An oblique illusion modulated by visibility: non-monotonic sensory integration in orientation processing.

Orientation perception is known to be anisotropic, with cardinal axes (i.e., horizontal and vertical) being privileged. Indeed, orientation sensitivity is greater near the cardinals, and small deviations from cardinal axes may be illusorily perceived in an exaggerated manner. Here, we quantified this illusory deviation from the cardinals at various visibility levels, by having participants reproduce the orientation of oriented Gabor stimuli whose visibility was manipulated by duration and masking. We found, first, that participants could reproduce quite accurately the orientation of very brief stimuli presented at lowest visibility levels. Second, the magnitude of the deviation followed a non-monotonic pattern, being maximal for stimuli of intermediate visibility, and lower for both the lowest and highest visibility levels. Thus, orientation processing at lowest visibility levels is noisier but paradoxically more faithful to the physical input. This counterintuitive result suggests that categorical processing of sensory information depends on perceptual awareness.