An eye movement analysis of multiple object tracking in a realistic environment

To study multiple object tracking under naturalistic conditions, observers tracked 1-4 sharks (9 in total) swimming throughout an underwater scene. Accuracy was high in the Track 1-3 conditions (>92%), but declined when tracking 4 targets (78%). Gaze analyses revealed a dependency between tracking strategy and target number. Observers tracking 2 targets kept their gaze on the target centroid rather than individual objects; observers tracking 4 targets switched their gaze back-and-forth between sharks. Using an oculomotor method for identifying targets lost during tracking, we confirmed that this strategy shift was real and not an artifact of centroid definition. Moreover, we found that tracking errors increased with gaze time on targets, and decreased with time spent looking at the centroid. Depending on tracking load, both centroid and target-switching strategies are used, with accuracy improving with reliance on centroid tracking. An index juggling hypothesis is advanced to explain the suboptimal tendency to fixate tracked objects.