Neural correlates of visual awareness at stimulus low vs. high-levels of processing.

A crucial view in the graded vs. dichotomous debate on visual awareness proposes that its graded or dichotomous nature may depend on the depth of stimulus processing (or level of processing) associated to the experimental task. In the present study, we explored the behavioral patterns and neural correlates of different degrees of awareness associated to different depths (i.e. low vs. high) of stimulus processing. The low-level stimulus condition consisted of detecting the location of the target based on its brightness characteristics, whereas the high-level stimulus condition consisted of identifying which of four possible targets (numbers/letters) had appeared. Behavioral results showed that both subjective ratings of awareness and accuracy levels increased linearly as a function of awareness and independently of the level of stimulus processing. Additionally, the electrophysiological recordings revealed two correlates of visual awareness: enhanced posterior negativity in the N200 time window (VAN, visual awareness negativity) and enhanced positivity in the P3 time window (LP, late positivity). Interestingly, we found evidence of awareness levels modulating N200/VAN amplitudes in a graded manner only for the low-level task, whereas P3/LP amplitudes were modulated in a graded manner for both low and high-level tasks. The finding that the early posterior correlate of visual awareness (VAN at 150-250 ms) was sensitive to level of processing is consistent with task effects occurring in the visual cortex and supports the view that it is mediated by attention to task-relevant features. The amplitudes of P3/LP in both tasks correlate more directly with graded awareness and behavioral accuracy.