remaining 24 participants was analysed. The QUEST procedure allocated a speed for each observer (mean = 5.34u/s). Results are shown in Figure 4. A repeated measures ANOVA with sphericity assumed (Mauchly’s test p = 0.68) indicated a main effect of condition (F(2, 46) = 71.0, p,0.001, partial g2 = 0.76). There was an effect of set size, with tracking accuracy greater for baseline trials than bilateral trials (t(23) = 6.72, p,0.001) as well as unilateral trials (t(23) = 11.8, p,0.001). There was also a hemifield effect, with tracking accuracy significantly higher for bilateral relative to unilateral trials (t(23) = 5.01, p,0.001). We compared our results to those of Experiment 2. A mixed 362 ANOVA with sphericity assumed (Mauchly’s test p = 0.46) showed a significant main effect of condition (F(2, 98) = 159, p,0.001, partial g2 = 0.77); no significant main effect of experi- ment (F(1, 49) = 0.13, p = 0.72, partial g2 = 0.96); and a significant interaction between condition and experiment (F (2, 98) = 4.10, p = 0.02, partial g2 = 0.08). Independent t-tests, however, indicated that there were no significant differences between Experiment 2 and 3 on baseline (t (49) = 20.73, p = 0.47); bilateral (t (49) = 0.09, p = 0.93); or unilateral trials (t (49) = 1.66, p = 0.10). This experiment replicates the findings of our previous two experiments. We have robustly demonstrated a bilateral tracking advantage for uniquely identifiable targets. Observers found it easier to track four targets when they are spread between two hemifields than confined to a single hemifield. We failed to demonstrate, however, that tracking is independent in the two hemifields. Observers found it easier to track two targets in a single hemifield (i.e. the baseline condition) than to simultaneously track two targets in each hemifield, suggesting in the former case they were able to utilise resources from both hemifields. This latter finding is not in agreement with Alvarez and Cavanagh [4]. To determine whether the contrast between Alvarez and Cavanagh’s results and our own was truly due to our targets having unique identities, we attempted to replicate their original MOT study in Experiment 4. Experiment 4 In an MIT experiment each target has an unique identity and so at the end of the trial the observer is asked to identify a particular target, e.g. the red one. Conversely, in MOT, all the targets are identically coloured and at the end of the trial the observer merely has to identify the location of the targets, without having to recall their identities. Thus, MOT does not require identity-location bindings. Figure 4. Mean tracking accuracy for the three conditions in Experiment 3. Broken line shows chance performance is 25%. Error bars are within-observers standard error. **Significantly different at p,0.001. doi:10.1371/journal.pone.0043796.g004 Hemifield Effects in Multiple Identity Tracking In this experiment we attempted to replicate the Alvarez and Cavanagh’s [4] MOT study. If our previous three experiments are genuinely displaying something specific to MIT, then we should get contrasting results here. If Alvarez and Cavanagh’s conclusion is correct, when using an MOT stimulus we would expect to see equal performance in the baseline and bilateral trials, and a large decrement in unilateral tracking relative to the other two conditions. Alternatively, if our previous experiments are simply demonstrating something generic to object tracking, we would expect the results of this experiment to be identical to those of Experiment 3. Method Participants. Twenty-nine people (21 female) aged 17–33 with normal or corrected-to-normal visual acuity took part. Twenty-six participants were University of Melbourne first year undergraduate psychology students recruited via posters who received course credit; the other three participants were personal contacts of author C.H. who did not receive reimbursement. 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. Unlike the previous experiments in which observers performed MIT, in this experiment observers performed MOT. As in Experiment 3 each quadrant always held four disks. In the quadrants that contained targets, two disks were identified as targets by briefly turning the same colour (either red, green, or blue) at the start of the trial. At the end of the trial, two disks from the same target quadrant were highlighted, one at a time, and the observers were asked whether or not they were targets. As no distinction was made between targets (since they were always the same colour as each other) this experiment did not require colour to be bound to location, and was thus a pure MOT experiment. Results & Discussion The data for three participants had to be excluded due to a computer error that corrupted their data. Data for the remaining 26 participants was analysed. The QUEST routine produced unique speeds for each participant (mean = 4.39u/s). Results are shown in Figure 5. A univariate ANOVA with sphericity assumed (Mauchly’s test p = 0.35) showed a main effect of condition (F(2, 50) = 132, p,0.001, partial g2 = 0.84). As expected, there was a hemifield effect, with bilateral tracking accuracy significantly greater than unilateral tracking accuracy (t(25) = 9.03, p,0.001). In contrast to the Alvarez and Cavanagh [4] study, baseline accuracy was significantly higher than accuracy in the bilateral condition (t(25) = 6.94, p,0.001). Baseline and bilateral tracking accuracy looked similar to Experiment 3, but unilateral accuracy appeared greatly reduced. We compared Experiment 4 to Experiment 3 using a mixed 362 ANOVA with sphericity assumed (Mauchly’s test p = 0.95). We found a significant main effect of condition (F(2, 96) = 192, p,0.001, partial g2 = 0.80); no main effect of experiment (F (1, 48) = 2.29, p = 0.14, partial g2 = 0.05), and a significant interaction between condition and experiment (F (2, 96) = 5.76, p = 0.004, partial g2 = 0.12). Between Experiment 3 and 4 there was no significant difference in baseline (t(48) = 1.09, p = 0.28) or bilateral (t(48) = 20.23, p = 0.82) trials. Tracking performance on unilateral trials was significantly lower in Experiment 4 than in Experiment 3 (t(48) = 3.90, p,0.001). As in our previous three experiments, we have found a set size effect with bilateral tracking accuracy being lower than baseline, PLOS ONE | www.plosone.org 5 August 2012 | Volume 7 | Issue 8 | e43796