Plato's Cave: Light Phenomena

Phenomena of Lightness, Brightness & Illuminance

The perception of form is intimately tied in with the perception of both "lightness" (= surface reflectance) as well as the perceived direction of illumination. Adelson presents a number of interesting examples of this interaction. In the first one (top left) the two circled edges are locally identical, although one is perceived as a form edge with no change in reflectance, while the other appears as a reflectance edge with no form edge.

In the next illusion (top center & top right) these two figures are identical except for a small difference at the circled corner. This local difference however propagates throughout the entire figure, generating a completely different percept of depth.

The last example (bottom row) shows how three figures which are topologically identical generate completely different spatial percepts, with form edges and illuminance edges interchanging in response to subtle differences in the global configuration.

The next figure (left) shows how a repeated pattern of light and dark produces simultaneously a percept of an overhead light source, and a three-dimensional percept of bumps and dimples. When the picture is inverted (right) the bumps become dimples, and the dimple becomes a bump, showing that the light source is preferentially perceived to be above, the usual location for light sources.
When the picture is rotated by 90 degrees, the percept becomes ambiguous, with the illuminant perceived either to the right or the left, creating bumps or dimples accordingly. Notice that the perception of the shapes and the illuminant are always synchronized, i.e. when one switches, then the other switches, in order to maintain a globally consistent percept.
What kind of model can explain these phenomena?

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