The effect of attention on brightness contrast and assimilation.

This paper reports three experiments that attempted to answer questions about the conditions under which brightness assimilation and brightness contrast are obtained. Brightness assimilation was found only under circumstances in which the gray portion of the visual display-the gray portion being compared with some other standard gray--was not the focus of attention. When attention was focused on this gray, brightness contrast was obtained. A theoretical explanation is offered in terms of the effect of attention on perceived average brightness. The phenomenon of simultaneous brightness contrast is well known: a gray patch on a black ground appears brighter than the same gray patch on a white ground. The phenomenon is pervasive and its parameters have been thoroughly studied.' In addition, the facts and theories about lateral inhibition2 seem adequate to account for brightness contrast. At the same time there are a number of annoying facts that are not easy to reconcile with the phenomenon of contrast. These facts have also been well known for a long time. Von Bezold, in 1874, described what he called a "spreading phenomenon" that has since come to be called brightness assimilation.8 It is, essentially, the opposite of brightness contrast, but it seems to occur only Received for publication January 8, 1970. The study was supported by Research Grant GB-8178 from the National Science Foundation, and Research Grant MH-16327 from the National Institute of Mental Health, to Leon Festinger. The authors also wish to thank Julian Hochberg and Lloyd Kaufman for their valuable suggestions. 1A. L. Diamond, Foveal simultaneous brightness contrast as a function of inducing and test-field luminances, J. exp. Psychol., 45, 1953, 304-314; A. L. Diamond, Foveal simultaneous contrast as a function of inducing-field area, J. exp. Psychol., 50, 1955, 144-152; A. L. Diamond, Simultaneous contrast as a function of test-field area, J. exp. Psychol., 64, 1962, 336-345; A. L. Diamond, Brightness of a field as a function of its area, J. opt. Soc. Amer., 52, 1962, 700-706; E. G. Heinemann, Simultaneous brightness induction as a function of inducing and test field luminances, J. exp. Psychol., 50, 1955, 89-96; H. Leibowitz, M. A. Mote, and W. R. Thurlow, Simultaneous contrast as a function of separation between test and inducing fields, J. exp. Psychol., 46, 1953, 453-456. 2A. L. Diamond, A theory of depression and enhancement in the brightness response, Psychol. Rev., 67, 1960, 168-199; F. Ratliff, Mach Bands: Quantitative Studies on Neural Networks in the Retina, 1965. 8 W. von Bezold, The Theory of Color and Its Relation to Art and ArtIndustry, S. R. Koehler (trans.), 1876.