Predictive disjunctive pursuit of virtual images perceived to move in depth

Human and nonhuman primates predictively use smooth pursuit and saccades to track visual targets that move in a fronto-parallel plane. This behaviour is believed to be facilitated by short-term memory of the target motion and/or an efference copy of the subject's motor effort. Subjects in our experiments tracked dichoptically viewed targets that appeared to move vertically, right or left and towards the subject. The virtual image of the target was tracked using disjunctive smooth pursuit and saccades. To reveal predictive tracking, targets were blanked 100 ms after the onset of motion for intervals of 800 ms. During the blanked interval, subjects initiated pursuit and predictively tracked the unseen virtual image using memory guided eye movements. Our data are consistent with recent electrophysiological studies that describe cells that encode target or eye movements in depth when a target is briefly blanked but pursuit is maintained. However, predictive pursuit of a virtual target with disjunctive eye movements poses a challenge for understanding how a short-term memory store might encode the desired eye movement, its coordinate frame, and how it is transformed into motor commands.