Behavior of horizontal semicircular canal afferents in alert monkey during vestibular and optokinetic stimulation.

The behavior of single vestibular nerve fibers from the lateral semicircular canal was recorded during sinusoidal oscillations of the head, during optokinetic stimulation with the head stationary, and during spontaneous oculomotor behavior in the alert monkey. The response of similar fibers to adequate vestibular stimulation was also studied in some of the animals under deeply anesthetized conditions. In the alert animals all units were spontaneously active and their discharge was modulated only by adequate vestibular stimulation. Ipsilateral horizontal rotations of the head were excitatory for all units. No modification of this basic vestibular response by visual stimulation including full-field striped drum rotation was observed. Furthermore no correlation of unit activity with oculomotor function including voluntary saccadic and pursuit eye movements was found in any of the units. The regularity of spontaneous discharge was the most consistent characteristic that differentiated the unit response into types. Most units were very regular in discharge, but a few were very irregular. The averaging of unit discharge over several cycles of oscillatory head rotation showed that the irregular type units were also consistently modulated by adequate vestibular stimulation. Both regular and irregular type units were found in the anesthetized animals. Unimodal distributions of the quantitative values for unit resting discharge rate, sensitivity, and phase relationship were found. The distributions for these three parameters were similar in the units recorded in the anesthetized animals. Thus at least these characteristics of semicircular canal response seem not to be affected by the vestibular efferent system which should be altered or eliminated in the case of the anesthetized animals.