Achromatic contrast and assimilation: The role of perceptual organization.

An account is provided for the role of perceptual organization in determining whether lightness contrast or assimilation is perceived. Assimilation occurs when the perceptual group is ambiguous or unstable, whereas contrast is perceived in the context of stable perceptual groupings elicited by Gestalt principles of perceptual organization. In two experiments it is shown that assimilation occurs when intentional effort is needed to disambiguate among possible perceptual groupings whereas contrast is observed when belongingness is unambiguously and spontaneously perceived.