Fixation-Related Potentials Reveal Category Selective Activity, Upon First, But Not Second, Fixations on Faces

Event-related paradigms (ERPs) provide control over experimental conditions yet ignore the natural dynamics of visual perception involving eye movements. Fixation-related potentials (FRPs) overcome this limitation yet require deconvolution of overlapping responses to temporally proximal fixations. We used EEG with simultaneous eye tracking in 12 subjects with two aims: (a) comparing the face-sensitive activity evoked by stimulus abrupt appearance (N170), with that evoked by self-controlled fixations and (b) explore whether such activity is apparent in ensuing fixations on the same object. We presented face and nonface stimuli in three conditions: (a) cued-saccade for analyzing guided eye movement FRPs, (b) free-viewing for analyzing self-guided eye movement FRPs, and (c) control for analyzing classical ERPs. All FRP analysis was done using a multiple regression generalized linear model framework for activity overlap correction. The face-selective (faces minus nonfaces) FRP component elicited by the first fixation on stimuli and the classic ERP showed overall similar spatio-temporal characteristics. Major topographical differences between the responses emerged, however, within category, and we present source simulations to provide possible interpretation of the results. Second fixations did not elicit any category-specific activity in any of the three conditions. While this may be explained by adaptation, the unaltered perception remains a puzzle.