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Peggy Gerardin, Marie de Montalembert, Pascal Mamassian; Shape from shading: New perspectives from the Polo Mint stimulus. Journal of Vision 2007;7(11):13. doi: 10.1167/7.11.13.
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The visual system exploits prior knowledge on the world to disambiguate a scene. In particular, the inference of shape-from-shading relies on the fact that light comes from above our heads. Recent studies have helped make that assumption more precise and suggested that the preferred light source position was further biased on the left of the vertical. We investigated the generality of this result in two experiments with a different stimulus and a novel task. The stimulus was shaped like a ring ( Polo Mint) divided in eight equal sectors. Depending on the assumed light source position, all sectors but one could be perceived as either convex or concave. In the first experiment, observers had to report the different shaped sector position (left or right side of the stimulus). In the second experiment, they had to classify the shape of the odd sector (convex or concave). Various amounts of blur were also applied on each stimulus. Results in both experiments confirm previous studies that observers prefer stimuli lit from the left rather than the right. We also demonstrate that left-lit stimuli give the observers a more sensitive perception of the shape of the objects. Finally, the second experiment confirms a preference for globally convex shapes, especially when the stimulus is severely blurred.
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