Smooth ocular pursuit during the transient disappearance of an accelerating visual target: the role of reflexive and voluntary control.

This study examined the extent to which human subjects predict future target motion for the control of smooth ocular pursuit. Subjects were required to pursue an accelerating target (0, 4 or 8 degrees/s2) that underwent a transient occlusion, and consequently reappeared with the same or increased velocity. Presentations were received in a random or blocked order. Subjects exhibited anticipatory smooth pursuit prior to target motion onset, which in blocked presentations was scaled to the velocity generated by the target acceleration. In random presentations subjects also exhibited anticipatory smooth pursuit, but this was reflected in a more generalized response. During the transient occlusion all subjects exhibited a reduction in eye velocity, which was followed in the majority by a recovery prior to target reappearance. In random presentations, eye velocity decayed and recovered to a level that followed on from the response to the initial ramp. In blocked presentations, there was evidence of improved scaling throughout, which culminated in a significant increase in eye velocity between the start and end of the transient occlusion (8 degrees/s2 only). These findings are difficult to reconcile with reflexive accounts of oculomotor control that perpetuate current eye motion, and hence generate a simple form of prediction using a direct efference copy ("eye-velocity memory"). Rather, they are more consistent with the scaling of smooth pursuit eye movements by means of a more-persistent velocity-based representation, which plays a significant role in both random and blocked stimulus presentations.