Independence of visual awareness from the scope of attention: an electrophysiological study

Recent brain imaging studies have revealed that increased neuralactivity along the ventral visual stream and parietal and frontalareas is associated with visual awareness. In order to studythe time-course and temporal aspects of awareness, we examinedelectrophysiological correlates of conscious vision in two maskingexperiments. The differences in event-related potentials (ERPs)between unmasked (consciously recognized) and masked (unrecognized)stimuli were considered to be electrophysiological correlatesof awareness. Two attentional conditions (global, local) wereincluded to examine the relationship between the scope of attentionand awareness. Two ERP-deflections were found to correlate withawareness. First, awareness was associated with a posteriornegative amplitude shift 130–320 ms after the stimulus.This effect was present in both attention conditions, suggestingthat it emerges independent of the scope of attention. Second,ERPs to unmasked stimuli became more positive as compared withmasked stimuli around 400 ms, peaking at parietal sites. Thiseffect was attenuated in the local attention condition, althoughthe participants were aware of the stimuli, suggesting thatthe late positivity does not directly correlate with visualawareness. The results imply that the earlier negativity isthe earliest and most direct correlate of visual awareness.