ABSTRACT
In many elementary textbooks of science, color is defined as a physical property of matter. Although that notion is misleading, it does have some truth in it because color is a visual percept that depends to a high degree on the spectral absorption characteristics of the surface being viewed, provided the conditions of illumination are not very unusual. Color tells us something about the nature of objects. More precisely, color is one of the many representations of the environment constructed by the brain processes that comprise the visual system. However, most of the visual information about objects is conveyed by the perception of spatial properties that are relatively independent of color, in particular shape and texture. But the perception of color, coupled to spatial perception, is useful to decode additional information in parallel. For example, in our everyday visual experience, we identify certain shapes and surface textures (viewed in the appropriate context) as specific types of food; simultaneously, we estimate the qualities of such edible objects to a large extent on the basis of their color-whether fruits and vegetables are fresh, green, ripe, or spoiled, whether breads are appropriately baked, whether meats are rare, medium, or well cooked, and whether colorant substances are dissolved in water. If we think of all the synthetic and realistic images profusely generated by current technology, the variety of examples showing the importance of color is practically end-
less. Therefore, it is of great interest to understand the visual processing of chromatic information and to elucidate the neural machinery involved in this task. One of the fundamental concepts of this endeavor is the distinction between chromatic and achromatic phenomena. As we discuss below, such a distinction is straightforward in physical terms, but it does not directly imply a clear distinction in terms of human perception, and that ambiguity has motivated many ongoing research efforts to arrive at psychophysical and neurophysiological distinctions between chromatic and achromatic phenomena.
