Unraveling the cerebral dynamics of mental imagery

Evidence from functional brain imaging studies suggests that mental imagery processes, like other higher cognitive functions, simultaneously activate different neuronal networks involving multiple cortical areas. The question of whether these different areas are truly simultaneously active or whether they are temporally distinct and might reflect different steps of information processing cannot be answered by these imaging methods. We applied spatiotemporal analysis techniques to multichannel event-related potential (ERP) recordings in order to elucidate the topography and chronology of brain processes involved in mental rotation. We measured 41-electrode ERPs in 12 healthy subjects who had to evaluate whether rotated letters were in a normal or mirror-reflected position. These figures were presented in the left, right, or central visual fields and were randomly rotated by 0 degrees, 50 degrees, 100 degrees, or 150 degrees. Behaviorally, we replicated the observation that reaction time increases with greater angles of rotation. Electrophysiologically, we identified a set of dominant electric potential distributions, each of them stable for a certain time period. Only one of these time segments (appearing between 400-600 msec) increased significantly in duration with greater angles of rotation mirroring reaction 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 lateralized to the right hemisphere.