Casey Becker¹, Witold Lipski¹, Mary-Kate Richey¹, Arish Alreja¹, Jorge Gonzalez-Martinez¹, Taylor Abel¹, Avniel Ghuman; ¹University of Pittsburgh

Since at least the seminal studies of Yarbus, we have known that people look at faces in stereotyped ways. Furthermore, disruptions of these gaze patterns are a hallmark of face processing deficits. Nonetheless, with a few notable exceptions, nearly all we know about the neurodynamics of face perception relies on paradigms where faces are presented abruptly at central fixation, overlooking the dynamic role of eye movements in natural vision. Here, we characterize the cortical and subcortical neurodynamics of face perception during unrestrained sequential fixations around a face using eye tracking and intracranial EEG in human participants. Participants free viewed static images of happy, angry, fearful, and neutral facial expressions for 1500 milliseconds. Behaviorally we found an initial freezing of eye movements upon stimulus onset, followed by a first saccade around 200ms consistently across participants. This timing aligns with the N170 timeframe and give rise to the question of how much of early face-related potentials are associated with this oculomotor freezing and the planning of the first saccade. Comparing responses across multiple fixations (fixation-related potentials) to traditional event-related potentials (ERPs) elicited by sudden onset faces uncovered large differences in the neurodynamics between the onset response and neural responses to subsequent fixations. Using Multi-Temporal Pattern Analysis (MTPA) we characterize distinct cortical and subcortical responses to different facial features, including presaccadic activity related to the facial feature participants saccade to next. These neurodynamics highlight the neural foundations of the dynamic active sensing process in face perception during free viewing.