Semantic cortical activation in dyslexic readers

The combined temporal and spatial resolution of MEG (magnetoencephalography ) was used to study whether the same brain areas are similarly engaged in r eading comprehension in normal and developmentally dyslexic adults. To extr act a semantically sensitive stage of brain activation we manipulated the a ppropriateness of sentence-ending words to the preceding sentence context. Sentences, presented visually one word at a time, either ended with a word that was (1) expected, (2) semantically appropriate but unexpected, (3) sem antically anomalous but sharing the initial letters With the expected word, or (4) both semantically and orthographically inappropriate to the sentenc e context. In both subject groups all but the highly expected sentence endi ngs evoked strong cortical responses, localized most consistently in the le ft superior temporal cortex, although additional sources were occasionally found in more posterior parietal and temporal areas and in the right hemisp here. Thus, no significant differences were found in the spatial distributi on of brain areas involved in semantic processing between fluent and dyslex ic readers. However both timing and strength of activation clearly differed between the two groups. First, activation sensitivity to word meaning with in a sentence context began about 100 msec later in dyslexic than in contro l subjects. This is likely to result from affected presemantic processing s tages in dyslexic readers. Second, the neural responses Were significantly weaker in dyslexic than in control subjects, indicating involvement of a sm aller or less-synchronous neural population in reading comprehension. Third , in contrast to control subjects, the dyslexic readers showed significantl y weaker activation to semantically inappropriate words that began with the same letters as the most expected word than to both orthographically and s emantically inappropriate sentence-ending words. Thus, word recognition by the dyslexic group seemed to be qualitatively different: Whereas control su bjects perceived words as wholes, dyslexic subjects may have relied on subl exical word recognition and occasionally mistook a correctly beginning word for the one they had expected.