Influence of predictability on control of extra-retinal components of smooth pursuit during prolonged 2D tracking.

We compared pursuit responses to 2D target motion in three separate conditions: predictable, randomised and randomised with timing cues. The target moved on a continuous quadrilateral path in which right-angle direction changes allowed anticipatory eye acceleration and deceleration in orthogonal axes to be assessed. Results indicated that whether the timing of direction changes was random or predictable, anticipatory acceleration, initiated by extra-retinal mechanisms, occurred in the new direction at approximately the same time as anticipatory deceleration in the terminating direction, but deceleration was of greater magnitude than acceleration. When path duration was randomised within a range of durations, the timing of acceleration and deceleration was almost constant irrespective of actual ramp duration but was dependent on the mean duration of the range. When ramp duration was predictable both deceleration and acceleration increased, the latter allowing peak velocity to be attained earlier than when randomised. When timing cues were given at a fixed time prior to direction change in randomised stimuli, this also resulted in higher anticipatory acceleration/deceleration. When both duration and velocity of sequential ramps were randomised, deceleration was dependent on target velocity, but acceleration remained constant. Altogether these findings show that although acceleration and deceleration in orthogonal axes occur almost simultaneously and are similarly affected by predictability, control of their magnitude is relatively independent. We suggest that deceleration and acceleration result from the switching off and on, respectively, of retinal and extra-retinal oculomotor components prior to direction change, with dynamics dependent on predictability of stimulus magnitude and timing.