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1. White's effect in lightness, color and motion

Summary: 1. White's effect in lightness,
color and motion
Stuart Anstis
In White's (1979) illusion of lightness, the background is a square-wave grating of
black and white stripes (Fig. 1a). Grey segments that replace parts of the black stripes
look much lighter than grey segments that replace parts of the white stripes. Assimi-
lation from flanking stripes has been proposed, the opposite of simultaneous contrast.
We use colored patterns to demonstrate that the perceived hue shifts are a joint func-
tion of contrast and assimilation. Simultaneous contrast was relatively stronger at low
spatial frequencies, assimilation at high. Both the chromatic and achromatic versions
of White's effect were stronger at high spatial frequencies. "Geometrical" theories
attempt to explain White's effect with T-junctions, anisotropic lateral inhibition, and
elongated receptive fields. But an isotropic random-dot illusion of lightness called
"Stuart's Rings" resists any anisotropic explanations. White's illusion also affects mo-
tion perception. In "crossover motion," a white and a black bar side by side abruptly
exchange luminances on a gray surround. Direction of seen motion depends upon the
relative contrast of the bars. On a light [dark] surround the black [white] bar is seen as
moving. Thus the bar with the higher contrast is seen as moving in a winner-take-all
computation. But if the bars are embedded in long vertical lines, the luminance of these


Source: Anstis, Stuart - Department of Psychology, University of California at San Diego


Collections: Biology and Medicine