saccade and fixation P3s were not significantly correlated with the corresponding differences in P3 amplitude, persistence, start time, or signal strength observed between the fixation and saccade conditions for each subject. (Difference in amplitude of P3 not significantly correlated with difference in image presentation time: r(5) = 0.45, p = 0.31; difference in start time of P3 not significantly correlated with difference in image presentation time: r(5) = 0.28, p = 0.54; difference in P3 persistence not significantly correlated with difference in image presentation time: r(5) = 0.50, p = 0.26; difference in P3 signal amplitude expressed as a t-statistic not significantly correlated with difference in image presentation time: r(5) = 20.26, p = 0.57; all tests non-directional.) In addition, a 3- way between subjects ANOVA (multiple regression) was applied in order to attempt to model the difference in image presentation time observed between saccade and fixation conditions (for the 7 subjects with significant P3s) as a function of the corresponding differences between fixation and saccade conditions in P3 amplitude, start time, and persistence. There were insufficient degrees of freedom to model interactions, therefore only main effects were modeled. None of the 3 main effects was significant. (P3 amplitude not significant: F(1, 3) = 3.65, p = 0.15; P3 start time not significant: F(1, 3) = 2.27, p = 0.23; P3 persistence not significant: F(1, 3) = 3.47, p = 0.16). Furthermore, three one-way within subject ANOVAs were applied to determine the effects of saccadic delay on the saccadic P3 parameters of amplitude, start time and persistence. A two-level factor of saccadic delay was created by calculating the median saccadic delay for each subject and then calculating the corresponding saccade P3 parameters for those trials in which the saccadic delay was greater than the median saccadic delay and those trials in which the saccadic delay was less than the median saccadic delay. For all three within subject ANOVAs, there were no significant effects of saccadic delay on saccade P3 parameters (effect of saccadic delay on saccade P3 amplitude not significant: F(1, 6) = 0.87, p = 0.39; effect of saccadic delay on saccade P3 start time not significant: F(1, 6) = 3.28, p = 0.12; effect of saccadic delay on saccade P3 persistence not significant: F(1, 6) = 1.96, p = 0.21). Note that the within subject ANOVAs were conducted without considering sphericity as correction can only result in higher p-values. Cluster Based Amplitude Comparisons of Fixation and Saccade P3s within Each Subject Figure 3 is a plot of the target minus distracter waveforms at midline electrodes Fz, Cz and Pz for all subjects for the saccade and fixation conditions. Importantly, although the results in Figure 3 are shown only at midline electrodes, note that all results cited below were derived from statistical tests applied across all electrodes as discussed in detail in Methods. In 7 of the 8 subjects, the target minus mean distracter data displayed no significant differences with p,0.05 between saccade and fixation trials. The one exception was S6, who had a cluster of significant amplitude difference between the fixation and saccade P3s (shaded region in Figure 3) in which the saccade P3 exceeded the fixation P3 in amplitude, with an associated two-tailed false alarm probability of p = 0.001. Note that S6 was one of the two subjects that were not na¨ıve regarding the goals of the experiment. It is therefore possible that the greater response in the saccade condition relative to the fixation condition observed in S6 could be due to attentional effects related to prior knowledge of the experiment. Although it was not a significant difference, S5 had a cluster of saccade versus fixation amplitude positivity associated with a two tailed false alarm probability of p = 0.094. Note that, across the subjects, no consistent pattern of fixation P3 amplitude exceeding The Fixation and Saccade P3 saccade P3 amplitude or vice versa existed. Furthermore, for all subjects other than S5 and S6, all clusters of amplitude differences between the saccade and fixation conditions had associated two- tailed false alarm probabilities of p.0.1. Comparing Fixation and Saccade P3 Grand Averages Grand averages across all subjects of the TMD signal in the saccade and the fixation conditions are shown in Figure 4. The waveforms in Figure 4A are the grand averages at central midline electrodes Fz, Cz, Pz and Oz. Importantly, although the waveforms are shown in Figure 4A only at midline electrodes, note that all results cited below were derived from statistical tests applied across all electrodes as discussed in detail in Methods. It can be seen that the target minus distracter positivity was widespread in both the fixation and saccade conditions, and the maximum positivity shifted from frontal to posterior electrode sites in both the fixation and saccade conditions. Non parametric tests as described in Methods revealed that there were no significant differences in start time and persistence between the fixation and saccade cross subject grand average (start time p = 0.75; persistence p = 0.81 two-tailed tests). There were no significant clusters of saccade versus fixation amplitude difference in the grand average identified via non- parametric cluster statistics. However, there was a period in which the saccade P3 was greater than the fixation P3 associated with a maximum effect size of 1.58 from approximately 300–450 ms (this cluster is most visible in electrode Pz and Oz time courses in Figure 4A) The above cited effect size of 1.58 was the maximum effect size observed at electrode Pz in the 300–450 ms time period. In addition, there was a period in which the opposite effect was observed (in which the fixation P3 was greater than the saccade P3) from approximately 520 to 660 ms with a maximum effect size of 21.53 observed at electrode Cz in this time window. Note again, however, that despite having substantial effect sizes these two clusters of amplitude difference did not reach significance. The saccade versus fixation positivity had an associated false alarm probability of p = 0.42 and the fixation versus saccade positivity had an associated false alarm probability of p = 0.27(two-tailed tests). Comparing Fixation and Saccade Target and Distracter Grand Averages Directly The EPs associated with targets in the saccade and fixation conditions and the EPs associated with distracters in saccade and fixation conditions are compared in Figures 5 and 6, respectively. Importantly, although the waveforms in Figures 5A and 6A are shown only at midline electrodes, note that all results cited below were derived from statistical tests applied across all electrodes as discussed in detail in Methods. The z-score waveforms in Figures 5 and 6 were derived by first expressing the amplitude at each electrode and time point as a single sample t-statistic (mean over all trials as determined at each time point divided by standard error as determined across all trials at each time point). The t- statistic waveforms were then converted to z-score waveforms prior to cross-subject averaging. When the non-parametric methods for comparison described in Methods were applied to compare saccade and fixation target trials, a cluster in which the grand average saccade target waveform was significantly greater than the fixation target waveform was found from 0 to approximately 200 ms with an associated false alarm probability of p = 0.001 (Figure 5). In addition, a significant cluster of fixation target versus saccade target positivity was found from approxi- mately 500 to 700 ms. Similarly, in the saccade distracter versus fixation distracter comparison shown in Figure 6, there was a PLOS ONE | www.plosone.org 11 November 2012 | Volume 7 | Issue 11 | e48761