UNRAVELING THE CEREBRAL DYNAMICS OF MENTAL-IMAGERY

Evidence from functional brain imaging studies suggests that mental im agery processes, like other higher cognitive functions, simultaneously activate different neuronal networks involving multiple cortical area s. The question of whether these different areas are truly simultaneou sly active or whether they are temporally distinct and might reflect d ifferent steps of information processing cannot be answered by these i maging methods. We applied spatiotemporal analysis techniques to multi channel event-related potential (ERP) recordings in order to elucidate the topography and chronology of brain processes involved in mental r otation. We measured 41-electrode ERPs in 12 healthy subjects who had to evaluate whether rotated letters were in a normal or mirror-reflect ed position. These figures were presented in the left, right, or centr al visual fields and were randomly rotated by 0 degrees, 50 degrees, 1 00 degrees, or 150 degrees. Behaviorally, we replicated the observatio n that reaction time increases with greater angles of rotation. Electr ophysiologically, we identified a set of dominant electric potential d istributions, each of them stable far a certain time period. Only one of these time segments (appearing between 400-600 msec) increased sign ificantly in duration with greater angles of rotation mirroring reacti on time. We suggest that the rotation of mental images is carried out during this time segment. A general linear inverse solution applied to this segment showed occipito-parietal cerebral activity that was late ralized to the right hemisphere. (C) 1997 Wiley-Liss, Inc.