by Oliver Sacks
If there was no discrete color center, so the thinking went, there could be no isolated achromatopsia either; thus Verrey’s case, and two similar ones in the 1890s, were dismissed from neurological consciousness—and cerebral achromatopsia, as a subject, all but disappeared for the next seventy-five years. 13
13. There is no mention of it in the great 1911 edition of Helmholtz’s Physiological Optics, though there is a large section on retinal achromatopsia.
There was not to be another full case study until 1974. 14
14. There were, however, brief mentions of achromatopsia in these intervening years, which were ignored, or soon forgotten, for the most part. Even Kurt Goldstein, although philosophically opposed to notions of isolated neurological deficits, remarked that he had seen several cases of pure cerebral achromatopsia without visual field losses or other impairments—an observation thrown off casually in the course of his 1948 book, Langvage and Language Disturbances.
Mr. I. himself was actively curious about what was going on in his brain. Though he now lived wholly in a world of lightnesses and darknesses, he was very struck by how these changed in different illuminations; red objects, for instance, which normally appeared black to him, became lighter in the long rays of the evening sun, and this allowed him to infer their redness. This phenomenon was very marked if the quality of illumination suddenly changed, as, for example, when a fluorescent light was turned on, which would cause an immediate change in the brightnesses of objects around the room. Mr. I. commented that he now found himself in an inconstant world, a world whose lights and darks fluctuated with the wavelength of illumination, in striking contrast to the relative stability, the constancy, of the color world he had previously known. 15
15. A perhaps similar phenomenon is described by Knut Nordby. During his first school year, his teacher presented the class with a printed alphabet, in which the vowels were red and the consonants black.
I could not see any difference between them and could not understand what the teacher meant, until early one morning late in the autumn when the room-lights had been turned on, and, unexpectedly, I saw that some of the letters, i.e. the AEIOUY ÅÅÖ, were now suddenly a darkish grey, while the others were still solid black. This experience taught me that colours may look different under different light-sources, and that the same colour can be matched to different grey-tones in different kinds of illumination.
All of this, of course, is very difficult to explain in terms of classical color theory—Newton’s notion of an invariant relationship between wavelength and color, of a cell-to-cell transmission of wavelength information from the retina to the brain, and of a direct conversion of this information into color. Such a simple process—a neurological analogy to the decomposition and recomposition of light through a prism—could hardly account for the complexity of color perception in real life.
This incompatibility between classical color theory and reality struck Goethe in the late eighteenth century. Intensely aware of the phenomenal reality of colored shadows and colored afterimages, of the effects of contiguity and illumination on the appearance of colors, of colored and other visual illusions, he felt that these must be the basis of a color theory and declared as his credo, “Optical illusion is optical truth!” Goethe was centrally concerned with the way we actually see colors and light, the ways in which we create worlds, and illusions, in color. This, he felt, was not explicable by Newton’s physics, but only by some as-yet unknown rules of the brain. He was saying, in effect, “Visual illusion is neurological truth.”
Goethe’s color theory, his Farbenlehre (which he regarded as the equal of his entire poetic opus), was, by and large, dismissed by all his contemporaries and has remained in a sort of limbo ever since, seen as the whimsy, the pseudoscience, of a very great poet. But science itself was not entirely insensitive to the “anomalies” that Goethe considered central, and Helmholtz, indeed, gave admiring lectures on Goethe and his science, on many occasions—the last in 1892. Helmholtz was very conscious of “color constancy”—the way in which the colors of objects are preserved, so that we can categorize them and always know what we are looking at, despite great fluctuations in the wavelength of the light illuminating them. The actual wavelengths reflected by an apple, for instance, will vary considerably depending on the illumination, but we consistently see it as red, nonetheless. This could not be, clearly, a mere translation of wavelength into color. There had to be some way, Helmholtz thought, of “discounting the illuminant”—and this he saw as an “unconscious inference” or “an act of judgement” (though he did not venture to suggest where such judgement might occur). Color constancy, for him, was a special example of the way in which we achieve perceptual constancy generally, make a stable perceptual world from a chaotic sensory flux—a world that would not be possible if our perceptions were merely passive reflections of the unpredictable and inconstant input that bathes our receptors.
Helmholtz’s great contemporary, Clerk Maxwell, had also been fascinated by the mystery of color vision from his student days. He formalized the notions of primary colors and color mixing by the invention of a color top (the colors of which fused, when it was spun, to yield a sensation of grey), and a graphic representation with three axes, a color triangle, which showed how any color could be created by different mixtures of the three primary colors. These prepared the way for his most spectacular demonstration, the demonstration in 1861 that color photography was possible, despite the fact that photographic emulsions were themselves black and white. He did this by photographing a colored bow three times, through red, green, and violet filters. Having obtained three “color-separation” images, as he called them, he now brought these together by superimposing them upon a screen, projecting each image through its corresponding filter (the image taken through the red filter was projected with red light, and so on). Suddenly, the bow burst forth in full color. Maxwell wondered if this was how colors were perceived in the brain, by the addition of color-separation images or their neural correlates, as in his magic-lantern demonstrations. 16
16. Maxwell’s demonstration of the “decomposition” and “reconstitution” of color in this way made color photography possible. Huge “color cameras” were used at first, which split the incident light into three beams and passed these through filters of the three primary colors (such a camera, reversed, served as a chromoscope, or Maxwellian projector). Though an integral color process was envisaged by Ducos du Hauron in the 1860s, it was not until 1907 that such a process (Autochrome) was actually developed, by the Lumière brothers. They used tiny starch grains dyed red, green, and violet, in contact with the photographic emulsion—these acted as a sort of Maxwellian grid through which the three color-separation images, mosaicked together, could both be taken and viewed. (Color cameras, Lumièrecolor, Dufaycolor, Finlaycolor, and many other additive color processes were still being used in the 1940s, when I was a boy, and stimulated my own first interest in the nature of color.)
Maxwell himself was acutely aware of the drawback of this additive process: color photography had no way of “discounting the illuminant”, and its colors changed helplessly with changing wavelengths of light.
In 1957, ninety-odd years after Maxwell’s famous demonstration, Edwin Land—not merely the inventor of the instant Land camera and Polaroid, but an experimenter and theorizer of genius—provided a photographic demonstration of color perception even more startling. Unlike Maxwell, he made only two black-and-white images (using a split-beam camera so they could be taken at the same time from the same viewpoint, through the same lens) and superimposed these on a screen with a double-lens projector. He used two filters to make the images: one passing longer wavelengths (a red filter), the other passing shorter wavelengths (a green filter). The first image was then projected through a red filter, the second with ordinary white light, unfiltered. One might expect that this would produce just an overall pale-pink image, but something “impossible” happened instead. The photograph of a y
oung woman appeared instantly in full color—“blonde hair, pale blue eyes, red coat, bluegreen collar, and strikingly natural flesh tones”, as Land later described it. Where did these colors come from, how were they made? They did not seem to be “in” the photographs or the illuminants themselves. These demonstrations, overwhelming in their simplicity and impact, were color “illusions” in Goethe’s sense, but illusions that demonstrated a neurological truth—that colors are not “out there” in the world, nor (as classical theory held) an automatic correlate of wavelength, but, rather, are constructed by the brain.
These experiments hung, at first, like anomalies, conceptless, in midair; they were inexplicable in terms of existing theory, but did not yet point clearly to a new one. It seemed possible, moreover, that the viewer’s knowledge of appropriate colors might influence his perception of such a scene. Land decided, therefore, to replace familiar images of the natural world with entirely abstract, multicolored displays consisting of geometric patches of colored paper, so that expectation could provide no clues as to what colors should be seen. These abstract displays vaguely resembled some of the paintings of Piet Mondrian, and Land therefore terms them “color Mondrians.” Using the Mondrians, which were illuminated by three projectors, using long-wave (red), middle-wave (green), and short-wave (blue) filters, Land was able to prove that, if a surface formed part of a complex multicolored scene, there was no simple relationship between the wavelength of light reflected from a surface and its perceived color.
If, moreover, a single patch of color (for example, one ordinarily seen as green) was isolated from its surrounding colors, it would appear only as white or pale grey, whatever illuminating beam was used. Thus the green patch, Land showed, could not be regarded as inherently green, but was, in part, given its greenness by its relation to the surrounding areas of the Mondrian.
Whereas color for Newton, for classical theory, was something local and absolute, given by the wavelength of light reflected from each point, Land showed that its determination was neither local nor absolute, but depended upon the surveying of a whole scene and a comparison of the wavelength composition of the light reflected from each point with that of the light reflected from its surround. There had to be a continuous relating, a comparison of every part of the visual field with its own surround, to arrive at that global synthesis—Helmholtz’s “act of judgement.” Land felt that this computation or correlation followed fixed, formal rules; and he was able to predict which colors would be perceived by an observer under different conditions. He devised a “color cube”, an algorithm, for this, in effect a model for the brain’s comparison of the brightnesses, at different wavelengths, of all the parts of a complex, multicolored surface. Whereas Maxwell’s color theory and color triangle were based on the concept of color addition, Land’s model was now one of comparison. He proposed that there were, in fact, two comparisons: first of the reflectance of all the surfaces in a scene within a certain group of wavelengths, or waveband (in Land’s term, a “lightness record” for that waveband), and second, a comparison of the three separate lightness records for the three wavebands (corresponding roughly to the red, green, and blue wavelengths). This second comparison generated the color. Land himself was at pains to avoid specifying any particular brain site for these operations and was careful to call his theory of color vision the Retinex theory, implying that there might be multiple sites of interaction between the retina and the cortex.
If Land was approaching the problem of how we see colors at a psychophysical level by asking human subjects to report how they perceived complex, multicolored mosaics in changing illuminations, Semir Zeki, working in London, was approaching the problem at a physiological level, by inserting microelectrodes in the visual cortex of anesthetized monkeys and measuring the neuronal potentials generated when they were given colored stimuli. Early in the 1970s, he was able to make a crucial discovery, to delineate a small area of cells on each side of the brain, in the prestriate cortex of monkeys (areas referred to as V4), which seemed to be specialized for responding to color (Zeki called these “color-coding cells”). 17
17. He was also able to find cells, in an adjacent area, that seemed to respond solely to movement. A remarkable account and analysis of a patient with a pure “motion blindness” was given by Zihl, Von Cramon, and Mai in 1983. The patient’s problems are described as follows:
The visual disorder complained of by the patient was a loss of movement vision in all three dimensions. She had difficulty, for example, in pouring tea or coffee into a cup because the fluid appeared to be frozen, like a glacier. In addition, she could not stop pouring at the right time since she was unable to perceive the movement in the cup (or a pot) when the fluid rose. Furthermore the patient complained of difficulties in following a dialogue because she could not see the movement of the face and, especially, the mouth of the speaker. In a room where more than two other people were walking, she felt very insecure and unwell, and usually left the room immediately, because “people were suddenly here or there but I have not seen them moving.” The patient experienced the same problem but to an even more marked extent in crowded streets or places, which she therefore avoided as much as possible. She could not cross the street because of her inability to judge the speed of a car, but she could identify the car itself without difficulty. “When I’m looking at the car first, it seems far away. But then, when I want to cross the road, suddenly the car is very near.” She gradually learned to “estimate” the distance of moving vehicles by means of the sound becoming louder.
Thus, ninety years after Wilbrand and Verrey had postulated a specific center for color in the brain, Zeki was finally able to prove that such a center existed.
Fifty years earlier, the eminent neurologist Gordon Holmes, reviewing two hundred cases of visual problems caused by gunshot wounds to the visual cortex, had found not a single case of achromatopsia. He went on to deny that an isolated cerebral achromatopsia could occur. The vehemence of this denial, coming from such a great authority, played a major part in bringing all clinical interest in the subject to an end. 18
18. A vivid account of Holmes’s negative influence has been provided by Damasio, who also points out that all of Holmes’s cases involved lesions in the dorsal aspect of the occipital lobe, whereas the center for achromatopsia lies on the ventral aspect.
Zeki’s brilliant and undeniable demonstration startled the neurological world, reawakening attention to a subject it had for many years dismissed. Following his 1973 paper, new cases of human achromatopsia began appearing in the literature once again, and these could now be examined with new brain-imaging techniques (CAT, MRI, PET, SQUID, etc.) not available to neurologists of an earlier era. Now, for the first time, it was possible to visualize, in life, what areas of the brain might be needed for human color perception. Though many of the cases described had other problems, too (cuts in the visual field, visual agnosia, alexia, etc.), the crucial lesions seemed to be in the medial association cortex, in areas homologous to V4 in the monkey. 19
19. The work of Antonio and Hanna Damasio and their colleagues at the University of Iowa was particularly important here, both by virtue of the minuteness of the perceptual testing, and the refinement of the neuroimaging they used.
It had been shown in the 1960s that there were cells in the primary visual cortex of monkeys (in the area termed V1) that responded specifically to wavelength, but not to color; Zeki now showed, in the early 1970s, that there were other cells in the V4 areas that responded to color but not to wavelength (these V4 cells, however, received impulses from the V1 cells, converging through an intermediate structure, V2). Thus each V4 cell received information regarding a large portion of the visual field. It seemed that the two stages postulated by Land in his theory might now have an anatomical and physiological grounding: lightness records for each waveband being extracted by the wavelength-sensitive cells in V1, but only being compared or correlated to generate color in the color-coding cells of V4.
Every one of these, indeed, seemed to act as a Landian correlator, or a Helmholtzian “judge.”
Color vision, it seemed—like the other processes of early vision: motion, depth, and form perception—required no prior knowledge, was not determined by learning or experience, but was, as neurologists say, a “bottom-up” process. Color can indeed be generated, experimentally, by magnetic stimulation of V4, causing the “seeing” of colored rings and halos—so-called chromatophenes. 20
20. Such chromatophenes may occur spontaneously in visual migraines, and Mr. I. himself had experienced these, on occasion, in migraines occurring before his accident. One wonders what would have been experienced if Mr. I.’s V4 areas had been stimulated—but magnetic stimulation of circumscribed brain areas was not technically possible at the time. One wonders, too, now that such stimulation is possible, whether it might be tried in individuals with congenital (retinal) achromatopsia (several such achromatopes have expressed their curiosity about such an experiment). It is possible—I am not aware of any studies on this—that V4 fails to develop in such people, with the absence of any cone input. But if V4 is present as a functional (though never functioning) unit despite the absence of cones, its stimulation might produce an astounding phenomenon—a burst of unprecedented, totally novel sensation, in a brain⁄mind that had never had a chance to experience or categorize such sensation. Hume wonders if a man could imagine, could even perceive, a color he had never seen before—perhaps this Humean question (propounded in 1738) could And an answer now.
