A novel signature of visual awareness independent of post-perceptual processing

What are the neural processes associated with perceptual awareness that are distinct from pre-conscious sensory encoding and post-perceptual processes such as reporting an experience? Here, we used electroencephalography (EEG) and a no-report visual masking paradigm with parametric manipulations of stimulus visibility to search for neural signatures associated with perceptual awareness independent from both early sensory processing and subsequent reporting. Specifically, we manipulated the time between stimuli and subsequent masks in a linear manner (i.e., 17ms, 33ms, 50ms, 67ms, and 83ms) such that observers awareness of the stimuli increased in a non-linear fashion (i.e., stimuli were never seen at the two shortest time intervals, always seen at the two longest intervals, and seen approximately 50% of the time at the intermediate interval). Moreover, we manipulated the task across blocks to create separate report and no-report conditions. Overall, we found one neural signal that was closely associated with perceptual awareness, independent from the task: a fronto-central event-related potential (ERP), from [~]250-300ms, that we refer to as the N2. In contrast, earlier ERP signals were linked with the linear manipulation of stimulus strength regardless of visibility, while later candidate signatures, such as P3b and temporal generalization of decoding, were present in the report condition but absent in the no-report condition suggesting a closer association with task performance than perceptual awareness. Taken together, these findings inform current debates regarding theories of consciousness and offer new avenues for exploring the neural mechanisms supporting conscious processing. Significance statementWhat differentiates conscious and unconscious processing in the brain? Here, we identify an electrophysiological signature of perceptual awareness using a combination of visual psychophysics and electroencephalography (EEG). In addition, we used a newly developed "no-report" paradigm, in which observers did not report anything about their perceptual experience to separate neural signals associated with consciousness from those associated with the act of reporting (i.e., memory, motor planning, etc.). Using this no-report paradigm was critical because several other candidate signatures of conscious processing were present when observers reported their experiences but completely disappeared when observers did not report their experiences. These findings open the door to future research interested in the neural mechanisms associated of conscious processing.