A WORLD WITHOUT COLOR appears to be missing crucial elements. And indeed it is. Colors not only enable us to see the world more precisely, they also create emergent qualities that would not exist without them. The color photograph at the left, for example, reveals autumnal leaves in the placid water of a fountain, along with the reflections of trees and of a dark-blue afternoon sky behind them. In a black-and-white picture of the same scene, the leaves are less distinct, the dark-blue sky is absent, the reflections of the light are weak, the water itself is hardly visible, and the difference in apparent depth among the sky, trees and floating leaves is all but gone.
Yet this role for color, and even the true nature of color, is not well recognized. Many people believe that color is a defining and essential property of objects, one depending entirely on the specific wavelengths of light reflected from them. But this belief is mistaken. Color is a sensation created in the brain. If the colors we perceived depended only on the wavelength of reflected light, an object’s color would appear to change dramatically with variations in illumination throughout the day and in shadows. Instead patterns of activity in the brain render an object’s color relatively stable despite changes in its environment.
Most researchers who study vision agree that color helps us discriminate objects when differences in brightness are insufficient for this task. Some go so far as to say that color is a luxury and not really needed: after all, totally color-blind people and many species of animals seem to do well without the degree of color perception that most humans have. The pathway in the brain that serves navigation and movement, for example, is essentially color-blind. People who become color-blind after a stroke appear to have normal visual perception otherwise. Such observations have been taken as support for the insular nature of color processing, suggesting it has no role in processing depth and form—in short, that color is only about hue, saturation and brightness.
But the study of illusory colors—colors that the brain is tricked into seeing—demonstrates that color processing in the brain occurs hand in hand with processing of other properties, such as shape and boundary. In our decade-long attempt to discern how color influences perceptions of other properties in objects, we have considered a number of novel illusions, many created by us. They have helped us understand how the neural processing of color results in emergent properties of shape and boundary. Before we begin our discussion of these illusions, however, we need to recall how the human visual system processes color.