Category-specific occipitotemporal activation during face perception in dyslexic individuals: an MEG study.
In dyslexia, it is consistently found that letter strings produce an abnormally weak or no response in the left occipitotemporal cortex. Time-sensitive imaging techniques have located this deficit to the category-specific processing stage at about 150 ms after stimulus presentation. The typically reported behavioral impairments in dyslexia suggest that the lack of occipitotemporal activation is specific to reading. It could, however, also reflect a more general dysfunction in the left inferior occipitotemporal cortex or in the time window of category-specific activation (150 to 200 ms). As early cortical processing of faces follows a sequence practically identical to that for letter strings, both in location and in timing, we investigated these possibilities by comparing face-specific occipitotemporal activations in dyslexic and non-reading-impaired subjects. We found that both the stage of general visual feature analysis at about 100 ms and the earliest face-specific activation at about 150 ms were essentially normal in the dyslexic individuals. The present results emphasize the special nature of the occipitotemporal abnormality to letter strings in dyslexia. However, in behavioral tests dyslexic subjects were slower and more error-prone than non-reading-impaired subjects in judging the similarity of faces and geometrical shapes. This effect may be related to reduced activation of the right parietotemporal cortex at about 250 ms after stimulus onset.