What is color? A speculative essay

zyxwvu COLOR FORUM What Is Color? A Speculative Essay When asked by Meno (in the Platonic dialogue of the same name’) what shape is, Socrates replies that it is the only thing that always accompanies color. When Meno, thinking this a naive definition, also asks for a definition of color, Socrates replies that color is “an effluence from shapes commensurate with sight and perceptible by it.” When Meno finds this to be an excellent answer, Socrates agrees that it is “a high-sounding answer,” but makes it clear that he prefers the phenomenological type of answer he provided for shape. But long before Socrates, thinkers had been attempting definitions of color. Pythagoras and his followers had defined color as the surface (chromos) of a body. Democritus, on the other hand, believed it to exist as a convention, i.e., in the mind, brought about by the only real entities: atoms of a particular shape and size. The nature, too, of color was a subject of argument from the beginning. Democritus, as a result of his “conventional” view, postulated color to occur in the mind as a result of the interaction of the atoms of effluences of bodies with the atoms of the soul. Aristotle, like Pythagoras, on the other hand, considered color to be associated with matter. It was a quality of materials, as were taste, smell, sound, and warmth. These qualities, transmitted by changes in an intervening medium, together with the primary ones, shape, size, motion, and solidity, can be apprehended by appropriately equipped organisms. According to Aristotle, the sensing takes place in the sensing organ, say, the eye, causing a change there. Thomas Aquinas believed the change not to be limited to the sensing organ but to also take place in the soul. The wide range of observed color phenomena led early commentators to place colors variously into objects, light, or the observer. But while disagreeing about the location of color, they were in general agreement about the existence of a sensing mechanism. A quite detailed theory of sensation was developed in the eighteenth century by David Hartley, a contemporary of Hume. According to Hartley, mental occurences such as the appearance of color in consciousness, are caused by vibration of minute particles in the brain, set off by outside stimulation. In later elaborations of this theory the result of sensations became “sensa” or “sense data.” In this view, a color patch in awareness is a sensum. something that exists aside from a functioning brain? Does mind (or color) have an existence? Or, more basically, what is color? The difficulty of this question has led most commentators to bypass it. The history of color science is full of explanations of how. The causes of color were elucidated by Newton and his followers.2 Descartes was one of the first to develop a theory of how the causes lead to color sensations. He was followed by Young, Grassmann, Helmholtz, and others. On the physiological side answers were provided by Galen, Alhazen, Ramon y Cahal, Polyak, and many others; on the neurophysiological side by de Valois, Hubel and Wiesel, and others. Psychological theories of how were provided by Fechner, Hering, the Hurviches, and others. When faced with the question of what color is, Hurvich responds: “Is color something that inheres in objects themselves? . . . Could it be that the light that falls on (a) leaf constitutes its color? . . . Is color a photochemical event that occurs in the receptor layer of the eye? Or is it perhaps a neural brain-excitation process? Or a psychical event? Color is all these things. . . .”3 It is quite evident that such an explanation addresses primarily the how, the process leading to the experience of color. It appears that we require help from philosophers if we are looking for an account of the what. Brain and Mind 0 1989 by John Wiley & Sons, Inc. Philosophers have provided a variety of answers concerning the relationship of body and mind over the centuries, but none has proved conclusive to date. The theological postulate of body and soul was formalized in the theory of dualism by Descartes in the seventeenth century. Accordingly, the mind is a nonphysical substance that can have a life separate from the body. Colors are occurences in the mind. Dualism exists in several shadings, ranging from the strong substance dualism (mind is a nonphysical substance) to the weaker property dualism (mind is a nonsubstantive property of the brain). The latter version is also known as epiphenomenalism. In between is the version of panpsychism (mind is a property of all matter). A tenet of epiphenomenalism is that while mind is caused by brain activity, the latter cannot be influenced by mind phenomena. This position is considered untenable by another group of property dualists, the interactionists. They take it for granted that mind phenomena can affect the brain, that it can make the brain and through it the body take some action. This may appear on the surface to be a sensible position, in agreement with our everyday experiences and the notion of free will. But it may be difficult to maintain an Volume 14, Number 4, August 1989 CCC 0361-2317/89/040207-04$04.00 zyxwv zyxwvut zyxwvu zyxwvutsrqpon Color: What and How The question of the essence of color is tied closely to the question of the relationship of brain and mind. Is mind 207 zyxwvutsrqp zyxwvutsrq interactionist position without granting a kind of mind to a thermostat that can ‘‘wil!” a furnace to turn on or shut off. A second major school of thought is that of materialism. Materialism admits only matter, energy, and void and considers mind to be explainable in terms of matter and processes, or not to exist at all. The latter is the behaviorist position and assumes the behavior of an organism to be simply its responses to stimuli. The main difficulty with this position is that it denies the qualitative aspects of our experiences, the so-called quafia (the “sweetness” of sugar, the “greeness” of a living plant leaf). The problems associated with behaviorism have resulted in the development of the central state identity theory. It equates mental events with neurophysiological events, i .e., a toothache is identical to and nothing but the firing of certain nerve cells of the tooth and the resulting events in the brain. To be in a certain mental state, e.g., to see red, is nothing but to be in a certain neurophysiological state. The uniqueness of this event can perhaps be represented by the firing of a particular type of cell in a particular brain location. The firing of such cells would be the necessary and sufficient condition for the organism to experience red. This theory has the advantage of preserving causality (the belief in cause and effect relationships) while not being incompatible with psychological facts. Central state theory is in some difficulty due to developments in computer science and artificial intelligence (AI) research. While it is by no means assured and continues to be debated furiously, a central assumption of some versions of A1 is that mental states can be created in ma~hinery.~ If this is possible, then a certain mental state cannot be only identical with a certain neurophysiological state but must be capable of being achieved by more than one means. The assumption is that if we appropriately program some complex machine based on a suitable technology, it is capable of experiencing, say, red. The theory that avoids these difficulties of central state identity theory is that of functionalism. It takes a view represented to some extent by the computer hardware/software situation. Digital computers, while currently based exclusively on silicone technology, could also be based on another technology. In the past electron tubes have been used to represent bits. Development of biological computers is envisaged. One can image the same computer program to be implementable on machines of widely different structural composition and yielding the same results. The psychology that any complex system represents, therefore, may not depend on the material it is made of but instead on the organization of the material. Until there is evidence for the reality of functionalism, i.e., until a meaningful duplication of a human mind function has been demonstrated in a machine (one that does not merely rely on speed of execution, such as a chess playing computer) it may be useful to regard central state identity theory as valid. What does this imply for our quest for the what of color? 208 The Color Experience It is perhaps useful at this point to provide an account (in a cursory and much simplified manner) of color perception. As the observer looks at her environment, photons from the sun scattered by the earth’s atmosphere and reflected from the surfaces of objects (as well as some photons perhaps streaming from an artificial light source) enter the eyes and are absorbed by receptor cells in the retina, by the inert macular pigment, or by the choroid. Those photons absorbed by the receptor cells trigger a multistage chemical reaction resulting in nerve cell firings through several cell layers in the retina, along the optic nerve to the lateral geniculate nucleus and to the primary visual area in the cortex at the back of the head. In the process the two-dimensional retinal representation has been separated into color signals, contour signals, depth clue signals, and motion detection signals. Each of these appear to be handled in separate layers of the visual cortex. The primary visual cortex, and perhaps later stages in the brain, convert the two-dimensional retinal representation into a “three-dimensional” mental image. It provides basic information to the observer as to where objects are located in the visual field, contours, surfaces, textures, and information about the nature of the objects in form of color data, in what is called the pre-attentive stage. If the scene is new, and the observer alert, this is most likely followed by the attentive stage where the available information is processed further, providing identification of objects and additional cognitive tasks resulting in thought and action. The specific path of color-causing impulses is via cone receptors and the lateral geniculate to the primary visual cortex, where they are processes in the blobs, then the thin stripes and continue to visual area 4 of the cortex. Exactly where and by what mechanism the experience of color is created remains for now a mystery. In the attentive stage the perceptual information is compared to information from memory and the perceptions are weighted with rational and emotional factors. Why was color vision added to the catalog of evolutionary developments? There is no doubt about the survival value of a color system enhancing general vision. Detection of food, enemies, friends, or situations that were pleasurable or dangerous were substantially improved. Special features of the color vision system, such as simultaneous contrast, logarithmic signal compression to aid discrimination of darker colors, adaptation, and others increased the information providing capabilities of the system. From this perspective color is simply an added level of information for the organism about the environment, enhancing those of contour, shape, distance, motion, speed, identification, etc. Its value can be gauged by comparing the information content of a blackand-white and acolor photograph of the same subject matter. For unknown objects or objects invisible to the naked eye the degree of information enhancement seems to be the same if the colors are “real” or if they are “false.” In the case of real colors the information provided relates to the nature of COLOR research and application z the objects as expressed in their reflectance or transmittance properties. Certain objects have certain reflectance or transmittance characteristics and by this means can be identified more easily on repeat encounters. In addition, in a scene several objects with comparable lightness but different spectral reflectance/transmittance properties can be distinguished much more easily. While more detailed spectral information (such as available in hearing and comparable to a diode array detector) might be useful, the compromise forced by the size of the visual field and the need for resolution and acuity restricted the detection system to three overlapping spectral detectors. Two overlapping detectors with an opponent system result in two unique hues while the addition of a third detector and appropriate wiring resulted in four unique hues. There is some evidence that the human color vision system may have had originally only two detectors resulting in color vision, the short-wave detector and a long-wave detector intermediate in response to those of the current mediumand long-wave detector^.^ What hues were experienced under those conditions is unclear. The additional detector and resulting two unique hues must have added to species survival of man since there is now no significant portion of humans with a two-detector system. But they apparently also provided sufficient information to assure (with many other factors) survival. Three detectors can theoretically result in six unique hues, but it is difficult to imagine what color space would look like under these conditions, not to mention the additional two unique hues. The general assumption is that unique hue experiences are associated with the specific firing of a given type of opponent color cell. * There is substantial neurophysiological and psychophysical evidence for this. However, the picture may be more complex than hitherto assumed.6 Depending on the stimulus or the conditions more than one type of opponent color cell associated with a given location of the visual field can fire at the same time, resulting in mixed sensations and resemblances, i.e., a hue sensation resulting out of the joint firing of two different types of opponent color cells has a certain resemblance to both unique hues associated with the two cell types. be identical to the firing of certain types of cells in a certain location of the brain (just as the experience of pleasure is probably the same as the firing of certain cells in certain brain locations). Other colors are identical to the firing of one or more other types of cells. We already know that many processes intervene from the point of absorption of the reflectedkransmitted light to the final destination of the created signal, modifying the initial information content. More fundamentally, there is no indication of any necessary connection between a given color experience and a given reflectanceltransmittance (between the redness of red and photons at 700 nm). Color seems to be a symbol representing information, without being the information itself. In a digital thermometer, the numbers flashed on the display are symbols representing information, variously modified on the way, about the energy content of the medium into which the probe is inserted. There is no necessary connection between the shape of the symbols and the energy content of the medium. Identification of the essence of a color with the firing of a particular type of neural cell in a particular place in the brain (with a particular brain state) may leave many readers dissatisfied. Indeed it does not say anything about the qualitative aspect (the quale) of the experience. However, at such a fundamental level no qualia have been explained in any satisfactory way by any theory of mind and matter. zyxwv zyxwvutsrq zyxwvuts Subjective vs. Objective Is color an objective property (is it an organism’s representation of reflectancekransmittance of materials) or is it a subjective experience? The former is a position continued to be maintained by some physicalist philosophers. According to the identity theory, color experience is identical to a certain neurophysiological state. While the evidence is not unequivocally in yet, the experience of unique red may *Complications arise because of the overlapping nature of the detectors and, therefore, of the opponent color responses. The unique hues do not correspond to the wavelengths of peak response of the opponent cells but to wavelengths where the second opponent system is at rest. Volume 14, Number 4, August 1989 Color as Phenomenon Some philosophers are entirely dissatisfied with a materialist explanation of the essence of experiences. They believe that a reduction of cognition to the neural level is impossible and that the problem should be tackled at a much higher, phenomenological level. It seems obvious, however, that a neural account of experiences should flow seamlessly into a high level account (or vice versa), unless the neural account is discounted completely. Neuropsychology and neuropharmacology have in recent years shown dramatic instances of the possibility of reduction of high level psychological experiences to events of neural chemistry. A phenomenological account of color involves determination of resemblances of colors and other controlled judgments. Work leading to uniform color order systems, global and local color scaling, and the hue cancellation and other experiments by Hurvich and Jameson are examples. It is obvious that a believable neurophysiological account of color cannot be in conflict with phenomenological facts.’ Color phenomenology is also connected to color semantics. Semantics is playing an increasing role in the study of cognition. A kind of phenomenology associated with semantics was introduced by the philosopher Wittgenstein. He pondered such questions as why there can be no transparent white or no reddish green.8 if one accepts the neurophysiological model sketched earlier, reductionist answers to these questions appear to be quite simple. On a high level phenomenological basis, answers seem to be difficult and awkward or to borrow ex post facto from reductionist ex- 209 z zyxw zyxwvuts zyxwvut planations. Perhaps it is useful here again to refer to the difficulties of a phenomenological explanation of complex psychological behavior, say, certain types of mania-depression. The associated behavior can be dampened or cured with a simple intake of lithium. A central state identity theory based on a specific neurophysiological action of lithium (while lacking at this point) seems to offer a much better chance for explanation than an attempted phenomenological one. There is an unresolved question if there is any necessary connection between phenomenological facts and words in human languages developed to designate these facts. Etymological studies of English color words indicate that the basic color words derive from the Indo-European language. Blue and black are said to derive from “bhel,” yellow and green from “gel,” red from “rudh.” How these early words developed and what their original meaning was is lost in history. Other color words are seemingly derived from resemblances, e.g., orange, from the Hindi word narangi, designating the fruit of that name; violet, after the flower of the color of the genus viola; pink, after the flower of a pink color of the species dianrhus and having the Old English name pynke; olive, from the Latin oliva for the tree carrying the fruits; etc. Berlin and Kay’ have shown that the development of basic color terms, even though they vary semantically, is quite uniform throughout humanity. Experiments in many population groups have indicated that basic color terms are focussed fairly narrowly in color space. These facts support the idea of uniformity of color experience and are consistent with a central state identity theory. There seem to be no insurmountable obstacles placed by phenomenology into the path of explanation by identity theory or functionalism. Our dissatisfaction with an identity theory type of explanation of the what of color may have to do with deepseated and automatic “coloring” of color experiences with memory and perhaps instinct. Goethe has remarked about yellowish red (scarlet): “The pleasant, cheerful feeling which orange is still providing for us is intensified all the way to unbearable violence in yellowish red.”” It is not too difficult to imagine uncounted experiences dealing with blood and fire resulting in fear or excitement, lodged deep within our common ancient memory, and leading to such a response. However, this has nothing to do with what color is. Symbols do not have to be universal. The symbolic value of certain colors can be drastically different, such as, for example, the color of mourning. In addition, there are the personal memories that are different for each of us and are bound to affect our perceptions strongly. The old search of popular psychology for universal meanings and effects of colors has, therefore, been unsuccessful. What is Color? At this point in our history our ideas about what color is are largely speculative. It may take, perhaps using Turing’s test, an aesthetically pleasing painting, unwittingly fashioned by a robot, to convince us that we know what color is. To achieve this, we would have had to unravel most or all of the marvels of cognition, resulting in a functionalist account. We have a long way to go. For now the most convincing account, the one that is in conflict with few if any facts, is a central state identity one. Color is identical to a particular brain state. When the organism is in that brain state it sees color in its visual field. The brain state is normally achieved under the influence of photons penetrating the eyes but can also be achieved by other means. Color is a symbol available to increase the information content the organism can gain from the influx of photons. The information increase, less than optimal, was at one point if not vital then certainly life enhancing. In a more and more complex and technological world it remains so. In addition, as with his other sensory facilities, man has used color vision for his own purposes of transmission of information in arts, crafts, and technology. zyxwvutsrq zyxw 210 ROLFG . KUEHNI 5217 Hillingdon Rd. Charlotte, North Carolina 28226 1. Plato, The CollectedDialogues, Princeton University Press, Princeton, zyxw 1961, 2. K . Nassau, The Physics and Chemistry of Color, Wiley, New York, 1983. 3. L. M. Hurvich, Color Vision. Sinauer Associates, Sunderland, Massachusetts, 1981. 4. S . Lem, “Non Serviarn,” A Perfect Vacuum, Harcourt Brace, New York, 1978, pp 167-196. 5. J . Nathans, The Genes for Color Vision, Sci. Am. 260, No. 2, 4249 (February 1989). 6. B. Drum, Hue signals from short- and middle-wavelength-sensitive cones, J. Opr. Soc. Am. A 6, 153-157 (1989). 7. C. L. Hardin, Color for Philosophers, Hackett, Indianapolis, 1988. 8. L. Wittgenstein, Remarks on Colour, University of California Press, Berkley, 1978. 9. B. Berlin and P. Kay, Basic Color Terms, University of California Press, Berkley, 1969. 10. J . W. Goethe, Farbenlehre, Verlag Freies Geistesleben, Stuttgart, 1979. Translation by the author. COLOR research and application