Effects of fixation target timing on smooth-pursuit initiation.

We measured smooth pursuit and anticipatory smooth eye movements in four normal subjects, using step-ramp stimuli. Between trials, subjects fixated a motionless central target which disappeared before, after, or at the same time as the ramp stimulus appeared. We found that gaps or overlaps in the relative timing of fixation target offset and ramp onset neither expedited nor delayed the initiation of smooth pursuit. In contrast, anticipatory and smooth pursuit eye accelerations were influenced by the presence of a stationary foveal target; both were significantly higher when the fixation stimulus disappeared before ramp onset than when it disappeared after ramp onset.