The Fixation and Saccade P3 Figure 5. Direct comparison of fixation target and saccade target EPs. In contrast to figure 4, when saccade target and fixation target EPs are compared directly, significant differences are observed between the saccade and fixation conditions, likely due to EOG and EEG saccade-related activity present in the saccade but not the fixation condition. A, Grand average across subjects of fixation and saccade target responses at midline electrodes Fz, Cz, Pz and Oz. Time is measured relative to image presentation (fixation condition) or relative to saccade landing (saccade condition). Time points belonging to clusters of significant difference are highlighted in gray. B, Topographies of target responses in fixation and saccade trials. doi:10.1371/journal.pone.0048761.g005 The P3 start time for each subject (which was defined as the first time point at which any electrode showed a significant TMD difference) did not differ significantly: 312638 ms for fixation trials and 321646 ms for saccade trials (paired t-test across subjects; t(6) = 20.21, p = 0.84, two tailed). Similarly, the persistence of the P3 (defined here as the temporal duration of the significant cluster of TMD positivity as determined across all electrodes) was found to not differ significantly between fixation and saccade trials across subjects (persistence in fixation trials 418658 ms; persistence in saccade trials 468645 ms; paired t-test; t(6) = 20.63, p = 0.55, two tailed). At every time point at which there was a significant P3 signal (significance determined by application of non-parametric cluster statistics as described in Methods), the mean TMD positivity was calculated only over those electrodes that were significant at that particular time point. A mean over all of the significant time points was then calculated. The P3 amplitude across all subjects as defined above was 4.3960.38 mV in fixation trials and 4.7860.53 mV in saccade trials. The difference in the amplitude PLOS ONE | www.plosone.org 9 November 2012 | Volume 7 | Issue 11 | e48761