A comparison of predictive and nonpredictive ocular pursuit under active and passive stimulation conditions in humans.

A technique has been developed for the comparison of predictive and nonpredictive ocular pursuit in human subjects, with the objective of estimating the contribution made by predictive processes to the normal pursuit response. Subjects were presented with a target moving at constant velocity in the horizontal plane and instructed to actively pursue the target or to passively stare at it. In the predictive mode (PRD), a step-ramp stimulus with velocity ranging from 12.5 to 50 degrees/s was presented at regular intervals of 1.728 s, in alternating directions, with target exposure durations (PD) that varied from 80 to 640 ms. In the interval between presentations, subjects were in complete darkness. In the nonpredictive mode (RND), similar step-ramp stimuli were presented but with randomized direction and timing of target exposure. In the nonpredictive mode, during both active and passive stimulation, the smooth component of eye velocity was initiated after a mean delay of 125 ms. In the predictive mode, eye velocity started to build up well before target onset, even during passive stimulation. It was found that the time of initiation of this anticipatory response was closely associated with the time at which the target would have changed direction even though the target could not be seen at this time. Eye velocity measured 100 ms after target onset was negligible in the nonpredictive mode, whereas in the predictive mode, it progressively increased with target velocity, reaching a maximum of 18 degrees/s when target velocity was 50 degrees/s and PD was greater than 240 ms. Examination of the difference in the eye velocity trajectories for the predictive and nonpredictive modes indicated that the greatest contribution of prediction occurred approximately 150 ms after target onset and its effects were evident in the predictive response for at least 300 ms. This effect was reflected in the reaction time between target onset and the occurrence of peak eye velocity. In the nonpredictive mode, this progressively increased from 250 ms to 400 ms as PD increased from 80 to 640 ms, whereas in the predictive mode peak velocity occurred an average of 50 ms earlier for all values of PD. The results demonstrate the significant contribution that predictive processes make to normal ocular pursuit behavior and the importance of timing control in this process. They also indicate that this process is not dependent on volitional control, but can be seen as an automatized response during passive stimulation.