TMS-EEG approach unveils brain mechanisms underlying conscious and unconscious face perception.

Background: Conscious perception of external stimuli has been related to recurrent activity in distributed cortical networks, although brain mechanisms controlling unconscious processing and stimuli access to conscious report need to be clarified.

Objective: This study aims at investigating modulations in cortical excitability related to conscious perception and unconscious processing of face stimuli with different visibility levels.

Methods: We used TMS-EEG over the right occipital face area (rOFA), or the right premotor cortex (rPMC) as control site, to measure cortical excitability during a backward masking paradigm with individually defined stimuli visibility.

Results: Event related potentials showed significant differences for faces compared to houses, and detected faces compared to missed ones, 200 ms post target onset. TMS over rOFA, but not over rPMC, triggered a relative positivity starting 150 ms post target when faces with high visibility were consciously reported. Moreover, rOFA TMS evoked differential responses for high versus low visible faces in conscious and unconscious processing at 290-390 and 180-240 ms, respectively.

Conclusion: Results unveiled a causal link between rOFA excitability and late responses related to access to conscious perception, suggesting a critical role of recurrent activity, but distinct components, for consciously perceived stimuli and unconscious face processing.