Mechanisms of color constancy under nearly natural viewing

Color constancy is our ability to perceive constant surface colors despite changes in illumination. Although color constancy has been studied extensiv ely, its mechanisms are still largely unknown. Three classic hypotheses are that constancy is mediated by local adaptation, by adaptation to the spati al mean of the image, or by adaptation to the most intense image region. We measure color constancy under nearly natural viewing conditions, by using a design that allows us to test these three hypotheses directly. By suitabl e stimulus manipulation, we are able to titrate the degree of constancy bet ween 11% and 83%, indicating that we have achieved good laboratory control. Our results rule out all three classic hypotheses and thus suggest that th ere is more to constancy than can be easily explained by the action of simp le visual mechanisms.