Three-dimensional curvature contrast--geometric or brightness illusion?

Simultaneous contrast effects have been found across a wide range of visual dimensions. We describe a simultaneous contrast effect--three-dimensional curvature contrast--in which the apparent curvature of a surface defined by shading and texture information is influenced by the curvature of a surrounding surface. The effect is strong and easily measurable. We asked whether the effect depends upon the presence of contrast at the level of the internal representation of surface curvature or whether it could be better explained in terms of local changes in the apparent brightness of regions within the test patches induced by luminance transitions at the borders. The experimental results suggest that, while these luminance-contrast-induced effects do contribute to the observed changes in perceived curvature, there are additional influences. In particular changes in perceived curvature induced by a pattern of curved patches were eliminated or considerably weakened when the inducing pattern was transformed into a photographic negative, a procedure which disrupts the apparent three-dimensional structure of the surface patches without changing their brightness contrast. This suggests a component of the illusion involves comparisons at the level of representation of surface curvature. The observation that three-dimensional curvature contrast persists when the inducing surfaces are spatially separate from the test surface suggests that shape perception involves global, as well as local, operations.