differences observed in the actual grand average EP across subjects. The value that was input into the distribution for each permutation was the maximum of the sum of the t-statistics observed in the permuted grand average EP over clusters of adjacent electrodes and time points that each met a single sample significance of p,0.05. The other details of the cluster statistics were the same as discussed previously except the test against the distribution to compare saccade and fixation TMD EPs was two-tailed. Effect size (g) associated with differences between the grand average saccade and fixation P3s at any given time point were quantified via the following formula [36]: gt~ Ps{Pf St : Where Ps and Pf are values of the saccade and fixation P3 target minus distracter EP waveforms, respectively, at the time point (t) at which the effect size is to be evaluated, and St is the pooled standard deviation across the saccade and fixation conditions calculated using: St2~ (ns{1)Ss2z(nf {1)Sf2 nsznf {2 : Where ns and nf are the number of observations of the saccade and fixation EP waveforms that contribute to the cross subject grand average (equal to the total number of subjects contributing to the cross subject grand average, in both cases). Ss and Sf are, respectively, the standard deviations across subjects of the saccade and fixation P3s at any given electrode and time point at which it is desired to estimate the effect size. The results of analyses described in this section are shown in Figures 4, 5 and 6. Quantifying Start Time and Persistence in Cross Subject Average For every cross-subject grand average P3 (target minus distracter) z-score waveform, the total positive area under the P3 waveform was found via integration. Start times of the cross- subject grand average P3 waveforms were defined at each electrode as the time that it took for the positive area under the curve to rise to a level at which five percent of the total positive area under the curve was covered. Similarly, the end time of the P3 was defined as the time that it took for the positive area under the curve to rise to a level at which ninety-five percent of the total positive area under the curve was covered. Persistence was defined as the difference in time between the start time and the end time as defined above. Non-parametric methods very similar to those described in Comparing the P3 in fixation and saccade trials across subjects were used to test the cross subject grand averages in the fixation and saccade conditions for differences in start time and persistence. After defining start time and persistence at each electrode in the grand average, the mean of each of these parameters across all electrodes was calculated. The difference between saccade and fixation for both parameters was calculated as the difference in the means across all electrodes as determined separately for the saccade and fixation conditions. To test the mean differences across all electrodes in start time and persistence for both parameters, a distribution of the differences expected between saccade and fixation conditions due to chance alone was created by performing pair-wise permutations within subjects of the saccade and fixation The Fixation and Saccade P3 waveforms (using the same permutation procedure described in Comparing the P3 in fixation and saccade trials across subjects). On each iteration, the grand average EPs of the randomly permuted data were calculated, and the mean difference between the two parameters of interest in the randomly permuted data were calculated, resulting in 2N values of the distribution. The actual differences in start time and persistence observed between the grand average saccade and fixation P3s could be tested against the start time and persistence distributions generated as described above (two-tailed tests). Results Behavioral Results The one second period after saccade landing in saccade trials and the one second period after image presentation in fixation trials were examined for microsaccades using a microsaccade detection algorithm [29]. Trials in which microsaccades exceeded 1u were excluded from both trial types. In addition, saccade trials were excluded if the saccadic delay on any given trial was longer than 300 ms, and also excluded if the saccadic delay on any given trial was below 100 ms or occurred prior to target presentation in the periphery. Furthermore, saccade trials were not included if eye movement data indicated that, on any given trial, the saccade from the central fixation point did not terminate on the target/distracter in the periphery. The above procedures resulted in retention of 8962% of fixation trials and 7566% of saccade trials on average across subjects (all results will be reported in the form of mean 6 standard error). During the 70 pre-experiment test trials to estimate the MSD for each subject, across subjects, the MSD was 24066 ms. During the experimental EEG trials, the MSD across subjects was 21366 ms. Therefore, on average across all subjects, the procedures to equate mean image presentation duration in the fixation condition and the saccade condition were able to minimize the difference in image presentation duration between conditions to 2766 ms. Because the difference in image presen- tation time between saccade and fixation trials was significant (paired t-test; p = 0.003, t(7) = 4.57), the differences between P3 parameters in the saccade and fixation conditions observed for each subject (P3 amplitude, start time, and persistence) were tested for correlations with the differences in image presentation times between the saccade and fixation conditions observed for each subject. No significant correlations between the difference in image presentation time between saccade and fixation conditions and the corresponding differences in P3 parameters were found (See Testing P3 parameters for dependence on the difference in image presentation time for further details). When subjects foveated the target and distracter stimuli, the two types of stimuli were designed to be very easily distinguishable, with subject button presses indicating that all subjects were correctly identifying targets and distracters with over 95% accuracy in both saccade and fixation trials. Note that the button presses were therefore included not for the purpose of behavioral analyses, but only to encourage subjects to remain as engaged as possible in the tasks. Any errors in subject button presses were therefore assumed to be manual or careless errors rather than difficulties in discrimination between target and distracter. The decision to design the target and distracter stimuli to be easily distinguishable was made due to the fact that the P3 signal’s amplitude has been shown to decrease if the target and distracter stimuli are not easily distinguishable [34], and it was desired to evoke a reliable P3 in both the fixation and saccade conditions. PLOS ONE | www.plosone.org 7 November 2012 | Volume 7 | Issue 11 | e48761