participants to attentively track the targets for the entire trial, as in a standard MOT task. Method Participants. Thirty-one people (20 female) ranging in age from 17–29 with normal or corrected-to-normal vision took part. Twenty-six of these participants were first year undergraduate psychology students from the University of Melbourne who received course credit for their participation; the remainder were personal contacts of author C.H. who did not receive re- imbursement. As before, 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. Stimuli & Procedure. Very similar stimuli to Experiment 1 were used. As in Experiment 1 the targets moved within their quadrant for eight seconds. For the first three seconds the targets were coloured red, green, and blue; for the remaining five seconds all were black. Participants responded in the same way as Experiment 1, and the same trial structure was used (10 practice trials; 50 trial Quest routine; 240 experimental trials). Results & Discussion Data for two participants was excluded because it was incomplete and two other participants’ data was excluded because a failure of the QUEST routine caused their baseline accuracy to be too high (.95%) thereby causing a ceiling effect. Data for the remaining 27 participants was analysed. The QUEST procedure allocated a unique speed to each participant (mean = 4.69u/s). Results are shown in Figure 3. A repeated-measures ANOVA with sphericity assumed (Mauchly’s test p = 0.38) indicated a main effect of condition (F(2, 52) = 95.9, p,0.001, partial g2 = 0.79). Tracking accuracy was significantly higher in baseline trials than bilateral trials (t(26) = 9.78, p,0.001) and unilateral trials (t(26) = 12.0, p,0.001). There was also a hemifield effect evident, with bilateral tracking accuracy significantly greater than unilat- eral tracking accuracy (t(26) = 4.36, p,0.001). We compared the results of Experiment 2 to those of Experiment 1a. A mixed 3 6 2 ANOVA with sphericity assumed (Mauchly’s test p = 0.06) indicated a significant main effect of condition (F(2, 100) = 138, p,0.001, partial g2 = 0.74) but no significant difference between the two experiments (F(1, 50) = 0.43, p = 0.51, partial g2 = 0.01). There was no significant interaction between condition and experiment (F(2, 100) = 0.06, p = 0.95, partial g2,0.01) indicating that tracking accuracy differed based Figure 3. Mean tracking accuracy for the three conditions in Experiment 2. The broken line indicates chance performance is 25%. Error bars are within-observers standard error. **Significantly different at p,0.001. doi:10.1371/journal.pone.0043796.g003 Hemifield Effects in Multiple Identity Tracking on trial type, but not due to target identities being visible versus concealed. This experiment replicates our findings from Experiment 1a and rules out the prospect that the bilateral advantage was merely a reflection of sustained tracking not being required due to the target identities being continuously visible. Accordingly, the data from both our experiments are incongruent with MOMIT [24]. One might therefore expect our data to replicate the Alvarez and Cavanagh [4] finding of independent tracking in the left and right visual hemifields. Specifically, they found that observers were as accurate at simultaneously tracking two targets in each hemifield (a total of four targets) as they were when just tracking two targets. Our data from Experiments 1 and 2 do not fit Alvarez and Cavanagh’s predictions as our observers were significantly worse at tracking four targets than two. There are two important differences between our stimuli and those used by Alvarez and Cavanagh that may account for this discrepancy. First, we used an MIT task whereas Alvarez and Cavanagh used an MOT task. Second, in our stimuli there were three disks in each quadrant whereas in their stimuli there were four disks in each quadrant. In Experiment 3 we will investigate the latter difference first by altering the number of disks to more closely resemble Alvarez and Cavanagh’s original display. Experiment 3 In the Alvarez and Cavanagh [4] experiment target quadrants held four disks; two targets and two distractors. When we adapted their display to MIT in Experiment 1 and 2, we used three unique disks in the target quadrants. We presumed this would be equivalent under the rationale that only two disks would need to be tracked as the identity of the third could then be deduced. In Experiment 3 we controlled for the possibility that this assumption may have been incorrect. We altered our display so that it had the same number of disks in target quadrants as Alvarez and Cavanagh’s experiment: two unique targets and two distractors. Method Participants. Twenty-five people (17 female) aged 18–30 with normal or corrected-to-normal visual acuity took part. One participant was a personal contact of author C.H.; the others were recruited via posters in the University of Melbourne psychology building and were reimbursed $15. As before, 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. Stimuli & Procedure. Very similar stimuli and procedure to the previous experiments were used, except that in each quadrant there were four disks. In the quadrants that contained targets, the two target disks were briefly coloured and the two distractor disks were always black. As before, targets were coloured red, green, or blue, and targets in the same quadrant were never the same colour as one another. The disks in the other quadrants were all black. As in Experiment 2, the targets’ colours were shown only for the first three seconds of the trial, after which they turned black and became indistinguishable from each other and the distractors. The same response method and trial structure as Experiment 2 was used. Results & Discussion Due to a problem with the QUEST staircase procedure, one participant’s baseline accuracy was greater than 95%, so this participant was excluded because of ceiling effects. Data for the PLOS ONE | www.plosone.org 4 August 2012 | Volume 7 | Issue 8 | e43796