Response patterns of primary vestibular neurons to thermal and rotational stimuli

Abstract

The neural responses of vestibular ganglion cells to quantified thermal and rotational stimulation were studied in anesthetized squirrel monkeys. All cells exhibited a spontaneous discharge rate with the average being 72 ± 27.8 spikes/sec. The observed increases and decreases in discharge rates during sinusoidal angular rotation were consistent with the morphological polarization patterns of the hair cells. Likewise, the ganglion cell responses resulting from thermal stimulation by a controlled temperature air stream directed on the lateral portion of the horizontal canal were in agreement with those expected on the basis of Barany's convective current theory and the polarization pattern of the hair cells. In an attempt to determine the relationship between thermal and rotational stimuli, the sensitivities of individual cells to both types of stimuli were determined. Those cells most sensitive to angular rotation were also most sensitive to caloric stimulation. An equivalency value for the two types of stimuli was defined and found to be 1.84 °C/radian·sec −1. The global response characteristics of the vestibular endorgan were studied by simultaneous thermal and rotational stimuli. The thermal stimulus was used to bias the cupula at a new equilibrium position while a rotational stimulus was used to determine the differential sensitivity at that cupular position. For the cells with a more regular spontaneous discharge rate, the differential sensitivity was observed to be a function of the instantaneous discharge rate. No such relationship was observed for the more irregular cells.