Evidence for a primary cortical origin of a middle latency auditory evoked potential in cats

The origin of the scalp-recorded auditory evoked potential, Pa, was examined in cats anesthetized with chloralose-urethane and immobilized with gallamine triethiodide. This potential is a prominent positive wave which peaks approximately 12--15 msec following click stimuli. Mapping revealed that Pa is distributed on the scalp in the region overlying cortical area AI, contralateral to the stimulated ear. The cortical potential recorded from AI was a surface-positive wave, restricted to the anterior portion of AI. Laminar analysis of the cortical evoked potentials demonstrated the existence of a dipole generator at that area. The onset of this potential coincided with the onset of the scalp-recorded Pa. Comparison of the scalp and the cortex-recorded potentials showed that both the amplitude-intensity function and the amplitude-rate function for the scalp-recorded potential closely paralleled those recorded from AI. Acute and chronic lesion studies showed that extirpation of AI (particularly the anterior part) almost completely abolished the Pa response. This evidence indicates that the scalp-recorded Pa of cats is generated almost entirely from the anterior part of the contralateral AI.