Electrophysiological signatures of visual awareness can be distinguished from other task-related processes by manipulating the level of processing of visual stimuli. In an EEG experiment, 36 subjects performed either color (low-level) or magnitude (high-level) evaluations of masked digits and rated their subjective visibility. Mean amplitude in the VAN component time window (140-240 ms) correlated with visibility ratings in both conditions. Mean amplitude in the LP time window (380-480 ms) correlated with visibility ratings only in the high-level condition. These results support a temporal unfolding of ERP markers of conscious processing: an early component reflects initial perceptual experience, while a late component correlates with conscious experience of non-perceptual information.
Transitions from unconscious to conscious perception may be more gradual for low-level visual features and more dichotomous for high-level features. In an fMRI study using backward masking, participants reported a target digit's magnitude (high-level) or its color (low-level) and rated visibility. Intermediate visibility was reported more often in the low-level task, supporting the levels-of-processing hypothesis. Visible targets activated insulo-fronto-parietal regions in both tasks, while visual areas showed higher activity in the low-level task across all visibility levels. The findings suggest that attentional modulation of feature-representing brain regions mediates how processing level shapes conscious perception.