Abstract
Motion perception relies on two fundamentally different motion systems. The energy-based system relies on early direction-selective neurons that automatically and pre-attentively extract motion within their receptive fields. The high-level tracking system rather relies on attentively tracking the position of an object. Two types of apparent motion stimuli were used to investigate the two motion systems. Since early direction-selective neurons only operate within a short temporal window, a temporal gap of 100 msec between the two frames was used to probe the tracking system. No temporal gap was used to probe the energy-based system. In order to explore the role of attention on both systems, the attentional resources dedicated to the stimuli were systematically manipulated using a cueing paradigm. Eight dots simultaneously appeared uniformly distributed on an annulus with a radius of 5 degrees of visual angle. Each dot was displaced in a different random direction (up, down, right or left) in a 2-frame sequence. Observers were asked to report the displacement direction of the randomly selected dot (the target) selected by a central cue. The level of attentional resources dedicated to the target was manipulated by varying the timing of the central cue relative to the offset of the target. Results demonstrated that manipulating attention had a similar effect on both types of stimuli: percentage of correct answers was near perfect when the central cue appeared before the target onset and gradually declined when the cue appeared with or after the target). This suggests that attention plays a similar role in both systems. As energy-based processing is pre-attentive, these results propose that attention plays a minor role in tracking and that the drop in performance with a late cue might not be due to motion processing per se. Instead, it could be related to iconic memory.