The visual system is able to recognize body motion from impoverished stimuli. This requires combining

stimulus information with visual priors. We present a new visual illusion showing that one of these

priors is the assumption that bodies are typically illuminated from above. A change of illumination

direction from above to below flips the perceived locomotion direction of a biological motion stimulus.

Control experiments show that the underlying mechanism is different from shape-from-shading and

directly combines information about body motion with a lighting-from-above prior. We further show

that the illusion is critically dependent on the intrinsic luminance gradients of the most mobile parts of

the moving body. We present a neural model with physiologically plausible mechanisms that accounts

for the illusion and shows how the illumination prior might be encoded within the visual pathway. Our

experiments demonstrate, for the first time, a direct influence of illumination priors in high-level motion

vision.