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Color constancy
The amount of light of any given wavelength reflected from a surface changes continually, depending upon the illuminant in which it is viewed. Yet the color of a surface remains the same, even though there will be changes in shade. The stability of colors is to be sought in the capacity of the brain to undertake an operation which makes it independent of the continual change in the wavelength composition of the light reflected from a surface, and hence of servility to the constant vicissitude of things; this in turn allows the brain to obtain knowledge about a certain property of surfaces in spite of continual variations in what reaches the eye from those surfaces.
The world would be a strange place if the color of a surface changed with every change in the wavelength composition of the light reflected from it; we would no longer be able to obtain knowledge about certain properties that they have and color would cease to be an efficient biological signaling mechanism. The mandatory involvement of the brain, the notion that it is the brain that undertakes an operation, was actually well stated by the combative Arthur Schopenhauer in a book, On vision and colors: an essay (1854). The importance of this work lies not in the details of the color theory which Schopenhauer produced (he supposed that the operation was entirely undertaken in the retina ). Rather, it is in the supposition that colors lie in the observer and not outside him. To Schopenhauer, color may be an immediate percept, and therefore intuitive, but "All intuitive perception [Anschauung] is intellectual, for without the understanding [Verstand] we could never achieve intuitive perception," thus implying a formal contribution of the brain to the Anschauung. In the second volume of his Die Welt als Wille und Vorstellung, Schopenhauer says that "perception is not only the source of all knowledge, but is itself knowledge... it alone is the unconditionally true genuine knowledge." We can interpret his use of the word Verstand to mean the operation undertaken by the brain and leading to the intuitive perception [Anschauung], since Schopenhauer tells us that "the forms underlying Verstand, the Verstand is a function of the brain."
Perceptual constancy
The brain is only interested in obtaining knowledge about those permanent, essential or characteristic properties of objects and surfaces that allows it to categorize them. But the information reaching the brain from these surfaces and objects is in continual flux. A face may be categorized as a sad one, thus giving the brain knowledge about a person, in spite of the continual changes in individual features or in viewing angle or indeed in the identity of the face viewed; or the destination of an object may have to be decided by its direction of motion, regardless of its speed or distance. An object may have to be categorized according to color, as when judging the state of ripeness of an edible fruit. But the wavelength composition of the light reflected from an object is never constant; instead it changes continually, depending upon the time of day, without entailing a substantial shift in its color. The ability of the brain to assign a constant color to a surface or a constant form to an object is generally referred to as color or object constancy.
Perceptual constancy is a much wider phenomenon. It applies as well, for example, to faces that are recognizable when viewed from different angles and regardless of the expression worn. There is also what we can call situational constancy, when the brain is able to categorize an event or a situation as a festive or a sad one, and so on, regardless of the particular event.
There is narrative constancy when, for example, the brain is able to identify a scene as the Descent from the Cross, regardless of variations in detail or the style of the painting. "Descent from the Cross," painted by Pedro Machuca, 1547 (left), Rembrandt van Rijn, 1634 (middle), and Max Beckmann, 1917 (right).
In the painting, Descent from the Cross (Deposition), in which Christ is taken down from the Cross, for burial. It is a general scene of mourning. Traditionally present are Joseph of Arimathea, the rich Sanhedrin member who gave up his tomb for Christ, Nicodemus (with myrrh and pincers to pull out the nails), the Virgin, usually overcome with emotion, Mary Magdalene (recognizable with her long hair, sometimes kissing Christ's feet) and St John. The scene has been gradually extended by artists, but it is always recognizable.
The brain, in each case, extracts from the continually changing information reaching it only that which is necessary for it to identify the characteristic properties of what it views; it has to extract constant features in order to be able to be able to obtain knowledge about them and to categorize them.
Vision, in brief, is an active process depending as much upon the operations of the brain as upon the external, physical, environment; the brain must discount much of the information reaching it, select from that information only that which is necessary for it to be able to obtain knowledge about the visual world and compare the selected information with its stored record of all that it has seen.
