Hemifield Effects in Multiple Identity Tracking Figure 1. Schematic depiction of the three trial conditions for Experiment 1. Targets are divided between both visual hemifields on bilateral trials, and confined to one hemifield on unilateral trials. doi:10.1371/journal.pone.0043796.g001 the proceeding 240 experimental trials, which comprised equal numbers of baseline, bilateral and unilateral trials randomly interleaved. Thus, for each of the three conditions 80 trials were run and the average accuracy was calculated. Results & Discussion Three participants had to be excluded due to a programming error that invalidated their data. Data for the remaining 25 participants was analysed. A unique speed was generated for each individual participant using the QUEST routine (mean speed = 12.1u/s). Figure 2 depicts our results. A repeated measures Analysis of Variance (ANOVA) with sphericity assumed (Mauchly’s test p = 0.15) indicated a significant main effect of condition (F(2, 48) = 53.4, p,0.001, partial g2 = 0.69). As expected, there was an effect of set size, with tracking performance decreasing signifi- cantly in the bilateral and unilateral conditions compared to the baseline condition (t(24) = 7.00, p,0.001; t(24) = 9.12, p,0.001, respectively). Contrary to the prediction of MOMIT, a hemifield effect was evident: tracking accuracy was higher when targets were arranged bilaterally compared to unilaterally (t(24) = 3.40, p = 0.002). Bonferroni corrections were not applied here or in the other experiments of this article because they are not needed provided three or fewer groups are being compared and an ANOVA has already demonstrated a main effect [30]. Figure 2. Mean tracking accuracy for the three conditions in Experiment 1. The broken line shows chance performance, which is 25% as two yes/no questions were asked per trial and both had to be answered correctly for the trial to be counted as correct. Error bars denote within-observers standard error [38]. *Significantly different at p,0.01; **Significantly different at p,0.001. doi:10.1371/journal.pone.0043796.g002 At odds with the serial, non-hemifield-specific tracking mech- anism predicted by MOMIT [24], our results indicate the existence of a bilateral advantage for tracking unique objects. It is possible that this unexpected hemifield effect was an artifact of participants mis-fixating vertically on bilateral trials, which would decrease the eccentricity of the objects and make the task easier. We addressed this concern by repeating the experiment with the addition of an Arrington Research eye tracker to monitor participants’ gaze on the fixation cross. Experiment 1B This experiment was a replication of the previous experiment using an eye tracker. Eight participants (two female; seven of them new, including author P.H.) aged 21–35 were recruited in the manner described above. The message ‘fixation broken’ was displayed whenever the observers failed to maintain fixation on the fixation cross, after which the trial was restarted. As before, the QUEST staircase routine produced a unique speed for each participant (mean speed = 21.7u/s), which ensured that accuracy in the baseline condition was approximately 75%. Mean accuracies were very similar to before: 72%, 57%, and 51% for the baseline, bilateral and unilateral conditions, respectively (for the previous experiment the corresponding accuracies were 77%, 61% and 54%). Although the overall accuracies were slightly lower when the eye-tracker was used, the relative accuracies were very similar. Crucially, the difference between the bilateral and the unilateral conditions was not significantly different between the two experiments (t(31) = 0.106, p = 0.92). This replication using an eye-tracker rules out the possibility that the bilateral advantage we observed in Experiment 1a was due to participants mis-fixating vertically on bilateral trials. Perhaps then the hemifield effect is a manifestation of participants ‘cheating’ rather than actively tracking? We addressed this possibility in Experiment 2. Experiment 2 As the targets’ identities were continuously visible in Experiment 1, perhaps sustained tracking was not actually required: it is possible that participants only needed to attend to the targets near the end of the trial to see their colour and final location. We addressed this possibility by repeating our previous experiment with one alteration. This time, we only briefly cued the targets’ unique identities at the start of the trial. After this point, all the disks turned black and were visually indistinguishable. This forced PLOS ONE | www.plosone.org 3 August 2012 | Volume 7 | Issue 8 | e43796