Dissociation of normal feature analysis and deficient processing of letter-strings in dyslexic adults

Neuroimaging studies have revealed that the functional organization of read ing differs between developmentally dyslexic and non-impaired individuals. However, it is not clear how early in the reading process the differences b etween fluent and dyslexic readers start to emerge. We studied cortical act ivity of ten dyslexic adults using magnetoencephalography (MEG), as they si lently read words or viewed symbol-strings which were clearly visible or de graded with Gaussian noise. This method has previously been used to dissoci ate between analysis of local features and pre-lexical word processing in f luent adult readers. Signals peaking around 100 ms after stimulus onset and originating in the postero-medial extrastriate cortex were associated with increasing local luminance contrast in the noise patches. These early visu al responses were similar in dyslexic and non-impaired readers. In contrast , the letter-string-specific responses peaking around 150 ms predominantly in the left inferior occipito-temporal cortex in fluent readers were undete ctable in dyslexic readers. Thus, while the early visual processing seems i ntact in dyslexic adults, the pattern of cortical activation starts to diff er from that of fluent readers at the point where letter-string-specific si gnals first emerge during reading.