Size contrast explained by the statistics of scene geometry
Standard presentations of size contrast stimuli include the well-known Ebbinghaus circles and the Delboeuf figures. When presented with such stimuli, observers perceive a target form surrounded by larger but otherwise similar forms to be smaller than the same target surrounded by smaller forms. Here we have examined the hypothesis that the anomalous perception of these stimuli is a consequence of a wholly probabilistic strategy of vision in which percepts accord with the probability distribution of the possible sources of the stimuli. To test this idea, we used a range image database acquired by laser scanning natural scenes to determine the probability distribution of the size of the real-world sources of the central targets in the size contrast stimuli. In good quantitative agreement with a large body of psychophysical evidence, the average physical size of the sources of a given form embedded in a context of larger surrounding forms in the image plane is smaller than the sources of the same target surrounded by smaller forms. Thus, the reason why the two identical central targets look different in size is because their possible physical sources are, in fact, different in size. These findings support the hypothesis that the size contrast effect is a signature of a fundamentally probabilistic process of vision perception.
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