Second somatosensory cortical area in macaque monkeys. I. Neuronal responses to controlled, punctate indentations of glabrous skin on the hand.

Responses of 492 single neurons from the second somatosensory cortical area of macaque monkeys were studied using computer controlled ramp indentations. Results were obtained using chronic recording techniques from lightly tranquilized or awake animals. Amongst those cells that were activated by punctate tactile stimuli, various subclasses of responses were identified that included neurons with phasic and sustained adaptation characteristics. In addition, several cells showed unusual firing patterns, such as delayed responses and reverberating afterdischarges. Latency measurements from 90 cells showed a modal latency to ramp stimuli of 34 ms and a second group of cells whose latency exceeded 75 ms. Measurements of response functions to different velocities of indentation revealed that some cells maintained relatively shallow ascending functions but that most cells were insensitive to the velocity variable. The response characteristics of these neurons in primates are discussed in reference to the hypothesis that in the somatosensory cortex, SII occupies a higher order, serially dependent region in a hierarchy from SI to other parts of the brain.