Mechanisms of color vision

We review the physiological and psychophysical research on mechanisms of color vision. Psychophysical work has led to the formulation of explicit theories of the early stages of color vision. The principal postulates of these theories have been confirmed by physiologists (e.g., the existence of three classes of receptors and second-stage mechanisms in which the signals from these receptors are compared), but some important features of the psychophysical scheme have found limited physiological support. One such issue is the absence of the unitary "achromatic" mechanism required by psychophysicists. We know a good deal less about the chromatic analyses that occur beyond these early stages. Although physiologists have devoted much effort to the study of cortical mechanisms, little of this work has been guided by clear ideas of the tasks performed by them. The provision of color constancy and the ability to segment scenes are perhaps the foremost concerns of chromatic mechanisms, and recent psychophysical work bearing on these problems offers physiologists clearer guidance on what to seek with their electrodes.