Effects of primary auditory cortex lesions on middle latency responses in awake cats

In order to clarify the role of the primary auditory cortex (AI) on middle latency responses (MLRs), we recorded the auditory evoked potentials (AEPs) from the vertex and the right and left AI areas of the skull simultaneously before and after creating serial lesions of the AIs contralateral and ipsilateral to the stimulated ear in 7 awake cats. The auditory brainstem responses (ABRs) and MLRs recorded from the vertex in normal awake cats revealed the presence of peaks 1-8, NA and PA within the analysis time of 50 msec. After there were serial AI lesions, (1) all the peaks remained at nearly the same latencies, (2) the amplitude of the NA was decreased significantly, that of the PA was slightly decreased and those of peaks 6, 7 and 8 were variable, and (3) the difference between the effects of the first operation (contralateral AI) and the second operation (ipsilateral AI) was not statistically significant. These findings indicated that the main, prominent effect of bilateral AI lesions on MLRs in the awake cat is a significant decrease in the NA amplitude.