Firing behavior of brain stem neurons during voluntary cancellation of the horizontal vestibuloocular reflex. I. Secondary vestibular neurons.

1. The single-unit activity of vestibular neurons was recorded in alert squirrel monkeys. The monkeys had been trained to track a small visual target by generating smooth pursuit eye movements and to cancel their vestibuloocular reflex (VOR) by fixating a head stationary target. The monkeys were seated on a vestibular turntable, and their heads were held in the plane of the horizontal semicircular canals. The responses of 45 type I vestibular neurons whose activity was related to ipsilateral horizontal head movements were recorded. In 19 of 30 cells tested, electrical stimulation (0.1-ms monophasic pulses, < or = 800 microA) of the ipsilateral vestibular nerve evoked a spike at a monosynaptic latency (0.7-1.3 ms). 2. The spiking behavior of each cell was recorded during several behavioral paradigms: 1) spontaneous eye movements, 2) horizontal smooth pursuit of a target that was moved sinusoidally +/- 20 degrees/s at 0.5 Hz, 3) horizontal VOR during 0.5-Hz sinusoidal turntable rotations +/- 40 degrees/s (VORs), and 4) voluntary cancellation of the sinusoidal VOR by fixation of a head-stationary target during 0.5-Hz sinusoidal turntable rotation at +/- 40 degrees/s in the light (VORCs). 3. The response of most (34) of the units was recorded during unpredictable 100-ms steps in head acceleration (400 degrees/s2) that were generated while the monkey was fixating a target light. The acceleration steps were generated either when the monkey was stationary (VORt paradigm) or when the turntable was already rotating, and the monkey was canceling its VOR (VORCt paradigm). Smaller eye movements were evoked when the acceleration step was generated during VOR cancellation. 4. Type I vestibular units were grouped into two classes on the basis of the relationship of their firing rate to eye movements. The discharge rate of 20 "pure vestibular" units was not clearly related to eye movements. The remaining 25 units were classified as position-vestibular-pause (PVP) neurons. PVP neurons increased their firing rate during contralateral eye movements and during ipsilateral turntable rotations, and paused during saccadic eye movements. 5. Most (17/20) pure vestibular neurons generated the same response to vestibular stimuli when the monkeys canceled their VOR as they did during the VOR in both the sinusoidal and acceleration step paradigms. 6. The head velocity sensitivity of most (19/24) PVP neurons was reduced by 20-60% during VORCs, compared with their response during the VORs. The PVP neurons whose sensitivity of head movements was reduced during VORCs also exhibited a reduced vestibular sensitivity during VORCt.(ABSTRACT TRUNCATED AT 400 WORDS)