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Influence of shading on biological motion perception: illusion and model

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Fedorov, L., & Giese, M. (2015). Influence of shading on biological motion perception: illusion and model. Perception, 44(ECVP Abstract Supplement): 2T2A003, 177.

Cite as: http://hdl.handle.net/21.11116/0000-0004-DBE4-4
Research on biological motion perception has focused on 2D motion and form cues, while shading cues were largely neglected. Using point-light stimuli with volumetric elements, we discovered a new visual illusion, where the change of the light source direction flips the perceived walking direction. We extended a neural model for biological motion perception (Giese, 2003) to account for this illusion. METHOD: Walker stimuli consisted of 3D conic shapes that were centered on the limbs and illuminated from different light-source positions. We studied the dependence of the perceived walking direction on the position of the light source. We extended the model by a new pathway, which processes shading cues by analysis of inner brightness gradients. RESULTS: The light source position has a strong influence on the perceived walking directions (F(16,176) > 178, p < 0.01), where illumination from below results in a flip of the perceived walking direction by 180 deg compared to the veridical direction. The model reproduces this illusion. A control experiment reveals the critical shading features that cause the illusion, consistent with predictions from the model. CONCLUSION: Biological motion perception is influenced by a lighting-from-above prior, similar to the perception of static shapes (Brewster, 1874; Ramachandran, 1988).