Shape from shading: New perspectives from the Polo Mint stimulus

Abstract

International audienceThe 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

Prior knowledge of illumination for 3D perception in the human brain

“…A repeatedmeasures ANOVA showed a significant main effect of light source position on the perceived shape [F(7, 48) = 8.79, P < 0.0001]. As expected from our previous behavioral work (20), the most ambiguous ring shape (the crossing points between convex and concave) did not occur for horizontal light directions: that is, halfway between images "b" and "c" and "f" and "g." This observation illustrates a bias of the observers to prefer a light source located to the left of the vertical. To quantify this bias, we fitted the data in Fig.…”

“…Using these methods, we test for fMRI sensitivity in discriminating Light and Shape across various visual, temporal, and parietal regions known to be engaged in the representation of shape from different depth cues (15)(16)(17)(18)(19). We then compare fMRI sensitivity for Light and Shape discrimination to that predicted from the behavioral data (20). Our fMRI results demonstrate that this left-light bias is processed at early stages of visual processing.…”

The light-from-above prior is intact in autistic children

“…As expected (cf. Gerardin et al, 2007, Sun and Perona, 1996, Thomas et al, 2010), average light source biases were to the left of vertical (autistic children: M  = −12.67°, SD  = 12.51; typical children: M  = −12.50°, SD  = 7.87; adults: M  = −12.06°, SD  = 16.72) and were significantly lower than zero for all groups [autistic children: t (17) = −4.44, p  < .001; typical children: t (17) = −6.72, p  < .001; adults: t (14) = −2.79, p  = .01]. A one-way analysis of variance (ANOVA) with group (autistic children, typical children, or adults) as a between-participants factor revealed no significant effect of group on the magnitude of the light bias, F (2, 48) = 0.10, p  = .99.…”

Anticlockwise or clockwise? A dynamic Perception-Action-Laterality model for directionality bias in visuospatial functioning

“…Another type of orientation effect has been demonstrated in both visual search and shape discrimination tasks, such as humans tend to perform best in visual search and 3D shape discrimination tasks under top-left lighting conditions (Elias & Robinson, 2005; Gerardin, de Montalembert, & Mamassian, 2007; Mamassian, Jentzsch, Bacon, & Schweinberger, 2003; Sun & Perona, 1998; Thomas, Nardini, & Mareschal, 2010). Sun and Perona (1998), for example, were the first to demonstrate that shaded targets are detected more quickly when the illumination position is between 30° and 60° to the left of vertical.…”