significant cluster in which the grand average saccade distracter EP was greater than the grand average fixation distracter EP from approximately 0 to 200 ms with an associated false alarm probability of p = 0.008. In addition, there was a significant cluster in which the fixation distracter waveforms were greater than the saccade distracter waveforms with an associated false alarm probability of p = 0.023 and a temporal extent from approximately 300 to 650 ms.). The periods of significant difference observed in Figures 5 and 6 described above almost certainly were partially attributable to EOG components that were present in the saccade conditions but not in the fixation conditions. Note that saccades produce spike artifacts at saccade onset and step-like EOG artifacts [28] due to the rotation of the cornea- retinal dipoles during saccades. Furthermore, the differences observed in Figures 5 and 6 could be at least partially attributable to the saccadic lambda occipital response known to occur after saccades [15,16]. Importantly, in contrast to the results shown in Figures 5 and 6, EOG and other saccade related EEG activity was expected to cancel out in the saccade target versus saccade distracter comparison shown in Figure 4. As was described in the previous section and as can be seen in Figure 4, no significant differences in the saccade and fixation P3 were observed. Due to the greater impact of EOG artifacts and other saccade-related EEG activity on Figures 5 and 6 relative to Figure 4, most of the conclusions pertaining to neural post-saccadic modulation that will be discussed in the following section rely upon the results of Figure 4 rather than Figures 5 and 6. Discussion Similarity of Saccade and Fixation P3s P3 amplitude, persistence and start time are shown here to not differ significantly between fixation and saccade conditions when efforts are made to equate the stimuli in both conditions (see Methods for details). Although one subject did show a significant cluster of amplitude difference between the saccade and fixation P3s on the individual subject level (Figure 3), no significant amplitude differences were observed in the cross-subject grand average (Figure 4) and 7 out of 8 of the subjects did not exhibit significant differences between the saccade and fixation conditions on an individual subject level (Figure 3). Both the saccadic and fixation P3 were widespread on the scalp, and showed a progression of TMD positivity that began at anterior scalp sites before shifting to posterior scalp sites. The shift in the maxima of the P3 from frontal to more posterior sites over time is consistent with past research investigating the neural sources of the P3. A joint fMRI/EEG study [37] and EEG studies [38] of the P3 indicate that a widespread network of brain regions is responsible for the P3 signal observed in EEG, with the target minus distracter positivity beginning in frontal regions and then progressing to more posterior regions. No Significant Modulations of the Saccadic P3 Importantly, in this study, in contrast to other electrophysio- logical studies in which post-saccadic amplitude modulations of neural activity were observed, rather than comparing post- saccadic neural activity directly to a baseline of neural activity recorded at fixation, we instead compared the difference of two post-saccadic signals (the saccade P3) to the difference of two signals measured at fixation (the fixation P3). Therefore, any general, saccade-related modulations in neural activity (including sensory responses and any saccade-related EOG artifacts) were expected to cancel out in the saccade target minus saccade The Fixation and Saccade P3 distracter subtraction. Due to the removal of the common, saccade related EEG and EOG activity in the saccade target minus saccade distracter comparison, we were able to focus on neural amplitude differences between the saccade and fixation conditions associated with subjects’ categorization of the presented images. Indeed, it was found that when the saccade target and saccade distracter neural responses were compared directly to the corresponding fixation target and fixation distracter neural responses, there were significant amplitude differences in the EP waveforms (Figures 5 and 6). The significant amplitude differences between the saccade and fixation EPs when they were compared directly in Figures 5 and 6 likely are at least partially attributable to the post-saccadic amplitude modulations that have been observed in monkey electrophysiology [6,7], but because the data herein are scalp EEG data it is not possible to distinguish any such post- saccadic amplitude differences from the multiple signals that are expected to be present in EEG data after any saccade but are not present at fixation, including EOG artifacts and neural activity such as the saccadic lambda occipital response [15,16,28]. As was seen in Figure 4, in contrast to Figures 5 and 6, when the saccade P3 was compared to the fixation P3 there were no significant differences observed. Importantly, this result does not contradict past work that has found post-saccadic modulations in visual responses after a saccade relative to visual responses measured at fixation. The results presented herein could be entirely consistent with past research if post-saccadic amplitude modulations affected the processing of saccade targets and saccade distracters equivalently. It has been found that amplitude modulations of visual saccadic responses relative to visual responses measured at fixation occur as early as the LGN [9]. Importantly, any amplitude modulations of post-saccadic respons- es relative to fixation responses originating in the LGN would be expected to alter saccade target and saccade distracter trials equivalently and cancel out in the saccade target minus saccade distracter subtraction. Therefore, the absence of an amplitude difference between the saccade and fixation P3 does not contradict saccade versus fixation amplitude modulations of visual responses originating in the LGN or other neural regions involved in the initial stages of visual processing prior to stimulus categorization. However, from the absence of a significant fixation and saccade P3 amplitude difference, it can be concluded that post-saccadic modulations are likely limited to sensory and other early neural responses after saccade landing, and that post-saccadic modula- tions have no or very limited effects on the P3. In [25], a very early (140 ms post-saccadic landing) significant target minus distracter difference was observed after saccades in a free viewing condition. The early difference observed after saccades in [25] preceded the conventional fixation P3 by approximately 100 ms, and had a topography over central/ parietal regions whereas the conventional fixation P3 generally begins at frontal electrodes [38]. Note that the post-saccade EPs in [25] were recorded in a free viewing condition in which it was relatively difficult to verify that target and distracter stimuli were indistinguishable to subjects prior to saccade landing. Therefore, one possible explanation of the results in [25] is that object recognition was not fully constrained to the time period after saccade landing, thereby resulting in an earlier, significant target minus distracter difference after saccade landing than at fixation. In contrast, in the experiments described here, saccades to the objects to be recognized were higher in amplitude (10u) as well as constrained in size and controlled for image visibility prior to saccade landing. Another, perhaps more critical difference between the experiments described here and those described in [25] is that, herein, target and distracter images were equated so PLOS ONE | www.plosone.org 12 November 2012 | Volume 7 | Issue 11 | e48761