Cognitive ambiguity, such as found in categorical judgments, increases behavioral response latency. Here we examined whether perceptual ambiguity for bistable stimuli, stimuli in which two perceptual interpretations were mutually competitive, also increased perceptual latency. We presented a bistable stimulus and measured the observer's reaction time to judge which of two possible percepts was seen. Perceptual ambiguity was systematically manipulated and how it affected the response latency was examined. The first experiment used a motion-defined rotating cylinder. The observers judged the rotation direction, and the perceptual ambiguity was controlled by binocular disparity. The second experiment used Rubin's vase. The observers judged whether the figure was a vase or faces, and the perceptual ambiguity was controlled by luminance of the surround. In both experiments, we found that the perceptual ambiguity caused only a small or no increase in reaction time and, presumably, in the perceptual latency included in the reaction time. These findings suggest that perceptual competition does not have a strong effect on the latency of the initial perception of bistable stimuli. Given that many perceptual problems are under-constrained as in the cases of bistable stimuli, it is presumably ecologically functional for the brain to establish perception as quickly as possible regardless of the presence of potential alternatives.
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