Craik-O'Brien-Cornsweet effect

In viewing this scene, people invariably perceive the surface of the top block to be darker than the bottom block. Click on the link below to see that these territories are in fact identical. Covering the center section, including the shaded gradients, shows that the source of the difference is this component of the scene.

(Image by Beau Lotto - www.lottolab.org)