A Source Analysis of the Late Human Auditory Evoked Potentials

The intracerebral generators of the human auditory evoked potentials were estimated using dipole source analysis of 14-channel scalp recordings. The response to a 400-msec toneburst presented every 0.9 sec could be explained by three major dipole sources in each temporal lobe. The first was a vertically oriented dipole located on the supratemporal plane in or near the auditory koniocortex. This contributed to the scalp-recorded N1 wave at 100 msec. The second was a vertically oriented dipole source located on the supratemporal plane somewhat anterior to the first. This contributed to both the Nl and the sustained potential (SP). The third was a laterally oriented dipole source that perhaps originated in the magnopyramidal temporal field. This contributed a negative wave to the lateral scalp recordings at the latency of 145 msec. A change in the frequency of the toneburst elicited an additional negativity in the scalp-recording -the mismatch negativity (MMN). When the frequency change was large, the mismatch negativity was composed of two distinct sources with sequential but partially overlapping activities. The earlier corresponded to the Nl dipole sources and the later to a more anteriorly located dipole with an orientation more lateral than Nl. Only the later source was active when the frequency change was small. MMN source activities peaked about 15 msec earlier in the contralateral hemisphere, while this difference was only 4 msec for the sources of the Nl.