Position tracking and identity tracking are separate systems: Evidence from eye movements

How do we track multiple moving objects in our visual environment? Some investigators argue that tracking is based on a parallel mechanism (e.g., Cavanagh & Alvarez, 2005; Pylyshyn, 1989), others argue that tracking contains a serial component (e.g. Holcombe & Chen, 2013; Oksama & Hyönä, 2008). In the present study, we put previous theories into a direct test by registering observers' eye movements when they tracked identical moving targets (the MOT task) or when they tracked distinct object identities (the MIT task). The eye movement technique is a useful tool to study whether overt focal attention is exploited during tracking. We found a qualitative difference between these tasks in terms of eye movements. When the participants tracked only position information (MOT), the observers had a clear preference for keeping their eyes fixed for a rather long time on the same screen position. In contrast, active eye behavior was observed when the observers tracked the identities of moving objects (MIT). The participants updated over four target identities with overt attention shifts. These data suggest that there are two separate systems involved in multiple object tracking. The position tracking system keeps track of the positions of the moving targets in parallel without the need of overt attention shifts in the form of eye movements. On the other hand, the identity tracking system maintains identity-location bindings in a serial fashion by utilizing overt attention shifts.