Cortical mapping of the auditory middle latency response in the unanesthetized guinea pig

Auditory middle latency responses (MLRs) were mapped on the cortical surface of the unanesthetized guinea pig. The MLR consists of two positive peaks, designated A and C, with latencies of approximately 11 and 29 msec respectively, and a negative trough B, at 18 msec. Results suggest that each MLR component appears to have different generator sites and specific stimulus-response characteristics. The present work suggests that positive MLR components A and C have different generator sources because each component was maximally recorded from a different location over the contralateral temporal lobe. Little MLR activity was evident outside an area roughly 4 mm X 4 mm for each component.