to cancel in the saccade target minus saccade distracter subtraction. Importantly, in addition, we equated the visual stimuli used for P3 targets and distracters [17], so that any post- saccadic modulations in sensory responses were expected to cancel out in the saccade target minus saccade distracter subtraction. It was therefore ensured that any differences that were observed between the fixation and saccade P3 could be disambiguated from post-saccadic modulations of sensory neural activity. The P3 has been used by multiple research groups as the basis of brain computer interfaces [18,19]. Therefore, in addition to determining whether or not the P3 is affected by post-saccadic modulations in neural activity, another main motivation for studying the P3 at saccade landing is that the saccadic P3 could be useful in the development of improved brain computer interfaces (BCIs) allowing for natural eye movements. Quantifying the characteristics of the saccadic P3, such as amplitude, persistence and start time, and, furthermore, quantitatively comparing saccadic and fixation P3 characteristics as is done here could provide useful information for use in the training of pattern classifiers used in BCIs [20,21]. There have been a few observations of P3 activity following saccadic landing [22–24] but, to the best of our knowledge, only one prior study has attempted to directly compare the fixation and saccade P3 [25]. However, as will be further explained in the discussion, in the aforementioned study, target and distracter visual stimuli were not equated [17], thereby leading to post saccadic differences between saccade and fixation conditions that were likely sensory in origin rather than P3 differences. Herein, when the saccade P3 and the fixation P3 were compared, it was determined that there were no significant differences in P3 start time, temporal persistence, or amplitude dependent on whether the P3 was measured at fixation or after a saccade. The similarity of the fixation and saccade P3 indicates that the saccadic P3 could be a viable signal for use in BCIs. The lack of significant differences between the saccade and fixation P3s does not contradict past research that has found post- saccadic modulations of early sensory neural responses (see Discussion). The results do, however, indicate that post-saccadic modulations are likely limited to sensory and other neural processes occurring in the early stages (approximately first 200 ms) after saccade landing. We conclude that post-saccadic modulations in neural activity do not significantly affect the P3. Materials and Methods Subjects Data were collected from eight subjects (four males and four females). All subjects signed informed consent forms prior to the start of experiments. The experiments were approved by the Committee for the Protection of Human Subjects at the University of California, Berkeley. All subjects ranged in age from 20–30 years, had normal vision without need of correction, and were right handed. All subjects except S1 and S6 were na¨ıve in regard to the aims of the study. Eye Position and EEG Data The Eyelink eyetracker was used to track horizontal and vertical eye position. Eye position was sampled at 1000Hz. An Active Two EEG System (Biosemi) was used to collect EEG data simultaneously with eye movement recording. The 64 electrodes were used in a headcap with the standard Biosemi 64 electrode layout. Details pertaining to the method of referencing of the Biosemi system during data collection can be found at: http:// www.biosemi.com/faq/cms&drl.htm. In addition to the 64 The Fixation and Saccade P3 electrodes on the scalp, two electrodes were placed on the mastoids during data collection. After data collection, all scalp electrode data were referenced to the mean of the two mastoid electrodes. Electrode P04 was excluded from analyses for S1, S2 and S7 and electrodes T7 and TP8 were excluded from analyses for S3 because it was noted during data collection that the aforementioned electrodes were not maintaining good contact, likely due to localized looseness of the electrode cap. All 64 electrodes were retained for within subject analyses for the other subjects. In cross subject analyses, only the 61 electrodes that were retained for all subjects were analyzed. Analog eye position signals from the Eyelink were fed into the Biosemi input box. In this manner, the horizontal and vertical eye positions recorded by the Eyelink were sampled along with the EEG at 512 Hz. The Biosemi system also recorded stimulus event markers generated by the stimulus presentation system (WinVis psychophysical and physiological testing toolbox). Target and Distracter Stimuli The definition of target and distracter was switched in a counterbalanced manner from one block of 70 trials to the next to rule out differences in the target/distracter neural responses that could be due to low level differences in the target and distracter images [17]. One of the images that could be either a target or a distracter consisted of a 2-D Gaussian multiplied by a rectangular wave (25.5 cycles/degree) on a black background. For both saccade and central fixation trials, the targets/distracters were defined as being either the image shown in the left of Figure 1A or just a Gaussian pedestal without the rectangular wave addition (Figure 1A, right). The two stimuli were designated as either the target/distracter on an equal proportion of trials for all subjects. In other words, image 1 was defined as a target just as often as image 2 for every subject, and therefore image 1 and image 2 were also defined as distracters for the same number of trials for each subject. Trial numbers were equated before trial exclusion procedures for artifact rejection. Before each block of 70 trials, subjects were informed of which image would be defined as the target and which image would be defined as a distracter for that block of trials. The 2-D Gaussians used for both targets and distracters had horizontal and vertical standard deviations of 1u. For both saccade and fixation trials, 20% of the total images presented were targets. Blocks of saccade and fixation trials were interleaved with randomized order of presentation. Prior to beginning each block of 70 trials, subjects were informed whether it would be a block of fixation or saccade trials. 560–840 trials were presented for each subject, with 50% of the trials being fixation trials, 25% involving leftward saccades, and 25% involving rightward saccades. Temporal Image Presentation Sequence Subjects had to foveate a fixation point (diameter 1.2u) for a variable period of 1–2 seconds. In saccade trials the fixation dot disappeared and the target/distracter stimulus appeared unpre- dictably on either left or right side at 10u distance for 150 ms plus the mean saccadic delay (MSD) estimated individually for each subject. (See following section for details of MSD estimation). This ensured that each subject had approximately 150 ms to distinguish a target from a distracter after saccadic landing. Similarly, in fixation trials, after the variable length fixation period, the fixation point disappeared and the screen went blank for a duration of the estimated MSD prior to a 150 ms long target/distracter presen- tation in the fovea. In both the fixation and the saccade trials, target/distracter presentation in the fovea and the periphery, respectively were followed by a replacement of the target/ PLOS ONE | www.plosone.org 2 November 2012 | Volume 7 | Issue 11 | e48761