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rhodopsin, on chromosome 3, show much larger differences, from each other and from the red and green genes. Presumably, some time in the distant past, a primordial visual pigment gave rise to rhodopsin, the blue pigment, and the common precursor of the red and green pigments. At a much more recent time the X-chromosome genes for the red and green pigments arose from this precursor by a process of duplication. Possibly this occurred after the time of separation of the African and South American continents, 30 to 40 million years ago, since old world primates all exhibit this duplication of cone pigment genes on the X-chromosome, whereas new world primates do not.
Cloning the genes has led to a spectacular improvement in our understanding of the various forms of color blindness. It had long been known that most forms of color-vision deficiency are caused by the absence or abnormality of one or more of the three cone pigments. The most frequent abnormalities occur in the red and green pigments and affect about 8 percent of males.
Because of the wide range of these abnormalities the subject is complex, but given our molecular-level understanding, it is fortunately no longer bewildering.
Very rarely, destruction to certain cortical areas can cause color blindness.
Most often this occurs as the result of a stroke.



                                THE HERING THEORY
In the second half of the nineteenth century, a second school of thought arose parallel to, but until recently seemingly irreconcilable with, the Young-Helmholtz color theory. Ewald Hering (1834-1918) interpreted the results of color mixing by proposing the existence, in the eye, brain, or both, of three opponent processes, one for red-green sensation, one for yellow-blue, and a third, qualitatively different from the first two, for black-white. Hering was impressed by the nonexistence of any colors—and the impossibility of even imagining any colors—that could be described as yellowish-blue or reddish-green and by the apparent mutual canceling of blue and yellow or of red and green when they are added together in the right proportions, with complete elimination of hue—that is, with the production of white. Hering envisioned the red-green and yellow-blue processes as independent, in that blue and red do add to give bluish-red, or purple; similarly red added to yellow gives orange; green added to blue gives bluish-green; green and yellow gives greenish-yellow. In Hering's system, yellow, blue, red, and green could be thought of as "primary" colors. Anyone looking at orange can imagine it to be the result of mixing red and yellow, but no one looking at red or blue would see it as the result of mixing any other colors. (The feeling that some people have that green looks like yellow added to blue is probably related to their childhood experience with paint boxes.) Hering's notions of red-green and yellow-blue processes seemed to many to be disconcertingly dependent on intuitive impressions about color, but it is surprising how good the agreement is among people asked to select the point on the spectrum where blue is represented, untainted by any subjective trace of green or yellow. The same is so for yellow and green. With red, subjects again agree, this time insisting that some violet by added to counteract the slight yellowish appearance of longwavelength light. (It is this subjective red that when added to green gives white; ordinary (spectral) red added to green gives yellow.)
We can think of Hering's yellow-blue and red-green processes as separate channels in the nervous system, whose outputs can be represented as two meters, like old-fashioned voltmeters, with the indicator of one meter swinging to the left of zero to register yellow and to the right to register blue and the other meter doing the same for red versus green. The color of an object can then be described in terms of the two readings. Hering's third antagonistic process (you can think of it as a third voltmeter) registered black versus white.
He realized that black and gray are not produced simply by absence of light coming from an object or surface but arise when and only when the light from the object is less than the average of the light coming from the surrounding regions. White arises only when the surround is darker and when no hue is present. (I have already discussed this in Chapter 3, with examples such as the turned-off television set.) In Hering's theory, the black-white process requires a spatial comparison, or subtraction of reflectances, whereas his yellow-blue and red-green processes represent something occurring in one particular place t in the visual field, without regard to the surrounds. (Hering was certainly aware that neighboring colors interact, but his color theory as enunciated in his latest work does not encompass those phenomena.) We have already seen that black versus white is indeed represented in the retina and brain by spatially opponent excitatory and inhibitory (on versus off) processes that are literally antagonistic.
Hering's theory could account not only for all hues and levels of saturation, but also for colors such as brown and olive green, which are not only absent I from any rainbow, but cannot be produced in any of the classical psychophysical color-mixing procedures, in which we shine spots of light on a dark screen with a slide projector. We get brown only when a yellow or orange spot of ' light is surrounded by light that on the average is brighter. Take any brown and exclude all the surround by looking at it through a tube, a black piece of rolled up paper, and you will see yellow or orange. We can regard brown as a mixture of black—which is obtainable only by spatial contrasts—and orange or yellow. In Hering's terms, at least two of the systems are at work, the black-white and the yellow-blue.
Hering's theory of three opponent systems, for red-green, yellow-blue, and black-white, was regarded in his time and for the next half-century as rivaling and contradicting the Young-Helmholtz three-pigment (red, green, and blue)
theory: the proponents of each were usually strongly partisan and often emotional. Physicists generally sided with the Young-Helmholtz camp, perhaps because their hearts were warmed by the quantitative arguments—by such things as linear simultaneous equations—and turned or cooled off by arguments about purity of colors. Psychologists often sided with Hering, perhaps because they had experience with a wider variety of psychophysical phenomena. Hering's theory seemed to be arguing for either four receptor types (red, green, yellow, and blue) or worse, for three (one subserving black-white, one yellow-blue, and one red-green), all in the face of mounting evidence for the original Young hypothesis. In retrospect, as the contemporary psychophysicists Leo Hurvich and Dorothea Jameson have pointed out, it seems that one difficulty many people had with the Hering theory was the lack, until the 1950s, of any direct physiological evidence for inhibitory mechanisms in sensory systems. Such evidence became available only a half-century later, with single-unit recordings.
By imagining the voltmeters to be measuring positive to the right and negative to the left, you can see why Hering's work suggested inhibitory mechanisms. In a sense, the colors yellow and blue are mutually antagonistic; together they cancel each other, and if the red-green system also reads zero, we have no color. Hering in some ways was fifty years ahead of his time. As has happened before in science, two theories, for decades seemingly irreconcilable, both turned out to be correct. In the late 1800s, nobody could have guessed that at one and the same time the Young-Helmholtz notions of color would turn out to be correct at the receptor level, whereas Hering's ideas of opponent processes would be correct for subsequent stages in the visual path. It is now clear that the two formulations are not mutually exclusive: both propose a three-variable system: the three cones in the Young-Helmholtz and the three meters, or processes, in the Hering theory. What amazes us today is that with so little to go on, Hering's formulation turned out to describe cell-level central-nervous-system color mechanisms so well. Nevertheless, color-vision experts are still polarized into those who feel Hering was a prophet and those who feel that his theories represent a fortuitous fluke. To the extent that I am slightly to the Hering side of center, I will doubtless make enemies of all the experts.
   
 




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