Abstract: PURPOSE: Our visual system possesses remarkable ability to extract summary statistical in- formation from groups of similar objects, referred as ensemble perception. Here, we first investigated whether ensemble perception hinges on our conscious awareness of the stim- uli. Moreover, we further explored whether conscious and nonconscious ensemble pro- cessing differentially influence our perceptual decision making in a neutral orientation dis- crimination task.
METHODS: In Experiment 1, a set of Gabor patches arranged in a circle were presented near the center of the visual field and rendered invisible using the Continuous Flash Sup- pression (CFS) paradigm. Another set of Gabor patches with near vertical orientations were presented outside the inward array. Participants were asked to judge only the average ori- entation of the outward array. Experiment 2 was identical to Experiment 1 except that the inward array was presented without CFS.
RESULTS: Despite that participants couldn't consciously perceive the inward array in Ex- periment 1, their perceptual judgments of the average orientation of the outward array were significantly biased to the average orientation of the inward array. However, when the inward array was consciously perceived by participants in Experiment 2, their perceptu- al decision makings were somewhat repelled from the average orientation of the inward array.
CONCLUSIONS: The current study demonstrates that ensemble perception can occur even when stimuli are suppressed from conscious awareness. Moreover, conscious and noncon- scious perception of ensemble statistics differentially influence our perceptual judgments, suggesting that the perceptual decision making processes based on ensemble statistics engage both bottom-up and top-down mechanisms in the human visual system.

Key words: ensemble perception, summary statistics, unconscious, conscious, orientation