were used to identify significant clusters of target minus distracter positivity (P3 signals). One-tailed tests were used due to the well- known positivity of the P3 when referenced to the mastoids [33– 35]. The results of analyses described in this section are shown in Figure 2. Comparing the P3 in Fixation and Saccade Trials within Single Subjects Non-parametric methods similar to those described above were used to compare the fixation and saccade P3s observed within each subject separately. For all single trial target epochs observed in the fixation and saccade data, the mean distracter evoked potentials (for fixation and saccade trials, respectively) were subtracted from the target single trial epochs. The procedure resulted in a set of target epochs with mean distracter signal subtracted out for fixation trials and a comparable set of target epochs with mean distracter signal subtracted out for saccade trials. The saccade target epochs with mean distracter EPs subtracted and fixation target epochs with mean distracter EPs subtracted were tested for significant differences using cluster statistics in exactly the same way that the target and distracter epochs within a single condition were compared (cluster statistics application described in previous section.) However, the test on the distribution was two-tailed rather than one-tailed. The results of analyses described in this section are shown in Figure 3. The Fixation and Saccade P3 Comparing the P3 in Fixation and Saccade Trials in Cross Subject Average In order to compare fixation and saccade P3s across subjects, as a first step, for each subject at every electrode and time point, the differences between target and distracter EP signals were expressed as t-statistics for both the fixation and saccade conditions. In order to account for inter-subject differences in SNR, the resulting t- statistic waveforms were converted into z-statistic waveforms before cross subject averaging. The procedure resulted in z-score waveforms for fixation trials and z-score waveforms for saccade trials for every subject, or 2N waveforms total (where N is the number of subjects, and there are 2 waveforms corresponding the 2 conditions per subject) at every electrode. The hypothesis to be tested was that the target minus distracter (TMD) z-score waveforms in the saccade case were equivalent to the TMD z- score waveforms in the fixation case. Comparisons between the TMD EPs in the fixation and saccade conditions were performed using non-parametric cluster statistics similar to those described previously. A distribution of differences between the saccade and fixation conditions expected due to chance alone was created by permuting the definitions of saccade and fixation TMD EPs within every subject independently and calculating the grand EP for every permutation. Note that the two conditions were interchanged only within subjects and not across subjects. The interchanging procedures therefore resulted in 2N (where N is the number of subjects, and there are 2 conditions: saccade and fixation) values of the distribution that could be compared to Figure 3. Saccade and fixation target minus distracter data at midline electrodes for individual subjects. Periods of significant difference between the saccade and fixation conditions are shaded. doi:10.1371/journal.pone.0048761.g003 PLOS ONE | www.plosone.org 6 November 2012 | Volume 7 | Issue 11 | e48761