The effects of pattern shape, subliminal stimulation, and voluntary control on multistable visual perception.

Spontaneous figure reversal of ambiguous patterns was analyzed in humans. A) With Necker-"cube"-like, or "drum"-like figures, having square or round shaped "front" and "rear" surfaces, and either large or small "depth", the perceptual intervals corresponding to both interpretations of "drum" were longer than those of "cube"; the perceived "depth" of the figures was less relevant for reversal timing (inter-reversal intervals were only slightly longer for the "deeper" figures). Although the shape of "front" and "rear" surfaces is not a crucial geometrical feature for figure reversal, it did influence its timing. More, or longer information-processing steps should probably be needed for perceptual representations of curvilinear patterns in comparison with rectangular ones. The underlying neural mechanisms are probably located at a relatively peripheral level in the visual system. B) With a modified Necker "cube"-like figure, having the two internal vertices coincident, and the long axis of the figure aligned horizontally, the effect of voluntary control on perception-reversal timing overcomes opposite effects due to either fixation-attention to pattern's focal zones, or subliminal stimulation by the pattern's biased versions, suggesting one or the other perception's possibility, while it is enhanced by concordant imagery. Voluntary control should intervene downward at a high-level processing, and should probably affect both a decision-making and a perception-stabilizing mechanism in the process of the pattern's unconscious interpretation. Results A and B are confronted with other results on both perceptual and binocular rivalry of up-to-date literature, in the frame of discussions on low-level bottom-up automatic stimulus-driven processing vs high-level top-down covert attention-driven processing.