Investigating electrophysiological correlates of nonconscious processing of facial expressions using dichoptic forward masking

Affective stimuli, such as fearful faces, are assumed to receive prioritized processing over neutral stimuli, making them easier to detect when perception is suppressed using techniques such as visual masking or binocular rivalry. Moreover, some studies suggest that fearful expressions can be processed outside of conscious awareness, as evidenced by enhanced early event-related potentials (ERPs) in response to subliminally presented fearful vs. neutral faces. This might be associated with increased detection of fear-related stimuli. However, it remains unclear whether increased cortical processing of unseen fearful faces is a necessary condition for detection advantages and whether effects are related to expression-related or low-level features of stimuli. In the present pre-registered ERP study, we addressed these questions using stereoscopic forward masking (FM). Forty-eight male and female participants were presented with fearful and neutral, intact and phase-scrambled faces. The behavioural results showed that masked fearful faces were detected better than neutral faces. Phase-scrambled faces did not show this effect, indicating that the perceptual advantage was not mediated by low-level spectral stimulus properties. ERPs elicited by consciously perceived faces showed typical expression effects in the form of enhanced occipito-temporal negativities in the time range of the N170 and the early posterior negativity (EPN). Crucially, these effects were absent for subjectively invisible faces, as corroborated by Bayesian statistics. Taken together, our results suggest that the perceptual advantage of fearful faces under dichoptic FM cannot be attributed to low-level stimulus properties. Furthermore, within this paradigm, the perceptual advantage does not coincide with the amplification of early ERPs to subjectively invisible faces, suggesting a dissociation between unconscious stimulus processing and stimulus detection.