Analysis of somato-sensory, auditory and visual averaged transcortical and scalp responses in the monkey

Abstract

Averaged evoked responses (AERs) to median nerve, auditory and photic stimulation were recorded with scalp vertex-to-nose or vertex transcortical electrodes in forty-five unanesthetized squirrel monkeys. Potentials were designated as follows: P 1 (11–14 msec), N 1 (16–30 msec), P 2 (36–115 msec), N 2 (92–140 msec), P 3 (158–160 msec) and N 3 (200–270 msec). The AERs of all three modalities were compared with respect to peak amplitudes and wave shapes in the same and different monkeys, and a characteristic pattern for each modality was constructed from the total group. The photic AER differed clearly from the other two, which were similar. The referentially recorded vertex-to-nose AER was identical to the single flash electroretinogram. There was no response to flash when the AER was recorded transcortically at the vertex. Transcortical recordings were made of AERs from 202 cortical points in twenty-one of the forty-five monkeys using shock, click and photic stimuli. The frontal cortex and vertex produced locally generated long latency waves to shock and click but not to flash. Only the occipital cortex was productive of true photic responses. The study of long latency AERs from small brained animals, such as the squirrel monkey and cat, is greatly simplified if bipolar electrodes are used to record across small distances. This method helps to eliminate the confusion that can arise from the unwanted addition of potentials arising from generators away from the presumed recording site.