Most of us take our color vision somewhat for granted—we have never known any differently. Still, it can take years of practice to discern the slight differences in temperature and value between two adjacent colors on the color wheel well enough to paint them. Color is so dominant in our daytime visual experiences that differences in value, or luminance, can be especially troublesome to recognize and match. This has to do with the two different light sensing cells in our eyes, the cones and rods, which play different roles in our vision and have different sensitivities to the light and luminance spectrum. Basically, the cones sense colors in daylight, while the luminance-sensing rods excel in dimly lit situations and provide our night vision. See The Purkinje Effect. Imagine then, what the world must be like for someone who can see no color at all, and is also extremely sensitive to bright sunlight.
Oliver Sacks was fascinated, even obsessed by the phenomenon of achromatopsia. In his book, The Island of the Colorblind, he shared his experiences with a special culture of people isolated on two remote islands of the Caroline chain in the South Pacific. Sacks took the long trip to visit the Micronesian islands of Pingelap and Pohnpei where a significant percentage of the population have complete colorblindness, a very rare condition not often encountered by most vision researchers. Accompanying him was Knut Nordby, a Norwegian scientist and vision researcher who, in addition to being an expert on the subject of achromatopsia, is also completely colorblind. Congenital achromatopes have no functional cones in their retinas, only rods. This means that, while they are extraordinarily sensitive to light and dark, they can perceive no color at all.
In the course of their travels, Knut related the story of his sister, Britt, also an achromatope: “Britt, to prove it could be done, had knitted a jacket in sixteen different colors. She had devised her own system for keeping track of the skeins of wool, by labelling them with numbers. The jacket had marvellous intricate patterns and images drawn from Norwegian folktales, . . . but since they were done in dim browns and purples, colors without much chromatic contrast, they were almost invisible to normal eyes. Britt, however, responding to luminances only, could see them quite clearly, perhaps even more clearly than color-normals. ‘It is my special, secret art,’ she says. ‘You have to be totally colorblind to see it.’ ”
The achromatopes that Sacks and Nordby met on these islands were aware that they were different, and they suffered from the constraints of their impairment. However, their visual lives were rich in textures and subtle gradations color-sighted people cannot perceive.Their condition even gave them some advantages over the color-sighted. As the last of the daylight begins to wane, their visual world expands and their visual experiences under moonlight and starlight are something we can only imagine.
Sacks wrote about Nordby’s acute sensitivity to the tones of the native flora of the island. “Knut was fascinated . . . by the richness of the vegetation, which he saw quite clearly, perhaps more clearly than the rest of us. For us, as color-normals, it was at first just a confusion of greens, whereas to Knut it was a polyphony of brightnesses, tonalities, shapes, and textures, easily identified and distinguished from each other. He mentioned this to James [a native of the island], who said it was the same for him, for all the achromatopes on the island—none of them had any difficulty distinguishing the plants on the island. He thought they were helped in this, perhaps, by the basically monochrome nature of the landscape: there were a few red flowers and fruits on the island, and these, it was true, they might miss in certain lighting situations—but virtually all else was green.”
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–John and Ann