Generators of event-related potentials (ERPs) studied by magnetoencephalography (MEG)

We recorded magnetic and electric responses simultaneously to elucidate the neural mechanism involved in the oddball paradigm in normal subjects and patients with intractable temporal lobe epilepsy. In the auditory oddball paradigm, target stimuli evoked the magnetic deflections at about 400 ms (M400) over the anterior temporal, superior temporal, and parietal areas on each hemisphere in normal subjects. With a time-varying multidipole model, the equivalent current dipoles (ECDs) were located in the mesial temporal, superior temporal, and inferior parietal regions on each hemisphere. ECDs were detected in patients (in the same regions as in normal subjects) before surgery. After the selective amygdalohippocampectomy and the standard temporal lobectomy, M400 disappeared only over the anterior temporal channels on the operated side, and ECD in the mesial temporal region on the resection side was lost. In the visual oddball paradigm, M400 was evoked over the similar areas as in the auditory paradigm, but not over the occipital area-ECD in the mesial temporal region was detected as in the auditory paradigm. In addition, ECDs were localized in the parietal regions and inferior temporal regions, which varied among subjects. These results suggest that the generating mechanisms underlying auditory and visual cognitive processing involved the combination of these multiple structures.