Context-dependent smooth eye movements evoked by stationary visual stimuliin trained monkeys

The appearance of a stationary but irrelevant cue triggers a smooth eye mov ement away from the position of the cue in monkeys that have been trained e xtensively to smoothly track the motion of moving targets while not making saccades to the stationary cue. We have analyzed the parameters that regula te the size of the cue-evoked smooth eye movement and examined whether pres entation of the cue changes the initiation of pursuit for subsequent steps of target velocity. Cues evoked smooth eye movements in blocks of target mo tions that required smooth pursuit to moving targets, but evoked much small er smooth eye movements in blocks that required saccades to stationary targ ets. The direction of the cue-evoked eye movement was always opposite to th e position of the cue and did not depend on whether subsequent target motio n was toward or away from the position of fixation. The latency of the cue- evoked smooth eye movement was near 100 ms and was slightly longer than the latency of pursuit for target motion away from the position of fixation. T he size of the cue-evoked smooth eye movement was as large as 10 degrees/s and decreased as functions of the eccentricity of the cue and the illuminat ion of the experimental room. To study the initiation of pursuit in the wak e of the cues, we used bilateral cues at equal eccentricities to the right and left of the position of fixation. These evoked smaller eye velocities t hat were consistent with vector averaging of the responses to each cue. In the wake of bilateral cues, the initiation of pursuit was enhanced for targ et motion away from the position of fixation, but not for target motion tow ard the position of fixation. We suggest that the cue-evoked smooth eye mov ement is related to a previously postulated on-line gain control for pursui t, and that it is a side-effect of sudden activation of the gain-controllin g element.