MAGNETOENCEPHALOGRAPHY REVEALS 2 DISTINCT SOURCES ASSOCIATED WITH LATE POSITIVE EVOKED-POTENTIALS DURING VISUAL ODDBALL TASK

The present investigation was undertaken in order to utilize magnetoencephalography to locate generator sources, modeled as equivalent current point dipoles, that account for the well-established late positive electrical potentials commonly measured along the midline (Cz, Pz, Fz, and Oz) in response to rare or task-relevant stimuli and their simultaneously recorded magnetic field components. Two simultaneous but spatially distinct sources were present in all six subjects. One source in which the magnetic flux exited and reentered the brain over the right hemisphere was localized in deep structures under the temporal cortex in the vicinity of the right hippocampal formation. The other occurred in the vicinity of the primary visual cortex in the occipital area with magnetic flux entering and exiting over the posterior aspects of the subjects' heads. These data correspond to depth and surface electrode studies that have demonstrated multiple generator sources.