Figure 1.

Experimental procedures. A, A single trial of the CFS task, with the presentation of a single picture at a given contrast level, overlaid by random texture masks flickering at 10 Hz. After a short interval, the participants reported a PAS score, reflecting their stimulus' subjective awareness level. The trial structure was identical for both the behavioral and fMRI sessions. B, In the pre-fMRI behavioral session, all masked stimuli belonged to the Living category. Each participant was presented with five trials for each of 21 image contrast levels, from full invisibility (−50%) to full visibility (+50%), with 5% increments. We plotted the number of responses (blue circles) with a PAS score of 3 or 4 [PAS(3,4)], indicating high to maximal subjective awareness. We then fitted a sigmoid function (see Materials and Methods), and used its point of inflection to define the subjective perceptual threshold. Six contrast levels were then set to be used in the fMRI session: a void-of-object baseline (Empty), two contrast levels below perceptual threshold (Thrm2, Thrm1), one at (Thr0), and two above (Thrp1, Full). C, In the fMRI session, the masked stimuli were either MOs or NOs. We looked for MO-specific BOLD responses that fitted: (bottom right) a subliminal, as opposed to a step or linear, profile of activation as a function of contrast [i.e., activation both below and above perceptual threshold (PT)]; (bottom left) a subliminal as opposed to a step profile as a function of PAS [i.e., activation both with and without subjective awareness (SA)].