12.06.2015

New manuscript accepted for publication in the Journal of Vision

"Testing the role of luminance edges in White's illusion with contour adaptation” by Torsten Betz, Robert Shapley, Felix Wichmann, Marianne Maertens

Abstract:

White’s illusion is the perceptual effect that two equiluminant gray patches embedded in a black-and- white grating appear different in lightness. A test patch placed on a dark stripe of the grating looks lighter than a test patch placed on a light stripe. Although the effect does not depend on the aspect ratio of the test patches, and thus, on the amount of border that is shared with either the dark or the light stripe, the context of each patch must, in a yet to be specified way, influence their lightness. We employed a contour adaptation paradigm (Anstis, 2013) to test the contribution of each of the test patches’ edges, edges parallel or orthogonal to the grating, to the perceived lightness of the test patches. Observers were adapted to flickering bars that were presented either at the position of the orthogonal or of the parallel edges. Observers then judged the lightness of the test patches. We found that adapting to the parallel edges slightly increased the perceived patch lightness, whereas adapting to the orthogonal edges abolished, or for some observers even reversed, the lightness illusion. We implemented a temporal adaptation mechanism in two spatial filtering models of lightness perception, and showed that the models cannot account for the observed adaptation effects. We conclude that White’s illusion is in most part determined by edge contrast across the edge orthogonal to the grating, whereas the parallel edge has little or no influence. We suggest mechanisms that could explain this asymmetry.