Isolating the proper neural correlates of visual awareness from its neural consequences

One of the major challenges in the neuroscience of consciousness is to disclose the timing and neural mechanisms underlying visual awareness, the subjective experience of seeing. Electroencephalography (EEG) studies investigating the time course of consciousness-related neural processes have found two potential correlates of visual awareness: the N2 and the P3 ERP components. However, recent works have suggested that only N2 correlates to visual awareness, whereas later neural activity expressed by the P3 component reflects post-perceptual processes related to subjects’ report or to accumulation of sensory evidence leading to decision-making. Building upon this observation, the objective of this study was to provide further evidence that the P3 component reflects a post-perceptual process. To this end, we designed two EEG experiments in which we directly manipulated decision making processes and visual awareness while keeping constant the physical property of visual stimuli. This experimental manipulation allowed us to identify the possible influences of post-perceptual processing over the time course of neural responses and determine the temporal window at which such influence occurs. In Experiment 1, we manipulated participant’s decision criterion by inducing, respectively, a liberal and a conservative decision bias in two different experimental sessions. The aim of this first Experiment was to determine whether our manipulation of the decision processes would produce a modulation of P3 ERP component. Experiment 2 was identical to Experiment 1 except that participants were not requested to adjust their decision criterion (own criterion session). The aim of this experiment was to examine whether in a condition in which there was no manipulation of post-perceptual processes, N2 and P3 ERP component would distribute differently. Electrophysiological and Behavioral results of Experiment 2 were then compared with those of Experiment 1. If the amplitude of the P3 reflects post-perceptual processes related to decision making processes, one would expect to find some differences in the topography or in the time-course of the P3 between the condition in which a decision criterion was imposed (Experiment 1) and the condition in which there was not a decision bias induced (Experiment 2). ERP results of Experiment 1 revealed that the amplitude of the N2 and the P3 components were enhanced for those trials were subjects reported to have seen the stimulus as compared to unaware trials. Importantly, the amplitude of the P3 was modulated by the decision criterion: it was enhanced when participants adopted the liberal criterion compared to the conservative criterion, suggesting that P3 reflects brain processes related to decision making that occurs after that awareness has emerged. ERP data of Experiment 2 confirmed that aware responses were associated with enhanced N2 and P3 amplitude than unaware responses. Interestingly, the decision criterion manipulation had an effect on P3 component revealing that the own and the liberal criteria were associated with an increased positivity over central areas if compared to the conservative criterion. In addition, we found that the amplitude of the N2 was enhanced for the own session if compared to conservative and liberal sessions. Overall these results suggest that when sensory information was relevant for the task (own session) a bigger N2 was observed. On the contrary, P3 amplitude was sensitive to the manipulation of the decision criterion, suggesting a critical role of neural activity expressed by the P3 component in decision making processes. These findings support the hypothesis that P3 might reflect post-perceptual processes that occur after that awareness has emerged while the N2 component reflects a proper correlate of visual awareness.