Combined smooth and saccadic ocular pursuit during the transient occlusion of a moving visual object

Accurate ocular pursuit during a transient occlusion interval would minimize retinal position and velocity error, and could provide an advantage when discriminating object characteristics at reappearance. This study was designed to examine how the smooth and saccadic response extrapolates the trajectory of a moving visual object during a transient occlusion. We confirmed that subjects could not maintain unity gain smooth pursuit during the transient occlusion. Eye velocity decayed significantly without visual feedback but then in the majority of subjects, there was a recovery that brought eye velocity back towards object velocity. However, eye velocity did not increase to a level that eliminated the developing position error. Subjects corrected for the resulting error in eye position by releasing saccades that generally placed the eye ahead of the occluded object's extrapolated position. The majority of saccadic correction occurred between 220 and 600 ms of the occlusion interval, and when combined with the smooth response enabled accurate pursuit of a 10 degrees/s object for up to 1,200 ms of occlusion. The lack of saccadic correction after 600 ms of occlusion combined with the reduced eye velocity resulted in significant undershoot of eye position at the moment of object reappearance when pursuing an 18 degrees/s object. We suggest that extra-retinal information regarding eye velocity and smooth eye displacement could be available from a continually updating efference copy of eye motion in MST, whereas a veridical representation of extrapolated object velocity and displacement could be obtained from persistent activity in FEF.