Human ocular pursuit during the transient disappearance of a visual target.

During the course of pursuing a moving target there are often periods of transient disappearance as it moves behind objects and surfaces. In experimental settings, eye velocity decays rapidly on the extinction of a moving target. However, eye velocity does not decay to zero if there is an expectation the target will reappear further along its trajectory. Increasing eye velocity to coincide with target reappearance could minimize the developing velocity error, but it remains to be empirically verified whether this can be achieved. The aim of the present study was to examine the influence of stimulus predictability, target velocity, and interstimulus interval (ISI) on ocular pursuit during the transient disappearance of a visual target. We confirmed that subjects (n = 9) did not maintain eye velocity close to target velocity for the duration of the ISI. In general, after an initial reduction in eye velocity the majority of subjects (n = 7) exhibited a significant increase before target reappearance. The timing of the velocity increase was not influenced by target velocity, stimulus predictability, or ISI. Consequently, for the 900-ms ISI the increase occurred too early and the eye was decelerating at the moment of target reappearance. These results are consistent with a reduction in gain being applied to the visuomotor drive when the target disappeared, followed by a reactivation in expectation of target reappearance. We modeled this process such that gain was modulated within a reafferent feedback system, hence preserving its output in the absence of negative visual feedback and enabling an anticipatory increase in eye velocity before expected target reappearance.