The Fixation and Saccade P3 Figure 1. Saccade and fixation stimuli. A, Two stimuli that were interchangeably used as target and distracter stimuli. B, Time course of horizontal eye position in fixation and saccade trials and visual stimuli corresponding to each temporal interval. The mean saccadic delay (MSD) was estimated prior to image presentation for each subject. The blank frames in the fixation trials were set to appear for a duration equal to the MSD prior to target/distracter presentation of 150 ms. The duration of image presentation in saccade trials was set to 150 ms plus the MSD in order to ensure that the image presentation duration after saccadic landing was also approximately 150 ms. doi:10.1371/journal.pone.0048761.g001 distracter image with a fixation point centered at the previous location of the target/distracter. Subjects were instructed to maintain fixation and delay their target versus distracter button press until the variable length fixation period (varying from 1 to 2 seconds) that followed target/ distracter presentation ended (signified by disappearance of the fixation point). Target and distracter button presses were done with the same (right) hand. (Figure 1B). Procedures to Approximately Equate Fixation and Saccade Image Presentation Times In order to compare the saccade and fixation P3, procedures were developed to minimize the difference between: 1) image presentation duration in the fixation trials and 2) the image presentation duration after saccadic landing in the saccade trials. In order to approximately equate image presentation duration in fixation trials and image presentation duration after saccadic PLOS ONE | www.plosone.org 3 November 2012 | Volume 7 | Issue 11 | e48761