Figure 5. Mean tracking accuracy for the three conditions in Experiment 4. The broken line shows that chance performance is 25%. Error bars are within-observers standard error. **Significantly different at p,0.001. doi:10.1371/journal.pone.0043796.g005 indicating a capacity limitation. This is in contrast to Alvarez and Cavanagh’s [4] findings, in which four targets could be tracked as well as two, provided the four targets were displayed bilaterally. In agreement with Alvarez and Cavanagh’s results, we found a large drop in accuracy when targets were confined to a single hemifield, relative to when they were divided between both hemifields. This hemifield effect is more pronounced than in our previous three MIT experiments. In the Alvarez and Cavanagh [4] experiment, when each trial ended, the observer was asked a single question: one disk was highlighted and the observer was asked whether or not it was a target. Conversely, in our Experiment 4, at the end of the trial the observer was asked two questions: two disks were highlighted in succession and the observer was asked whether both of these disks were targets. A trial was only counted as correct if observers correctly answered both questions. Accordingly, by taking the square root of our mean accuracy data, we can find the equivalent probability of getting one question per trial correct, facilitating a direct comparison of our data to that of Alvarez and Cavanagh. The probability of being able to answer a single question correctly in our Experiment 4 was 86% for the baseline condition, 77% for the bilateral condition, and 60% for the unilateral condition. Overall accuracy was slightly higher in the Alvarez and Cavanagh study, presumably because their disks were slightly larger than ours and their disk density was slightly less. Both factors would tend to increase tracking accuracy [31]. Their accuracies for the baseline, bilateral and unilateral conditions were 92%, 87%, and 66%, respectively. The difference in accuracy between the baseline condition and the bilateral condition in Alvarez and Cavanagh’s study, however, was similar to that in Experiment 4. In our study, the accuracy difference between the baseline and the bilateral condition was 9% whereas for Alvarez and Cavanagh it was 5%. Given the confidence intervals associated with these two measure- ments, these two values are consistent with each other. Similarly, the accuracy difference between the baseline and the unilateral conditions was also very similar for the two studies, in both cases being 26%. Thus, although the overall accuracy was slightly higher in the Alvarez and Cavanagh study, the relative accuracies of the three conditions were very similar in both studies, so we have successfully replicated their experiment. The only difference between our results and those of Alvarez and Cavanagh [4] is that we found the difference between the bilateral and baseline conditions to be statistically significant, whereas Alvarez and Cavanagh did not. This difference appears to be due to our study having greater statistical power caused by our larger sample size (26 versus 8). Alvarez and Cavanagh also found Hemifield Effects in Multiple Identity Tracking accuracy to be less in the bilateral condition than in the baseline condition – they were just unable to prove that this difference was statistically significant. General Discussion In this study we conducted a novel investigation into whether the identity-location bindings that are necessitated in tracking unique objects (MIT) would eliminate the bilateral tracking advantage that Alvarez and Cavanagh [4] reported when only locations needed to be tracked (MOT). We compared tracking performance for four targets located within one visual hemifield with tracking performance for four targets distributed between the left and right visual hemifields. According to the logic of the paradigm, no difference in tracking performance between these two arrangements would indicate a non-hemifield-specific cognitive tracking resource. Conversely, higher accuracy for the bilateral arrangement would indicate a tracking resource that is at least limited in its capacity to be dynamically allocated across the visual field. Across four experi- ments our results show that the presence of identity-location bindings reduces, but does not entirely remove, the bilateral advantage (Experiments 1–3) relative to that observed when only locations need to be tracked (Experiment 4). In Experiment 1, using a typical MIT display in which targets’ unique identities remained visible for the duration of movement, we found performance for the bilateral and unilateral arrange- ments to be unequal: tracking accuracy was greater when targets were divided between the left and right hemifields. In accordance with a set size effect [12], tracking accuracy for the baseline condition, in which observers tracked only two targets, was higher still. We ruled out the possibility that the bilateral advantage was a reflection of participants not maintaining fixation on the fixation cross in bilateral trials by replicating our finding using an eye- tracker. In Experiment 2 we briefly cued the targets’ unique identities but kept them hidden for the remainder of the trial to ensure that observers had to continuously track the targets and could not ‘cheat’ by using the targets’ unique colours to recover their positions during the course of the trial. Still, we found a significant hemifield effect with tracking accuracy being again greater in the bilateral condition than in the unilateral condition. In Experiment 3 we changed the number of targets to more closely reflect Alvarez and Cavanagh’s [4] original experiment (two unique targets and two distractors in each target quadrant). We found the same pattern of tracking performance as before and again observed a strong hemifield effect. In Experiment 4 we conducted an MOT experiment, in which identity-location bindings were not required. This experiment was a close replication of Alvarez and Cavanagh’s [4] original experiment. We found a much stronger bilateral advantage than in our previous three MIT experiments. Unlike Alvarez and Cavanagh we found that accuracy in the baseline condition was significantly greater than that in the bilateral condition. This difference between the two studies seems to be due to our study having greater statistical power because our sample size was larger. Alvarez and Cavanagh also found tracking accuracy to be less in the bilateral condition than in the baseline condition, however for them the difference was not statistically significant. This finding is important because it shows that, even for MOT, tracking is not completely independent in the left and right visual hemifields. The fact that tracking accuracy is greater when tracking two targets in one hemifield (the baseline condition) than tracking two targets in each hemifield (the bilateral condition) shows that in the former PLOS ONE | www.plosone.org 6 August 2012 | Volume 7 | Issue 8 | e43796