Scene complexity affects lightness constancy with respect to changes in object slant and surround reflectance

Sarah Allred
Department of Psychology / University of Pennsylvania

David Brainard
Department of Psychology / University of Pennsylvania

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     Last modified: July 13, 2007

Abstract
Purpose: The visual system achieves partial lightness constancy across a variety of scene and illumination changes. What scene variables mediate this constancy?

Methods: Observers viewed scenes illuminated by a single directional light source. Two spatially separated cards provided the immediate surround for reference and test spots presented on them. Observers matched the apparent lightness of the two spots. The slant and reflectance of the card surrounding the reference spot were fixed, and we parametrically varied the slant and reflectance of the card surrounding the test spot. Reflectance variations of spots and their immediate surrounds were accomplished via simulation using a hidden projector. We also varied the complexity of the global scene.

Results and Conclusion: Scene complexity affected lightness matches. In more complex scenes, observers exhibited higher degrees of lightness constancy: they were better able to discount changes in illumination induced by variations in slant and better able to ignore changes in spot contrast induced by variations in local surround reflectance. Even in the most complex scene, however, observers failed to show complete lightness constancy.