Adaptation Mechanisms in Color and Brightness

In order for color to be a useful property for segmenting and identifying objects in a scene it must remain constant in spite of variations in intensity and color of the ambient light, and we know that the visual system achieves an elegant deconfounding of incident light and the reflectance of objects. The last decade has seen considerable advances in the formal analysis of color constancy. These advances have not been matched, however, by advances in understanding whether or how the nervous system carries out these computations. Models of constancy generally require some sort of ‘knowledge’ or computation of the illuminant (see review by Lennie & D’Zmura, 1988). This knowledge may be realized in the visual system to a large extent by mechanisms of light adaptation which can compensate for the changes in the illuminant so that objects of constant reflectance will have a constant effect following adaptational transformations (D’Zmura & Lennie, 1986). Light adaptation is generally acknowledged to play a major role in both color and brightness constancy, but the nature of the adaptational transformations and how they contribute to constancy are poorly understood. In this chapter we will report on preliminary experiments which attempt to specify how light adaptation mechanisms operate to determine the color and brightness of lights. First, we will describe what we know about light adaptation from measurements of sensitivity and then describe how this might be extended to account for color appearance and brightness.