DIPOLE-MODELING OF THE VISUAL-EVOKED P300

We collected visual event-related potentials (ERPs) from 6 normal subj ects using an ''oddball'' paradigm. Subjects were required to count th e occurrences of matching shapes presented in the left and right visua l field. Shapes matched on 20% of trials. ERPs were recorded from 20 o r 43 electrodes distributed over the scalp. A multiple spatio-temporal equivalent dipole (ED) model was used to fit the early sensory and P3 00 component. A latency window to analyze the P300 was determined usin g the global field power statistic. The spatial topography of the P300 over this window was characterized by a midline positivity that decre ased in amplitude with spatial distance from the peak. After sensory c omponents were fit, the source of P300 could be accounted for by 1 or 2 EDs, which were usually located near medial temporal areas. This res ult is at odds with evidence from depth recordings during the oddball paradigm, showing that multiple regions of the brain are active during this interval.