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Color Vision Deficiency
Contrary to popular belief and intuition, colors do not actually exist. Our perception of color is the product of our minds. The vast extent of this shared illusion in the human experience is probably why most people act as though objects do indeed have color. So, if there is no organism with color vision in the forest-- is the forest green? It is not green if there is no one or no organism with color vision there to perceive it. Our eyes are equipped with three receptor types that allow us to create the perception of color. Human vision is known, therefore, as trichromatic.
A rat, which has only one type of receptor, cannot perceive color. The rat's vision is known as monochromatic. These ideas can be difficult to comprehend. Here is a brief analogy that may, at least, aid in understanding. If you walk up to your television set, if it is a color set, you can notice that there are three types of 'colored' diodes. It is necessary to have these three different types in order to enable the perception of color from a television. Old black-and-white models had only one type of diode. Hence, our perception with these old televisions, like a rat's, was in black and white. Because three retinal receptor types are necessary to create the perception of color in just one perceptual point in space, there exists a trade-off between color vision and acuity. As any organism's color vision becomes more complex, acuity is sacrificed. The color sensations that humans experience with so-called 'normal' color vision can be broken down into six necessary and essential sensations: blue, yellow, red, green, black, and white.
Those of us with 'normal' color vision have three separate color systems: a yellow-blue system, a red-green system, and a black-white system. The first two systems are opponent or opposites. This means that neither red nor green can be perceived in the same perceptual point in space. Thus, we have no word for or perception of a 'reddish green' nor are there red-green crayons. Similarly, we cannot perceive 'bluish yellow'. The remaining system, however, is not opponent since we can perceive a blackish-white. We call this gray. Since colors do not exist in the environment, but, instead, are created within humans (probably other organisms too), the term colorblind is inaccurate. 'Color different' is more precise. Most people who are born 'colorblind' are either missing the red-green system or have a red-green color system that is working at less than 100 percent efficiency. In either case, this deficiency results in a difference in color perception.
Approximately 8 percent of American males are color deficient. Females may also be color deficient. However, this is rare. Color deficiency is a sex-linked trait carried on the X-chromosome. Thus, for males, those who are color deficient get the 'defective' gene from their mothers. The sisters of these 'color deficient' males tend to be carriers. They have no color vision deficiency, having one normal X chromosome and one with the 'color deficient' trait. Statistically speaking, half of the children of these carrier females are likely to be 'color deficient'. A female can only be color deficient if she gets two color defective X genes, one from each parent.
So what does the ‘colorblind’ child perceive?
Most people who have congenital defects in color vision have a fully functioning yellow-blue system, a fully functioning black-white system, and a not-so-good red-green system. Most can still create the perception of red and green but the reds will appear 'washed out' and yellowish. Similarly, greens will tend to be 'washed out' and bluish. For those who are completely missing the red-green system, the world appears as combinations of blue, yellow, black, and white. Thus, the sky is blue, the grass is blue, and fire engines are yellow.
People with color deficiencies learn how to adapt and have never known anything else. For this reason, most are quite good at color naming of objects even though their perception is quite different than 'normal'.
Diseases of the eye can also affect color perception. Since these changes occur after birth, the effects are more difficult to quantify. However, it is generally accepted that diseases like glaucoma and macular degeneration primarily affect the blue-yellow system. In addition, most cataracts cause a 'yellowing' of the lens inside the eye and the perception is like looking through a dirty yellow filter. Rarely, people are born without color systems at all. These people are known as rod monochromats and are 'blinded' under normal lighting conditions and see best at dawn or dust or with very dark sunglasses. They have both poor acuity and lack any color vision. Some drugs, for example, Viagra can also affect color vision. So far, these effects seem to be temporary and to be affecting the blue-yellow system.