An Anthropologist on Mars (1995) (5 page)

10. In 1877, Gladstone, in an article entitled “On the Colour Sense of Homer”, spoke of Homer’s use of such phrases as “the wine-dark sea.” Was this just a poetic convention, or did Homer, the Greeks, actually see the sea differently? There is indeed considerable variation between different cultures in the way they will categorize and name colors—individuals may only “see” a color (or make a perceptual categorization) if there is an existing cultural category or name for it. But it is not clear whether such categorization may actually alter elementary color perception.

Nor did he (now) have any difficulties reading. Testing up to this point, and a general neurological examination, thus confirmed Mr. I.’s total achromatopsia.

We could say to him at this point that his problem was real—that he had a true achromatopsia and not a hysteria. He took this, we thought, with mixed feelings: he had half hoped it might be merely a hysteria, and as such potentially reversible. But the notion of something psychological had also distressed him and made him feel that his problem was “not real” (indeed, several doctors had hinted at this). Our testing, in a sense, legitimized his condition, but deepened his fear about brain damage and the prognosis for recovery.

Although it seemed that he had an achromatopsia of cerebral origin, we could not help wondering whether a lifetime of heavy smoking could have played a part; nicotine can cause a dimming of vision (an amblyopia) and sometimes an achromatopsia—but this is predominantly due to its effects on the cells of the retina. But the major problem was clearly cerebral: Mr. I. could have sustained tiny areas of brain damage as a result of his concussion; he could have had a small stroke either following, or conceivably precipitating, the accident.

The history of our knowledge about the brain’s ability to represent color has followed a complex and zigzag course. Newton, in his famous prism experiment in 1666, showed that white light was composite—could be decomposed into, and recomposed by, all the colors of the spectrum. The rays that were bent most (“the most refrangible”) were seen as violet, the least refrangible as red, with the rest of the spectrum in between. The colors of objects, Newton thought, were determined by the “copiousness” with which they reflected particular rays to the eye. Thomas Young, in 1802, feeling that there was no need to have an infinity of different receptors in the eye, each tuned to a different wavelength (artists, after all, could create almost any color they wanted by using a very limited palette of paints) postulated that three types of receptors would be enough.
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11. “As it is almost impossible to conceive each sensitive point of the retina to contain an infinite number of particles, each capable of vibrating in perfect unison with every possible undulation”, Young wrote, “it becomes necessary to suppose the number limited, for instance to the three principal colours, red, yellow, and blue. ”

The great chemist John Dalton, just five years earlier, had provided a classic description of red-green colorblindness in himself. He thought this was due to a discoloration in the transparent media of the eye—and, indeed, willed an eye to posterity to test this. Young, however, provided the correct interpretation—that one of the three types of color receptor was missing. Dalton’s eye still resides, pickled, on a shelf in Cambridge.

Lindsay T. Sharpe and Knut Nordby discuss this and many other aspects of the history of colorblindness research in “Total Colorblindness: An Introduction.”

Young’s brilliant idea, thrown off casually in the course of a lecture, was forgotten, or lay dormant, for fifty years, until Hermann von Helmholtz, in the course of his own investigation of vision, resurrected it and gave it a new precision, so that we now speak of the Young-Helmholtz hypothesis. For Helmholtz, as for Young, color was a direct expression of the wavelengths of light absorbed by each receptor, the nervous system just translating one into the other: “Red light stimulates the red-sensitive fibres strongly, and the other two weakly, giving the sensation red.”
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12. In 1816, the young Schopenhauer proposed a different theory of color vision, one that envisaged not a passive, mechanical resonance of tuned particles or receptors, as Young had postulated, but their active stimulation, competition, and inhibition—an explicit “opponens” theory such as Ewald Hering was to create seventy years later, in apparent contradiction of the Young-Helmholtz theory. These opponens theories were ignored at the time, and continued to be ignored until the 1950s. We now envisage a combination of Young-Helmholtz and opponens mechanisms: tuned receptors, which converse with one another, are continually linked in an interactional balance. Thus integration and selection, as Schopenhauer divined, start in the retina.

In 1884, Hermann Wilbrand, seeing in his neurological practice patients with a range of visual losses—in some predominantly the loss of visual field, in others predominantly of color perception, and in still others predominantly of form perception—suggested that there must be separate visual centers in the primary visual cortex for “light impressions”, “color impressions”, and “form impressions”, though he had no anatomical evidence for this. That achromatopsia (and even hemi-achromatopsia) could indeed arise from damage to specific parts of the brain was first confirmed, four years later, by a Swiss ophthalmologist, Louis Verrey. He described a sixty-year-old woman who, in consequence of a stroke affecting the occipital lobe of her left hemisphere, now saw everything in the right half of her visual field in shades of grey (the left half remained normally colored). The opportunity to examine his patient’s brain after her death showed damage confined to a small portion (the fusiform and lingual gyri) of the visual cortex—it was here, Verrey concluded, that “the centre for chromatic sense will be found.” That such a center might exist, that any part of the cortex might be specialized for the perception or representation of color, was immediately contested and continued to be contested for almost a century. The grounds of this contention go very deep, as deep as the philosophy of neurology itself.

Locke, in the seventeenth century, had held to a “sensationalist” philosophy (which paralleled Newton’s physicalist one): our senses are measuring instruments, recording the external world for us in terms of sensation. Hearing, seeing, all sensation, he took to be wholly passive and receptive. Neurologists in the late nineteenth century were quick to accept this philosophy and to embed it in a speculative anatomy of the brain. Visual perception was equated with “sense-data” or “impressions” transmitted from the retina to the primary visual area of the brain, in an exact, point-to-point correspondence—and there experienced, subjectively, as an image of the visual world. Color, it was presumed, was an integral part of this image. There was no room, anatomically, it was thought, for a separate color center—or indeed, conceptually, for the very idea of one. Thus when Verrey published his findings in 1888, they flew in the face of accepted doctrine. His observations were doubted, his testing criticized, his examination regarded as flawed—but the real objection, behind these, was doctrinal in nature.

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 1890
s
, were dismissed from neurological consciousness—and cerebral achromatopsia, as a subject, all but disappeared for the next seventy-five years.
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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.
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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.
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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.
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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.)