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Observers experience a three dimensional world through two dimensional retinal images. Nevertheless, the geometry of visual surfaces can be effortlessly recovered from pictorial cues, available in monocular, stationary images. Shading is one such cue, which aids the internal reconstruction of the 3D shape of solid surfaces by exploiting the fact that the parts of the surface facing the light source are brighter than those facing away from it. When the position of the light source is ambiguous, observers assume that the light is coming from above, possibly reflecting the fact that the illumination is usually placed above the observer. Surprisingly, human observers assume that the light source is not coming directly from above, but rather, from a direction displaced to the left of the zenith. It is not yet clear why the assumed light source is biased to the left, and a couple of proposals have been put forward. The bias may reflect a learnt statistical regularity in the environment, or may be a result of an internal constraint, such as hemispheric asymmetry for lateralized attentional processes. We have examined the effects of cultural and biological factors, known to affect attentional biases and hemispheric asymmetries, testing different populations, including patients, children and older adults. Our results suggest that both, cultural and biological factors, modulate the assumed light source bias. The study of shape from shading provides a simple way to examine how attention shapes perception.

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