SOME DISCHARGE CHARACTERISTICS OF SINGLE NEURONS IN THE INFERIOR COLLICULUS OF THE CAT. II. TIMING OF THE DISCHARGES AND OBSERVATIONS ON BINAURAL STIMULATION

Abstract

This, the second paper of a pair, emphasizes the timing of unitary discharges the effects produced by binaural stimulation when single units in the inferior colliculus of anesthetized cats were studied with tone and click stimuli. Many neurons discharge an early onset spike; some produce such a spike only under certain conditions; and some start to discharge late. If an early spike is consistently discharged, its latent period is a function of stimulus intensity and often a function of stimulus frequency. It is concluded that different stimuli within the effective frequency-intensity domain may activate a neuron at systematically different times. A tone which is excitatory upon stimulation of one ear may produce inhibitory effects if presented bilaterally. Inhibition may affect the entire firing pattern of a neuron, or affect preferentially either the onset burst or the spikes generated during the period of sustained firing. There is some evidence that an inhibitory stimulus may cause also some excitatory events. The degree of facilitation produced by binaural stimulation with low frequency tones varies, at a given frequency and intensity, depending upon whether the stimuli are in or out of phase and thus displaced in time by a fraction of a millisecond with respect to each other. It is suggested that the degree of the resulting interaction depends upon small differences in timing of the activating impulses.