condition resources from both hemispheres were used. Thus, our results show that tracking is only partially independent in the left and right visual hemifields and the degree of independence is greater for MOT than MIT. MOMIT [24] is currently the only model of MIT. It proposes that identity-location bindings require a mechanism that serially cycles through each target, only ever attending to one target at a time. Every time a target is attended, its location is memorised. When it is time to re-attend to a given target the model assumes that whichever object is closest to the target’s previously remembered position is the target. The more targets there are, the longer it will take a given target to be re-attended, and hence the more chance that an error will occur. In this way, the model can explain why accuracy decreased with increasing set size, and hence why in all our experiments accuracy was greater in the baseline condition than in the other two conditions. Because the model assumes that there is just a single attentive resource responsible for tracking all the objects in the visual field, it predicts no difference in tracking accuracy as a function of target arrangement. In particular, it predicts that tracking accuracy would be equal in the bilateral and unilateral conditions, contrary to the data of Experiment 1–3. One could envisage a double serial model, in which there is a separate serial tracking mechanism for each hemifield. Assuming that the two tracking mechanisms operated independently, this could explain the bilateral advantage observed in our experiments. Unfortunately, this model would predict complete independence in tracking in the two hemifields so would predict that accuracy in the baseline condition would necessarily be equal to that in the bilateral condition. In none of our experiments was this observed. Experiment 4 investigated an MOT stimulus. There have been a large number of models of MOT (for reviews see [6,32]). For example, the FINST model of Pylyshyn and Storm [1] suggests that each target is tracked by a mental pointer known as a FINST. Because these FINSTs operate independently, the arrangement of the targets should not affect tracking accuracy. Thus, this model cannot explain why in Experiment 4 tracking accuracy was greater in the bilateral arrangement than in the unilateral arrangement. The FLEX model of Alvarez and Franconeri [12] is similar to the FINST model in that it also assumes that each target is tracked by a mental pointer, although each pointer can be flexibly allocated a variable amount of a mental resource based on current tracking demands. The pointers are referred to as FLEX’s and each FLEX draws upon a common limited resource pool. The less resource the FLEX receives, the less able it is to track its target. The FLEX model assumes that there is a single, shared resource that all FLEXs draw upon, regardless of the arrangements of the targets. As such, the FLEX model must also incorrectly predict that tracking accuracy would be equal in the bilateral and unilateral arrangements in Experiment 4. Franconeri and colleagues [14,33] have proposed an alternative theory of MOT. According to their theory, tracking errors are primarily caused by spatial interference between targets, rather than any spatial interference between the targets and distractors, the speed at which the targets move, or the duration of movement. As long as there is adequate spacing between targets, they posit that we could theoretically track an unlimited number of them [33]. Of crucial relevance to the current study, spatial interference between two targets is assumed to occur less when they are in different hemifields than when they are in the same hemifield. This theory can accordingly explain why tracking is easier when targets are arranged bilaterally versus unilaterally. Holcombe and Chen [34] have recently challenged the assumption that spatial interference between targets is the main Hemifield Effects in Multiple Identity Tracking cause of tracking errors. Their display had two pairs of disks that rotated about a common central axis, rather than moving freely. Each pair contained a single target. The inner pair moved on a circular track that had a radius of 2u and the outer pair moved on a circular track with a larger radius of either 4u or 9u. Tracking performance did not change with the radius of the outer pair, even though this caused the separation between the two targets to vary. Holcombe and Chen’s finding therefore challenges Franconeri and colleagues’ [33] proposition that tracking errors are always caused by spatial interference between the targets. Battelli, Alvarez, Carlson, and Pascual-Leone [35] have pro- posed an alternative explanation for the bilateral advantage in object tracking. They used transcranial magnetic stimulation (TMS) to temporarily disrupt neural activity unilaterally in the intraparietal sulcus (IPS), an area that has been identified as strongly associated with object tracking in imaging studies [36,37]. Tracking performance for targets in the visual hemifield contra- lateral to the site of the TMS interference decreased when targets were presented bilaterally. Conversely, when targets were pre- sented unilaterally – regardless of whether they were ipsilateral or contralateral to the TMS disruption – there was no effect on tracking accuracy. The authors concluded that the left and right IPS have a contralateral tracking bias, but have the ability to track objects in both the contralateral and ipsilateral visual hemifields. Battelli and colleagues explained this as a process of mutual competition between the left and right IPS counterparts. According to this logic, the left and right IPS inhibit each other, so that each predominantly tracks objects in the contralateral hemifield. Interfering with IPS activity in one hemisphere necessarily removes its inhibitory influence on the other IPS. Provided that the targets are presented unilaterally, this thereby allows the uninhibited IPS to track targets in either the contralateral or ipsilateral visual hemifield. When targets are presented bilaterally, however, the uninhibited IPS will display its contra- lateral bias at the expense of the ipsilateral targets. Battelli and colleagues’ [35] hypothesis can readily explain why it is easier to track objects bilaterally than unilaterally. It can also explain why tracking is not completely independent in the left and right hemifields: to some extent the IPS can track objects ipsilaterally. While this is a compelling hypothesis, it was developed only for MOT and needs to be extended before it can be applied to MIT and thus be able to explain all of our results. All existing models of object tracking have been about either MOT (e.g. FLEX; [12]) or MIT (e.g. MOMIT; [24]). There are, however, obvious similarities between the two tasks: both require observers to sustain attention on multiple objects as their locations change. Indeed, Cohen and colleagues [22] have found evidence of a common resource for the location and identity aspects of the MIT task. They found that participants were able to choose to prioritise either the location or the identity aspect of an MIT task, and that prioritising one ultimately degraded performance on the other. This finding supports the idea that object tracking may be best conceptualised using a generalised model that addresses both MOT and MIT. We therefore propose a working hypothesis for a combined model of MOT and MIT. We envisage a two-stage model, involving a first stage that segregates the targets from the distractors (i.e. MOT) and a second stage that associates a unique identity with each target (i.e. MIT). In the first stage, targets are segregated from distractors via multifocal attention, in a manner similar to that described by the FLEX model [12]. Unlike the original FLEX model, we assume that tracking operates separately in the left and right cerebral hemispheres, with each cerebral PLOS ONE | www.plosone.org 7 August 2012 | Volume 7 | Issue 8 | e43796