Abstract
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).
