Spatial and temporal aspects of cognitive influences on smooth pursuit.

It is well known that prediction is used to overcome processing delays within the motor system and ocular control is no exception. Motion extrapolation is one mechanism that can be used to overcome the visual processing delay. Expectations based on previous experience or cognitive cues are also capable of overcoming this delay. The present experiment was designed to examine how smooth pursuit is altered by cognitive information about the time and/or direction of an upcoming change in target direction. Subjects visually tracked a cursor as it moved at a constant velocity on a computer screen. The target initially moved from left to right and then abruptly reversed horizontal direction and traveled along one of seven possible oblique paths. In half of the trials, a cue was present throughout the trial to signal the position (as well as the time), and/or the direction of the upcoming change. Whenever a position cue (which will be referred to as a timing cue throughout the paper) was present, there were clear anticipatory adjustments to the horizontal velocity component of smooth pursuit. In the presence of a timing cue, a directional cue also led to anticipatory adjustments in the vertical velocity, and hence the direction of smooth pursuit. However, without the timing cue, a directional cue alone produced no anticipation. Thus, in this task, a cognitive spatial cue about the new direction could not be used unless it was made explicit in the time domain.