Abstract
We investigated the influence of distractors on the control of smooth pursuit eye movements. In a series of experiments distractor velocity, direction of motion and distance to target were manipulated to bias active smooth pursuit. Our results suggest an important role of weighted vector averaging as shown for pursuit initiation (Lisberger & Ferrera, 1997), but with slight modifications. A distractor primarily affected those components of the smooth pursuit involved in tracking the target. This implies that smooth pursuit can be selectively disturbed, depending on the smooth pursuit and distractor motion being composed of either horizontal, vertical or both movements. One smooth pursuit velocity component (vertical or horizontal) is increasing, if the relative velocity component between target and distractor is larger than zero, smooth pursuit velocity component is decreasing if the relative velocity component is smaller than zero. If relative velocity component between target and distractor is zero, smooth pursuit will remain nearly unaffected in this component. The velocity alteration is approximately 10 per cent of target velocity. If a distractor is presented along target trajectories, the quantitative influence is larger than if the distractor is not on target trajectories. Our explanation is that after pursuit initiation visual attention becomes an important rule in controlling active smooth pursuit, so it is not only stimulus triggered. It might be possible that visual attention is averaged along the target trajectories rather than on an equable area around the target, so distractors along target trajectories are more salient than others.