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Peggy Gerardin, Marie de Montalembert, Pascal Mamassian; Polo mint shading. Journal of Vision 2006;6(6):261. doi: https://doi.org/10.1167/6.6.261.
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The visual system exploits prior knowledge on the world to infer shape from shading, in particular the fact that light comes from above rather than from below. Recent studies have suggested that the assumption on the light source position was further biased on the left of the vertical (Sun and Perona, 1998, Nature Neuroscience, 1, 183–184; Mamassian and Goutcher, 2001, Cognition, 81, B1–B9). We investigated the generality of this result with a different stimulus and a novel task. The stimulus was shaped like a polo mint candy divided in eight equal sectors. All but one sectors had the same curvature, namely convex or concave. Upon a brief presentation of a stimulus, observers had to report the side (left or right) that contained the odd curved sector. Stimuli were generated using one of four possible light source positions and were then blurred using one of ten low-pass filters. Results show a strong asymmetry in accuracy in favour of stimuli lit from the left rather than the right. This asymmetry was found both for naïve and experienced observers. There was also an advantage to detect a concave sector amongst convex ones. However, various amounts of blur did not produce any systematic differences. Our results therefore replicate previous studies that found a consistent leftward bias for the assumed light source position. The possible origins of such a preference are discussed.
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