3-DIMENSIONAL ANALYSIS OF AUDITORY-EVOKED POTENTIALS IN RAT NEOCORTEX

A 8 X 8-channel microelectrode array was used to map epicortical field potentials evoked by bilaterally presented click stimuli from a 8 X 8-mm2 area in the right parietotemporal neocortex of four rats. In two rats, a 16-channel microelectrode array was also inserted into primary auditory cortex to record the laminar profile of auditory evoked potentials (AEP). The epicortical responses began with a positive-negative fast wave followed by a positive-negative slow wave, similar to the previously reported P1, N1, P2, N2 complex. Topographical distributions of the potentials at the peak of each of these waves were distinct, suggesting that they were produced by separate but overlapping populations of cells. Laminar recording revealed the asynchronous participation of supragranular and infragranular pyramidal cells in the generation of the evoked-response complex. The surface-recorded P1 was primarily produced by a supragranular cells and the N1, by infragranular cells. The P2 and N2 were produced by temporally overlapping contributions from both cell groups. We conclude that middle-latency components of the AEP complex are produced by both sequential and parallel activation of subpopulations of pyramidal cells in primary auditory cortex.