Lightness anchoring: One anchor or multiple anchors?

Abstract

Gilchrist and colleagues¹ distinguish two processes subserving lightness perception: scaling and anchoring. Scaling maps the range of real-world reflectances onto the range of perceived reflectances (lightness values). Anchoring determines where particular achromatic colors, such as white, gray, or black, lie within the perceptual range. Previous results suggest that the human visual system employs an anchoring rule based on the assumption that the highest luminance in the scene is white. Contemporary anchoring theories supplement this highest luminance rule with new anchoring principles including the Area Rule;¹ compromise between global and local highest luminance anchoring;¹ and compromise between anchoring to the global highest luminance and to the luminances of regions contiguous with the target.² These theories deal with the growing complexity of known lightness phenomena either by adding additional anchoring rules, or by positing that the old anchoring rules are applied within multiple anchoring frameworks. I will present examples of lightness phenomena that challenge existing multiple anchor theories, including evidence that the luminances of target non-contiguous regions having luminances lower than the target luminance can influence the target lightness,³ and that increasing the luminance or area of a region having luminance intermediate between the target luminance and the highest luminance can affect the target lightness in a non-monotonic manner.⁴ These lightness phenomena can all be accounted for by a mechanism involving perceptual edge integration and contrast gain control acting between edges. Once this mechanism has been taken into account, a single anchoring rule is all that is needed: the surface associated with the largest integrated edge value appears white.

1. Gilchrist, etal. (1999). Psychol. Rev., 106, 795-824.

2. Bressan. (2006). Psychol. Rev., 113, 526-553.

3. Zemach Rudd. (2005). J. Vision, 5, 983-1003.

4. Arrington Rudd and Zemach Rudd. (2001). Vision Res., 41, 3649-3662; (in press) JOSA A.