PATTERNS OF ACTIVITY IN SINGLE CORTICAL UNITS FOLLOWING STIMULATION OF THE DIGITS IN MONKEYS

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Abstract
The discharge evoked in single units of the somatosensory cortex by mechanical and electrical stimulation of the forelimb were studied in Macaca mulatta. The response of a unit varied. In general, units fired only after peripheral stimulation. Most units responded at about the peak positivity of the surface primary evoked response; a few fired during subsequent repetitive waves. No unit fired during both the primary evoked and the generalized secondary response. The probability of unit discharge was a joint function of the intensity of the stimulus and its position in the unit''s receptive field. The size of the receptive field varied directly with stimulus intensity. Response latency was a function of stimulus locus and strength. Mean latencies usually decreased as stimulus strength increased. Latency dispersion was least when a unit was fired from the peripheral point giving the shortest mean latency. The number of spikes per discharge was a function of stimulus intensity; in a few units the mean number of spikes was unchanged unless the stimulus was very weak. The interspike intervals tended to increase irregularly later in the series. In 3 units firing at a low tonic rate, cutaneous stimulation blocked the discharge for about 100 milliseconds. The unit then resumed discharging at a greater than normal rate, and after about 80 msec, returned to its normal rate. In 40 units the evoked discharge was prevented by prior or simultaneous stimulation at a nearby peripheral point, even one incapable of firing the unit. The duration of this inhibitory effect was proportional to the intensity of the testing stimulus and the spatial separation of the stimuli. This effect is not a result of sharing at the cortical unit. In 7 units, facilitation occurred when two separate peripheral points were stimulated simultaneously or successively, but was of much shorter duration than the inhibitory phenomenon.Authors.