Early ERPs dissociate subjectively nonconscious low- and high-level face processing.

Abstract

There is an ongoing debate about the extent to which faces are processed if they are not consciously perceived. In the present study, we used event-related potentials (ERPs) to investigate neural responses to faces and two types of control stimuli (monochrome color-matched ovals and Fourier phase-scrambled faces), which allowed us to dissociate low-level and high-level face processing. Based on a pre-registered sequential Bayesian sampling protocol, we recorded the electroencephalogram (EEG) from 40 participants and compared the average amplitude of early components of the ERP (P1, N170) between faces, scrambles, and blanks presented for 17 ms, while the mask followed directly or 200 ms after the target stimulus. Participants were asked to rate their subjective perception after each trial on a perceptual awareness scale, and only trials with the lowest rating in the masked condition were considered as subjectively nonconscious. Matching the pre-registered hypotheses, P1 amplitudes were higher for faces and scrambles compared to blanks but did not differ between faces and scrambles. This pattern was found for conscious and nonconscious faces, however, with smaller yet reliable differences in the latter case. In contrast, the N170 reliably differentiated between faces and both types of control stimuli (scrambles and blanks), again for conscious and, with attenuated differences, also for nonconscious faces. Findings support the hypothesis of two early stages of face processing, which are at least partially independent of consciousness awareness of stimuli. The P1 stage is associated with low-level processing, while the N170 reflects processing of face-related configural information.