The time course of semantic category processing in the cerebral hemispheres: an electrophysiological study

Using visual half-field presentations of words to the right (RVF) and to th e left visual field (LVF), this study investigated the time course of the h emispheric involvement in the processing of semantic category information. Multi-channel event related brain potentials (ERPs) were recorded from 15 h ealthy subjects during a categorisation task of sequentially presented word pairs. Subjects had to judge mentally after the appearance of the second w ord whether the words of a pair were semantically related (SR) or not (SU). ERPs were computed, from 100 ms before the onset of the second word to 600 ms, for SR and SU conditions in the LVF and in the RVF separately. The tem poral segmentation of ERP map series into sequences of quasi-stable map con figurations revealed a total of seven segments in each visual held of which only the first five (S1-S5, appearing between 70 and 400 ms) showed differ ent map configurations as a function of visual field but presented a simila r temporal sequence in both visual fields. By contrast, of the last two seg ments (S6 and S7) which appeared between similar to 400 and similar to 600 ms, only S7 differentiated SR and SU conditions in terms of its duration. S ource localisation analysis of the segments: showed that following the init ial activation of posterior brain regions as a function of the visual field of presentation, a common neural network was activated in the left hemisph ere (LH) although the dynamics of activation varied as a function of visual held. Concerning the role of the right hemisphere (RH) in lexico-semantic processing, the results presented here appear to be compatible with a 'call osal relay model' and suggest that, in healthy subjects. information is tra nsferred rapidly (similar to 150 ms) from the RH to the language dominant-L H. (C) 2001 Elsevier Science B.V. All rights reserved.