Abstract
In multiple object tracking (MOT), observers track a subset of haphazardly moving and featurally-identical objects. The fact that MOT is possible in the first place is often taken as implicit evidence that the moving targets are all attended in parallel, but there has never been a direct experimental test of this critical hypothesis. We tested this possibility using novel combinations of MOT and probe detection. In the MultiProbe task, observers detected small simultaneous 80ms probes that appeared in the centers of targets once on each trial. In particular, observers determined whether there were as many probes as targets, or one fewer. Performance was well above chance when tracking multiple targets among an equal number of distractors — an ability that would be impossible without simultaneous access to each of the targets. In the Slot-Machine task, observers tracked three targets among three distractors, but these objects were not identical. Instead, each objects color changed every 250ms, and each object possessed a distinct color throughout a trial (ensuring that tracking was still necessary for target identification). At one key moment in two thirds of trials, however, either two or three of the targets' colors momentarily matched — as in the congruence of wheels on a slot-machine (jackpot!). Observers readily determined whether the match involved two, three, or none of the targets — an ability that would not be possible without sustained parallel access to each of the targets' features. (This result is also striking due to the fact that observers generally have very poor access to objects' surface features during MOT.) This novel slot-machine method can also be used to directly explore the extent to which these parallel resources are divided between noncontiguous regions of the display. Overall, this work begins to reveal the underlying attentional dynamics that make MOT possible.