Neural evidence for a two-stage model of conscious perception

Are we conscious at any moment? Postdictive effects, where later events influence the perception of earlier ones, suggest a two-stage model of conscious perception: extended periods of unconscious information processing (stage 1) occur before a discrete conscious percept emerges (stage 2). These effects are evident in the Sequential Metacontrast Paradigm (SQM), where a sequence of diverging lines is displayed. When one line is spatially offset (i.e., vernier), the offset appears to propagate along the entire stream, even if the vernier is actually presented later. Furthermore, when two verniers with opposite offsets are presented in the stream, they cancel each other and cannot be individually reported. Here, we analyzed electroencephalography (EEG) data during the SQM, presenting either one or two opposite verniers at different locations within the stream, and a control condition with only straight lines. We sought to disentangle the neural correlates of stage 1 —when individual verniers are unconsciously processed— and stage 2 —when an integrated conscious percept emerges. Using temporal decoding, we found that the presence of vernier in the stream (one or two verniers vs. none) can be decoded from occipital EEG activity patterns. Moreover, the earlier the vernier is presented, the earlier it can be decoded. Since this holds true even for conditions with two verniers, where neither is consciously accessible, we interpret this as neural correlates of unconscious processing (stage 1). We then showed that behavioral reports can be instead decoded from parietal EEG activity patterns, emerging later. Since this decoding relates to the reported percepts, we interpret this as neural correlates of conscious perception (stage 2).