Synchronization of Neural Activity across Cortical Areas Correlates with Conscious Perception

TLDR

Subliminal stimuli elicit deep processing in brain areas similar to those activated by consciously perceived stimuli, and transient neural synchronization has been proposed as a correlate of conscious perception. The study aims to determine which neural activity signatures differentiate conscious from unconscious processing by testing the transient synchronization hypothesis in a delayed matching‑to‑sample task with visible and invisible words. The authors compared electrophysiological responses during a delayed matching‑to‑sample task for visible and invisible words. Both perceived and nonperceived words increased local gamma oscillations, but only perceived words induced transient long‑distance gamma synchronization, followed by divergent electrographic signatures, and perceived words also produced enhanced frontal theta during maintenance, increased P300, and increased gamma power and phase synchrony before the test word,.

Abstract

Subliminal stimuli can be deeply processed and activate similar brain areas as consciously perceived stimuli. This raises the question which signatures of neural activity critically differentiate conscious from unconscious processing. Transient synchronization of neural activity has been proposed as a neural correlate of conscious perception. Here we test this proposal by comparing the electrophysiological responses related to the processing of visible and invisible words in a delayed matching to sample task. Both perceived and nonperceived words caused a similar increase of local (gamma) oscillations in the EEG, but only perceived words induced a transient long-distance synchronization of gamma oscillations across widely separated regions of the brain. After this transient period of temporal coordination, the electrographic signatures of conscious and unconscious processes continue to diverge. Only words reported as perceived induced (1) enhanced theta oscillations over frontal regions during the maintenance interval, (2) an increase of the P300 component of the event-related potential, and (3) an increase in power and phase synchrony of gamma oscillations before the anticipated presentation of the test word. We propose that the critical process mediating the access to conscious perception is the early transient global increase of phase synchrony of oscillatory activity in the gamma frequency range.