Chromatic adapting effect of achromatic light

Consider a small patch of light superimposed on a larger long-wavelength "red" background field. Chromatic adapting effects of the "red" background are well known. We reported recently that the color of the patch is affected additionally by introducing achromatic light surrounding the background (ARVO, 1989). This is surprising because the same achromatic light presented alone (no "red" background) does not change the color of the patch. We consider here the mechanism by which achromatic light alters the color appearance of a remotely located patch. First, the space-averaged illumination in the complete visual field was changed by restricting the achromatic light to only a thin ring, concentric with the central patch but well within and superimposed upon the larger "red" background (1% test patch, 5% background, 12-ft.-wide achromatic ring of diameter 4°). The achromatic ring still caused a large shift in color appearance. Next, the achromatic incremental ring was replaced by a decremental ring (a dark gap) of the same size. The results were very different, indicating the achromatic ring does not merely segment the background field. Next, the achromatic incremental ring was replaced with a "red" incremental ring of somewhat greater luminance. The effect of the "red" ring was much smaller than th«: effect of the achromatic ring. These and other experiments suggest the change in color appearance caused by introducing achromatic light is related to the new information it provides about the range of chromaticities in the visual scene.