THE INITIAL VESTIBULOOCULAR REFLEX AND ITS VISUAL ENHANCEMENT AND CANCELLATION IN HUMANS

The gain (ratio of eye velocity to head velocity) of the initial horiz ontal vestibule-ocular reflex (VOR) was calculated in 12 normal subjec ts over 350 ms during impulsive, unpredictable whole body rotation und er three conditions: (1) darkness; (2) visual enhancement of the VOR, while the subjects fixated a stationary target; and (3) visual cancell ation of the reflex, while subjects fixated a target that rotated with the head. The gain of the initial 80 ms of compensatory eye movement increased significantly during visual fixation in 5 subjects and decre ased during attempted VOR cancellation in 3 subjects, when compared wi th VOR gain in darkness. Compensatory vestibular smooth eye movements were slowed, becoming curved at the onset of VOR cancellation, at mean latencies ranging from 78 to 149 ms in individual subjects (group mea n 128 ms). At about 190 ms, quick phases moved the eyes in the same di rection as head and target motion. The subsequent vestibular eye movem ents were about 50% slower than the initial smooth eye movements, indi cating more effective cancellation. Visual enhancement of the VOR can occur prior to the onset of pursuit, providing evidence that fixation and smooth pursuit are distinct ocular motor systems. Visual cancellat ion of the VOR also begins prior to smooth pursuit initiation and beco mes more effective after the latency of smooth pursuit.