Visual tracking in monkeys: evidence for short-latency suppression of the vestibuloocular reflex.

1. Monkeys normally use a combination of smooth head and eye movements to keep the eyes pointed at a slowly moving object. The visual inputs from target motion evoke smooth pursuit eye movements, whereas the vestibular inputs from head motion evoke a vestibuloocular reflex (VOR). Our study asks how the eye movements of pursuit and the VOR interact. Is there a linear addition of independent commands for pursuit and the VOR? Or does the interaction of visual and vestibular stimuli cause momentary, "parametric" modulation of transmission through VOR pathways? 2. We probed for the state of the VOR and pursuit by presenting transient perturbations of target and/or head motion under different steady-state tracking conditions. Tracking conditions included fixation at straight-ahead gaze, in which both the head and the target were stationary; "times-zero (X0) tracking," in which the target and head moved in the same direction at the same speed; and "times-two (X2) tracking," in which the target and head moved in opposite directions at the same speed. 3. Comparison of the eye velocities evoked by changes in the direction of X0 versus X2 tracking revealed two components of the tracking response. The earliest component, which we attribute to the VOR, had a latency of 14 ms and a trajectory that did not depend on initial tracking conditions. The later component had a latency of 70 ms or less and a trajectory that did depend on tracking conditions. 4. To probe the latency of pursuit eye movements, we imposed perturbations of target velocity imposed during X0 and X2 tracking. The resulting changes in eye velocity had latencies of at least 100 ms. We conclude that the effects of initial tracking conditions on eye velocity at latencies of less than 70 ms cannot be caused by visual feedback through the smooth-pursuit system. Instead, there must be another mechanism for short-latency control over the VOR; we call this component of the response "short-latency tracking." 5. Perturbations of head velocity or head and target velocity during X0 and X2 tracking showed that short-latency tracking depended only on the tracking conditions at the time the perturbation was imposed. The VOR appeared to be suppressed when the initial conditions were X0 tracking. 6. The magnitude of short-latency tracking depended on the speed of initial head and target movement. During X0 tracking at 15 deg/s, short-latency tracking was modest. When the initial speed of head and target motion was 60 deg/s, the amplitude of short-latency tracking was quite large and its latency became as short as 36 ms.(ABSTRACT TRUNCATED AT 400 WORDS)