Seeing Is Believing; January 2004; Scientific American Mind; by Vilayanur S. Ramachandran and Diane Rogers-Ramachandran; 2 Page(s)

The visual image is inherently ambiguous: an image of a person on the retina would be the same size for a dwarf seen from up close or a giant viewed from a distance. Perception is partly a matter of using certain assumptions about the world to resolve such ambiguities. We can use illusions to uncover what the brain's hidden rules and assumptions are. In this column, we consider illusions of shading.

In (a), the disks are ambiguous; you can see either the top row as convex spheres or "eggs," lit from the left, and the bottom row as cavities - or vice versa. This observation reveals that the visual centers in the brain have a built-in supposition that a single light source illuminates the entire image, which makes sense given that we evolved on a planet with one sun. By consciously shifting the light source from left to right, you can make the eggs and cavities switch places.