The role of prediction in head-free pursuit and vestibuloocular reflex suppression

In a recent experiment, the predictive mechanisms of the head-fixed ocular pursuit reflex were revealed by using the technique of repeated transient stimulation to show the changes in the temporal characteristics of the response.' Subjects were instructed to follow the motion of a constant-velocity target during repeated brief periods of stimulation that were separated by periods of darkness. Smooth eye velocity was observed to build up to an asymptotic level over the first three or four presentations of the target, while simultaneously becoming more phase advanced with respect to the onset of target illumination. When the target suddenly and unexpectedly changed velocity or frequency, an inappropriate predictive eye velocity trajectory was initiated that was highly correlated in peak velocity and timing with the characteristics of the preceding part of the stimulus. We have now used the same technique to examine the response during head-free pursuit of a moving target in order to establish whether the same predictive mechanisms govern the coordination of head
and eye movements.