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Abstract

Most research on biological motion perception has focused on the influences of 2D motion and form cues,and sometimes also of binocular disparity, while the influence of shading has been largely neglected.The perception of 3D static shapes from 2D images is strongly influenced by a lighting-from-aboveprior (Brewster, 1847; Ramachandran, 1988). We observed that for biological motion stimuli withperceptually ambiguous walking direction (Vanrie et al. 2004) the illumination direction can radicallyalter the perceived walking direction of walkers that consist of volumetric moving elements at thejoints. METHOD: We replaced the dots of a walker by volumetric elements that are rendered withdifferent positions of the illuminating light source. We studied the dependence of the perceived walkingdirection on the position of this light source. RESULTS/DISCUSSION: We found a radical change ofthe perceived walking direction (corresponding to a rotation by 180 deg) between lighting from aboveand lighting from below, while the physical structure of the walker remained exactly identical. Thisillusion demonstrates that biological motion perception is also substantially influenced by shading cues,where the processing of these cues is dependent on a lighting-from-above prior.