Abstract

Abstract Dyslexia is most often atributed 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. Pereception of words as specific units thus seems to be imparied in dyslexics.