The relationships among perceptual elements in a visual field determine both contrast and assimilation phenomena: Perceptual differences are enhanced in contrast and decreased in assimilation. Gestalt psychologists raised an intriguing paradox by explaining both phenomena as the result of perceptual belongingness; in fact, Benary proposed that belongingness determines contrast, whereas Fuchs suggested that it determines assimilation. We propose that both grouping stability and grouping intentionality are related to this paradox. In four experiments we manipulated both stability and intentionality, to verify whether contrast or assimilation will occur. We found that intentionality and multi-stability elicit assimilation; whereas non-intentionality and stability elicit contrast. Results are discussed within the previous literature on the relationship between lightness induction and perceptual belongingness.
