hemisphere in principle capable of tracking objects in both the left and right visual hemifields. In practice, due to competitive inhibition, each hemisphere is mainly (but not entirely) responsible for tracking targets in the contralateral visual hemifield, thereby creating the hemifield advantage observed in our experiments [35]. The first stage feeds the target locations to the second stage. The second stage builds on this information by continuously associating a unique identity with each target. We propose that this binding process is achieved by a central serial mechanism, shared between the two hemispheres, in a manner similar to that of MOMIT [24]. (The second stage differs from MOMIT in that it is explicitly informed of the target locations, so does not need to segregate the targets from the distractors.) Each target would be attended to in turn and its identity remembered. When it is time to reattend a given target, it would be assumed that whichever target is closest to that target’s previously remembered location is the target. If this is not the case, then an error will be made and the wrong identity would be associated with the wrong target. Since the second stage builds upon the output of the first stage, it is entirely possible for the second stage to make an error even though the first stage performed correctly. If this were to happen, the observer would be able to successfully segregate the targets from the distractors, but would not be able to differentiate the targets from each other. In practice, this is a common occurrence [19]. This two stage model can also explain why hemifield effects are larger for MOT than MIT. When observers are asked to perform MOT there is no need to associate unique identities with the References 1. Pylyshyn ZW, Storm RW (1988) Tracking multiple independent targets: evidence for a parallel tracking mechanism. 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Morey RD (2008) Confidence intervals from normalized data: A correction to Cousineau (2005). Tutorial in Quantitative Methods for Psychology 4: 61–64. PLOS ONE | www.plosone.org 8 August 2012 | Volume 7 | Issue 8 | e43796