Regional differences in evoked response latencies in the cat motor cortex (area 4 gamma)

The latencies of responses evoked in the cat motor cortex (area 4 gamma) on contralateral forelimb nerve stimulation were measured. In the caudolateral part, the latencies were 4-5 ms on ulnar and superficial radial nerve stimulation. Further, rostromedially the response latencies were longer, 7-9 ms. The latency-to-peak of the evoked responses was also measured and showed a parallel increase. In many cases, inflexions could be observed on the rising phase of the evoked potential, indicating that it consisted of several components. In most experiments, four components (I-IV) were observed. Their mean latencies were 4.3, 6.0, 7.2 and 8.5 ms, respectively. The early components were largest in the caudolateral part of area 4 gamma and decreased when recording further rostromedially such that, in the anterior sigmoid gyrus, usually only component IV could be recorded. The findings suggest that the cat motor cortex is functionally divided into regions which can be identified by their peripheral input latencies.