Inhomogeneous surrounds, conflicting frameworks, and the double-anchoring theory of lightness.

The empirical question of whether or not the lightness of a region is accounted for purely by the average luminance of its surround has a complex answer that depends on whether such a region is an increment, a decrement, or intermediate relative to the luminances of the contiguous surfaces. It is shown here that a new model of lightness, based on anchoring principles, predicts and clarifies such intricacies. In this model, the luminance of the target region determines its lightness in two ways: indirectly, by causing it to group with parts of its surround and thus defining the nested frameworks to which it belongs; and directly, by anchoring it to the highest luminance and to the average surround luminance in each of these frameworks. Inter- and intraindividual differences in lightness assessment are shown to emerge under grouping conditions that create unstable, conflicting frameworks.