Contribution of y group of vestibular nuclei and dentate nucleus of cerebellum to generation of vertical smooth eye movements.

Abstract

The role of the y group and dentate nucleus in the production of vertical smooth eye movements in response to visual and vestibular stimuli was investigated by recording extracellular single-unit activity and applying electrical stimulation in this region of alert rhesus macaques. The monkeys tracked a small visual target while undergoing passive sinusoidal whole-body rotation (0.2-1.0 Hz). In the y group, dentate nucleus and adjacent white matter, 51 neurons whose activity was correlated with upward head and eye velocity (UHEV) were found. If the rotating monkey fixated a target moving with him so that his eyes remained stationary in his head and his vestibuloocular reflex (VOR) was suppressed, the UHEV units fired approximately in phase with an in proportion to head velocity. During sinusoidal smooth pursuit, > 2/3 of the UHEV units fired approximately in phase with an in proportion to eye velocity. The activity of .apprx. 1/3 of the UHEV units was related to the position and eye velocity. For each UHEV unit lacking eye position sensitivity, the sensitivities to head and eye velocity were similar, but usually the eye velocity sensitivity was greater. During combined head and eye movement the activity of an UHEV unit was the sum of its activity during each alone. When the head and eyes moved equal amounts in opposite directions there was a marked reduction or absence of modulation. During horizontal smooth pursuit, 32% of the 25 UHEV units tested showed no significant modulation. Only 2 units (8%) showed greater modulation during horizontal than vertical smooth pursuit. During saccadic eye movements most UHEV units gave bursts of activity, especially for upward saccades. The only other unit type commonly encountered among the UHEV units had activity related to downward eye position; its discharge was not modulated during rotation in the sagittal plane when the VOR was suppressed. These 14 units were called down-eye-position units; peculiarities in their firing behavior suggest that they may respond best under untested stimulus conditions. The down-eye-position units were recorded mainly in the dentate nucleus. Electrical stimulation applied at sites were UHEV units had been recorded elicited upward smooth eye movements. The eye velocity increased with stimulus pulse frequency. Effective sites were localized to the region containing UHEV units, and stimulation at sites dorsal and ventral to this region failed to elicit upward eye movements. A model is suggested in which UHEV units receive inhibitory modulation from downward gaze velocity Purkinje cells of the flocculus and excite neurons in the oculomotor nucleus. The UHEV signal, when combined with the signal reaching oculomotor neurons via the medial longitudinal fasciculus, could produce the smooth eye movement responses to visual and vestibular stimuli.