VISUAL WORD RECOGNITION: THE FIRST HALF SECOND

 Piers Cornelissen, Morten Kringelbach, Ian Holliday, Peter Hansen

Abstract 

Ten skilled adult readers carried out a lexical decision task while cortical responses were recorded with MEG. Signals were analysed for each individual using SAM (synthetic aperture magnetometry) in overlapping time windows up to 500msec and overlapping frequency bands up to 50Hz, with a reference period of 200msec pre-stimulus onset. Within 200msec of seeing a word, activity is restricted to primary visual cortical areas together with predominantly left hemisphere (LH) BA 18/19, consistent with the extraction of visual features within a retino-topic framework (Tarkiainen et al, Brain 2002). Between 50-250msec, there begins a parallel activation in left hemisphere cortical areas associated with phonological processing (IFG, BA 44/6). Not until 150-350msec do we see activity in the visual word form area as defined by Cohen et al. (2002). Starting around 200msec, activity extends to include left posterior MTG (BA 37/39) accompanied by co-activation in angular and supramarginal gyri (BA 39/40) and the superior temporal operculum. Posterior MTG, supramarginal gyrus and the angular gyrus may play a role in cross-modal association. We suggest therefore that the posterior temporo-parietal regions that are activated between 300-550msec are cross-modal association areas for the convergence of orthographic, phonological and semantic information represented earlier in a parallel-distributed network, thereby modulating the sequence of later activity in anterior fusiform gyrus.