V&ion Res. Vol. 32, No. 9, pp. 1623-1634, 1992 Printed in Great Britain. All rights reserved 0042~6989/92 $5.00 + 0.00 Copyright Q 1992 Pergamon Press Ltd Color Perception Within a Chromatic Context: Changes in Red/Green Equilibria Caused by Noncontiguous Light MICHAEL F. WESNER,* STEVEN K. SHEVELL* Received 2 August 1991; in revised form 18 March 1992 We measured changes in the color appearance of one light caused by another light presented in a well-separated region. Observers viewed a lo test field superimposed on a 3’, 540 or 660 nm adapting field (32 or 320 td). The change in appearance due to noncontiguous light was determined by surrou~ing the 3Oadapting field with a continguous 3Oi.d., 5’ o.d. ring of either 32 or 320 td. The ring was 540,660 nm or achromatic (tungsten-halogen “white”). The test was an admixture of 549 and 660 nm light, and varied from 6 to 1000 td. The observer adjusted the ratio of 549 to 660 nm test light so the test appeared neither reddish nor greenish. A 540 or 660 nm ring had a chromatic inducing effect on the small test that mimicked a simple surround contiguous with the test. Results with an achromatic ring were more complex: an isolated achromatic ring (no adapting field present) had virtually no effect on the color appearance of the test, but the same achromatic ring surrounding a chromatic adapting field shifted the test toward the color appearance of the adapting light (e.g. introducing a ‘khite” ring surrounding a “green” adapting field shifted the test toward greenness). A thin ~nciI-width band of “white” light su~~rn~~ on a larger 5* adapting field had an effect similar to a “white” 3-5O ring. These results demonstrate (1) strong effects of the remote noncontiguous lights and (2) that the change in color appearance they cause is not a simple function of only the light in the noncontinguous region. The change depends on other lights in view. The visual processes revealed in these experiments are considered in terms of inferred illumination and surface reflectances of objects in natural scenes. Color perception Color appearance Chromatic adaptation Color context Color contrast Induction Red/green equilibrium INTRODUCTION In natural scenes one encounters many different lights reflected from various objects in view. The color appear- ance of any one particular area in the scene may be influenced not only by contiguous bounding regions (e.g. color contrast) but also by more remote, noncontiguous areas. Most investigators studying color perception of complex stimuli have been interested in developing or testing computational models of color and brightness constancy. Often these studies used arrays of different stimuli (e.g. Mond~ans) in order to better understand how a visual system, which is posited to seek stable color percepts of objects, might process the regional inter- actions of an elaborate scene (Brainard & Wandell, 1986; zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHG Brill & West, 1986; Land, 1983; Land & McCann, 1971; Reid & Shapley, 1988; Valberg & Lange-Malecki, 1990; Walraven, Benzschawel, Rogowitz & Soesterberg, 1987; *Departments of Psychology and of Ophthalmology & Visual Science, University of Chicago, 939 East 57th Street, Chicago, IL 60637, U.S.A. see Pokorny, Shevell & Smith, 199 1, for a review), While many of these models approximate color constancy, the neural substrates of the models’ computations are con- sidered only rarely (D’Zmura & Lennie, 1986; Reid & Shapley, 1989). A great deal is known, on the other hand, about processes mediating the perceived color of simple visual stimuli. Many studies have focused on the influence of one light on the appearance of another (e.g. Cicerone, Krantz & Larimer, 1975; Drum, 1981; Hayhoe, 1990; Jameson & Hurvich, 1959, 1972; SheveIl, 1978, 1982; Shevell & Humanski, 1988; Walraven, 1976; Ware & Cowan, 1982). Typically, two fields are arranged so that a test patch is within an adjacent surround or superim- posed upon a background. Peripheral mechanisms have been proposed to account for the effects of chromatic contrast and adaptation. For example, the von Kries Coefficient Law (von Kries, 1905) describes a receptor- based mechanism. More recently, the two-process theory of chromatic adaptation added an incrementa (or decre- mental) contribution from the adapting light (Jameson 1623