duration of movement and are then masked at the end of the trial (e.g. [16–19]). While this might initially seem like a trivial distinction, MIT may entail fundamentally different processing to MOT. While both tasks require objects to be tracked as their locations change, MIT additionally requires objects’ unique identities to be bound to their locations. Thus, the observer must solve the ‘binding problem’ [20]. Maintenance of identity-location bindings has been conceptualised as an effortful process that requires sustained attention [21]. This has led some authors to conclude that two separate tracking systems are required, one for identities and another for locations [16], while others have found evidence in support of a common-resource model [17,22]. Crucially, the mechanism that solves the binding problem has classically been considered a serial mechanism, meaning that each object must be attended to sequentially to refresh its identity- location binding [20,23]. Indeed, the only model of MIT to date, the Oksama and Hyo¨ na¨ [24] model of multiple identity tracking (MOMIT), posits that tracking unique objects cannot occur in parallel. Oksama and Hyo¨ na¨ argue that binding identity to location is a demanding task that can only be achieved via a serial switching mechanism that cycles through the tracked targets, attending to each one in turn. The identity-location bindings are held in an episodic buffer and each binding deteriorates if attention takes too long to return to a target. MOMIT [24] accordingly predicts that the bilateral advantage observed by Alvarez and Cavanagh [4] when observers were required to track identical objects (i.e. MOT) will not exist when observers are required to track unique objects (i.e. MIT) because it is indicative of parallel processing. The model proposes that for MIT, tracking performance is limited by a single resource that is shared between the two hemispheres. Thus, there should be no difference in tracking performance regardless of whether all the targets are located within one hemifield or divided between two hemifields. Although this is a bold prediction, it accords with Alvarez and Cavanagh’s suggestion ([4], p. 642) that hemifield independence would not occur in tasks that involve attending to identity information, because such processes are mediated by brain areas further along the visual processing pathway, where in- formation from the two visual hemifields is not separated. The MOMIT prediction is also consistent with the findings of a related change-detection study by Delvenne [25]. When observers performed a memory change-detection task that did not involve identity-location bindings, a bilateral hemifield advantage was observed. No such advantage was found, however, when observers performed another memory change-detection task that did involve identity-location bindings. The primary aim of the present study was to test the prediction of MOMIT [24] that the hemifield effect reported by Alvarez and Cavanagh [4] when observers performed MOT should not occur when observers perform MIT. To preview our results: The following four experiments demonstrated a reduction of the hemifield effect in MIT relative to MOT. Crucially, even when performing MIT a robust hemifield effect is still observed, contrary to previous findings [4]. The theoretical implications of these findings are reviewed in the Discussion. Experiment 1A In this experiment, we adapted Alvarez and Cavanagh’s [4] MOT paradigm to a typical MIT display, in which the unique identities of objects were visible for the duration of the movement phase. As in the previous study, there were three conditions: a baseline condition in which observers tracked two targets, both Hemifield Effects in Multiple Identity Tracking located in the same quadrant; a bilateral condition in which there were four targets in total, two in each visual hemifield; and a unilateral condition in which four targets were confined to just one hemifield. MOMIT [24] predicts that a) tracking accuracy will be higher in the baseline condition than in the other two conditions because it requires fewer targets to be tracked, and b) tracking accuracy for the bilateral and unilateral conditions will be equal. Method Participants. Twenty-eight participants (24 female) aged 19– 34 with normal or corrected-to-normal vision completed the experiment. All observers provided informed written consent and the study was approved by the Department Human Ethics Advisory Group in the School of Psychological Sciences at the University of Melbourne. Observers were reimbursed $15 for their time. Equipment. Participants had normal or corrected-to-normal visual acuity as tested by a Good-Lite near vision chart at a distance of 40 cm. Stimuli were presented on a 2199 Sony or Dell CRT monitor at a resolution of 128061024 with a refresh rate of 85 Hz. Observers viewed the stimuli from a distance of 60 cm. The experiment was run on a Windows XP operating system using Psychophysics Toolbox routines (version 3 [26,27]) for MATLAB (Natick, MA, USA). Participants completed the experiment individually in a small testing room with the lights off. Stimuli. The stimuli were adapted from Alvarez and Cavanagh [4]. Observers saw a white square display that subtended 23u623u of visual angle. There was a central black fixation cross that subtended 0.48u60.48u. Two thick grey lines with a subtended width of 3u, one aligned vertically and the other horizontally, divided the display into four equal quadrants that each subtended 10u610u. Moving disks with subtended diameters of 0.76u occupied each quadrant. The maximum distance a disk could travel from fixation was 15.8u. Each disk moved in straight lines except when bouncing after reaching a border or coming within 2u of another disk. Each quadrant contained three disks (Figure 1). In the baseline condition, the disks in one quadrant were coloured. In the other two conditions, the disks in two quadrants were coloured. These two quadrants were arranged either bilaterally or unilaterally. Procedure Participants were instructed to fixate on the central cross and track the coloured disks. Participants were instructed to employ a strategy of tracking two targets in a quadrant because the identity of the third could then be deduced. This was done to ensure that all participants used the same tracking strategy. The targets’ colours remained visible while the disks moved within their quadrant for eight seconds. The disks then stopped moving and were concealed by a multi-color square mask for 50 ms before reappearing in black. One of the target disks was then shown in red, green, or blue. Participants responded to the question ‘‘Was this colour target at this position?’’ using the ‘y’ key for yes and the ‘n’ key for no. This process was repeated for a second target from the same quadrant. If an error was made, the message ‘‘[one/two] mistake[s]’’ was displayed at the end of the trial. The experiment began with five practice trials. A 50 trial QUEST staircase routine followed [28,29], which found the disk speed at which each participant could achieve 75% tracking accuracy in the baseline condition. This allowed floor and ceiling effects to be avoided and controlled for individual differences in tracking ability [9]. Each participant’s unique speed was used for PLOS ONE | www.plosone.org 2 August 2012 | Volume 7 | Issue 8 | e43796