Achromatic contrast and assimilation: The role of perceptual organisation

In the visual world, the perceptual features of an object are affected by the context in which that object is perceived. An increase in perceived similarities between the object and its surround is called an assimilation effect, whereas an increase in their perceived differences is called a contrast effect. An account is provided for the role of perceptual organisation 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 organisation. By using psychophysical methods, in three 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.