Spontaneous unit activity of the monkey caudate nucleus under chronic experimental conditions

Abstract

Spontaneous unit activity and changes in its statistical parameters during electrical stimulation of the palms at 25 or 50/min were studied in four monkeys under chronic experimental conditions. Altogether 337 recordings of the activity of 64 neurons for durations of 2 and 3 min were studied. The mean firing rate was relatively low (5.7 spikes/sec). As a rule the unit activity remained stable for several tens of minutes. It could change spontaneously to new patterns and again remain stationary. These transitions took place during recording of spontaneous activity and also during stimulation of the animal, but they were independent of it. Interspike interval histograms were varied and most were polymodal. Their pattern changed with changes in the firing rate of the neurons. The position of the modes along the time axis and the type of distribution were the most conservative characteristics of the histograms. The probability of appearance of intervals was highest at 30, 60, 90, and 210–240 msec. The presence of stable intervals with increased probability of appearance may be the result of the existence of chains of neurons in the caudate nucleus with fixed temporal parameters.