Are we conscious at all times? Neural evidence for a two-stage model of visual processing

Postdictive effects, where later events influence the perception of earlier ones, suggest that conscious perception is not a continuous stream but occurs at discrete moments in time, preceded by extended periods of unconscious processing. This is evident in the Sequential Metacontrast Paradigm (SQM), where a stream of lines and vernier offsets is unconsciously integrated over several hundred milliseconds before a conscious percept emerges. Regardless of whether one or multiple verniers are shown, only a single offset is perceived. Postdictive phenomena offer a unique opportunity to study the neural correlates of unconscious and conscious stages of perception because unconscious and conscious processing are well separated in time. Here, using EEG recordings during the SQM, we identified two distinct stages of neural activity. Early occipital EEG activity patterns (∼200 ms after the initial vernier presentation) capture unconscious processing, while later centro-parietal EEG patterns (∼400-600 ms after the onset of the stimulus stream) are associated with conscious perception, aligning with behavioral results. We propose that the transition between these distinct neural topographies reflects the discrete moments when conscious perception emerges.