Abstract
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.
Highlights
The visual system is able to recognize body motion from impoverished stimuli
In this paper we investigated the influence of the such shading gradients on the perceived locomotion direction from a biological motion stimulus that consists of volumetric elements
We presented a new psychophysical illusion that provides evidence that the perception of body motion is influenced by a lighting-from-above prior
Summary
The visual system is able to recognize body motion from impoverished stimuli. This requires combining stimulus information with visual priors. While natural body motion stimuli often specify many cues for the disambiguation of the three-dimensional body structure, it has been shown that humans effortlessly recognize three-dimensional body motion even from strongly impoverished two-dimensional stimuli[7] This requires the combination of ambiguous stimulus information with perceptual priors that are encoded by the visual system. We present a new perceptual illusion that implies that the perceived locomotion direction of body motion stimuli critically depends on the prior assumption that such bodies typically are illuminated from above Such ‘lighting-from-above priors’ have been previously found for the perception of static shapes[8,9,10,11,12,13,14,15,16,17]. Our experiments motivate a computational model that accounts for the illusion, and which proposes a way how the underlying visual prior might be encoded by physiologically plausible neural mechanisms within the visual pathway
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