Cerebellar responses to teleceptive stimuli in alert monkeys.

Discharges of single Purkinje cells in the intermediate and lateral zones of the cerebellar cortex and of neurons in the interpositus and dentate nuclei were recorded in alert monkeys during the presentation of intense auditory and visual stimuli. Concomitant monitoring of the electromyogram (EMG) demonstrated that these stimuli evoked characteristic startle responses in most instances. Firing patterns of cerebellar nuclear cells to auditory stimuli could be categorized into four types, the most common of which consisted of a short-latency acceleration of discharge, followed by a decrease in activity, and in most cells by a later period of facilitation. Simple spike discharge patterns of Purkinje cells consisted largely of prolonged increases or decreases in firing rate, although more complex patterns were seen. In almost 50% of the Purkinje cells tested, complex spikes were evoked by the auditory stimuli. Comparison of simple spike responses of Purkinje cells and of the discharges of cerebellar nuclear cells to auditory and visual inputs revealed that, except for a longer latency, the discharge pattern evoked by flash stimuli was identical to that evoked by sound in all instances. By contrast, in about one-third of the Purkinje cells with related complex spike discharge, complex spikes were evoked by stimuli of only a single modality. Comparison of the times of changes in nuclear and Purkinje cell activity suggests that the initial change in nuclear cell discharge was due to an increase in mossy fiber activity, while the subsequent decrease resulted from Purkinje cell inhibition evoked by mossy and climbing fiber inputs. The absence of increases in nuclear cell discharge at the time of most decreases in Purkinje activity indicates that removal of Purkinje inhibition does not have a major effect on the discharge rates of individual nuclear cells. The data also suggest that excitation of nuclear cells via climbing fiber collaterals played only a minor role in influencing their discharge. Since most EMG changes occurred after or at about the same time as the initial changes in cerebellar discharge, it is unlikely that the initial changes in cerebellar activity were a result of feedback from contracting muscles. It is proposed that the similar discharge patterns of cerebellar neurons to auditory and visual input results from a convergence of these inputs on a structure which projects to the cerebellum as mossy fibers.