The Influence of Cast Shadows on Visual Search.
Ronald A. Rensink and Patrick Cavanagh; Harvard University, Cambridge MA, USA.

Submitted to Perception.

Abstract

We show that cast shadows can have a significant influence on the speed of visual search. In particular, we find that search based on the shape of a region is relatively slow when the region is darker than the background and corresponds to a shadow formed via lighting from above. This pattern of results supports the proposal that an early-level system rapidly identifies regions as shadows and discounts them, making their borders more difficult to access. The constraints used by this rapid system are mapped out in regards to the luminance and texture of the shadow region, and the nature of the item casting the shadow. Among other things, this system is found to distinguish between line elements (items containing only lines) and surface elements (items containing visible surfaces), with only the latter deemed capable of casting a shadow.

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