Neural processing around 200 ms after stimulus-onset correlates with subjective visual awareness

Abstract

Does visual awareness correlate with early activity in visual cortex or with later wide-spread neural activation? This question was studied by presenting liminal targets in one of the four quadrants of the screen and asking the participants to make forced-choice localization responses and to rate their subjective visual awareness of each target, while electroencephalography was measured. In the analyses of event-related potential (ERP) correlates of awareness, response accuracy was kept constant so that only the subjectively rated awareness varied between high-awareness ('seen' rating) and low-awareness ('unseen' rating) targets. High-awareness-correct trials were associated with enhanced contralateral N200 at 180-280 ms as compared with low-awareness-correct trials. This effect (visual awareness negativity, VAN) also correlated with aware sensitivity to the presence vs. absence of the stimulus (d'). In addition, high-awareness-correct trials were associated with later enhanced P3 amplitudes (after 350 ms), but this effect correlated only with the response bias (c). ERPs to low-awareness-correct trials elicited larger contralateral N200 than ERPs to low-awareness-incorrect trials, and this effect correlated with conservative response bias, suggesting that it reflected weak awareness rather than unconscious processing. The results suggest that the enhanced N200 correlates with graded awareness. The results support theories of visual awareness in which early activity in visual cortex gives rise to subjective visual experiences.

Keywords: Awareness; Consciousness; EEG; ERP; Visual cognition.