Brightness contrast: multiple cues

When diverse statistical information about the sources of a visual stimulus is mutually consistent, two equiluminant surfaces can be made to appear dramatically different than they do in standard presentations. In this case, the empirical information indicates a high probability that the two surfaces are differently reflective materials under different levels of illumination, making the same patch in one part of the scene look very dark gray, and in another part of the scene very light gray.

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