Face Selective Neural Activity: Comparison Between Fixed and Free Viewing

Abstract

Event Related Potentials (ERPs) are widely used to study category-selective EEG responses to visual stimuli, such as the face-selective N170 component. Typically, this is done by flashing stimuli abruptly at the point of static gaze fixation. While allowing for good experimental control, these paradigms ignore the dynamic role of eye-movements in natural vision. Fixation-related potentials (FRPs) obtained using simultaneous EEG and eye-tracking, overcome this limitation. Various studies have used FRPs to study processes such as lexical processing, target detection and attention allocation. The goal of this study was to compare face-sensitive activity evoked by stimulus abrupt appearance with that evoked by self-controlled gaze fixations on a stimulus. Twelve participants were studied in three experimental conditions: Free-viewing (FRPs), Cued-viewing (FRPs) and Control (ERPs). We used a multiple regression approach to disentangle overlapping activity components. Our results show that the N170 face effect (face vs. non-face) is evident for the first fixation on a stimulus, whether it follows a self-generated saccade or stimulus appearance at fixation point. Furthermore, the N170 exhibits category-specific adaptation in free viewing. The N170 face effect had similar topography across viewing conditions, but there were major differences within each stimulus category. We ascribe these differences to an overlap of the fixation-related Lambda response and the N170. We tested the plausibility of this account using dipole simulations. This study establishes the use of the N170 face effect as a signature of face detection in free-viewing experiments while highlighting the importance of accounting for eye-movement related effects.