Dr. Greene’s Answer:
The dazzling experience of color begins when light strikes a canvas of tightly-packed nerve cells in the back of the eye. These rods and cones, as they are commonly called, fire a storm of nerve impulses in response to the light, which then travel down the optic nerve to the visual centers of the brain. The rods are the “black-and-white” receptors; they photograph the ever-changing patterns of light and darkness that are before our eyes. The cones are responsible for the wonder of color vision.
Cones come in three varieties: red, blue, and green. Red light stimulates the red cones, and simultaneously inhibits the surrounding green cones. Green light does the exact opposite (green and red are each other’s opponent colors). Blue light stimulates the blue cones and inhibits both red and green cones (red and green light, mixed, form yellow light — blue’s opponent color).
All the rainbow of colors we see are a combination of these three primary colors of light. Note — there is nothing inherent about the primary colors that makes them primary — it is only that we have these three types of cones, and that the entire spectrum of visible light can be coded for by using only these three reference points. Another species could use a different number or group of colors as primaries.
In kindergarten, children learn that red, yellow, and blue are the primary colors. These are the primary colors of paints or pigments — when you add pigments together, fewer wavelengths of light are reflected to the eye, and if you mix them all you can get black. The primary colors of light are red, blue, and green. As you add light together you get more wavelengths of light, and if you mix them all together you can get white light. Projection televisions use red, blue, and green projectors, since we have red, blue, and green cones.
More than five million cones line the postage-stamp of tissue at the back of the eye we call the retina. Underlying this is a complex network of intermediary cells (bipolar cells, horizontal cells, and amacrine cells) that work much like a computer to rapidly interpret the wealth of data generated by the proportionately different stimuli to the cones. The result is that the eye is able to pick out a pinpoint of color. As quick as a glance, the patterns change, and the eye is able to seamlessly generate another precision picture of the world around us.
The ability to see colors is relatively rare among vertebrates. Humans and other primates see in color, but most other mammals do not. Most fish and amphibians do see in color, as well as some birds and reptiles. Unlike most insects, butterflies and bees have color vision to guide them on their journeys.
We humans are all born colorblind! The cones don’t begin functioning until a baby is about 4 months old. At that time the baby undergoes a gradual transformation that is as remarkable as the scene in the Wizard of Oz when Dorothy leaves the black-and-white world of Kansas for the brilliant colors of Oz. About one out of 40,000 babies never develops cones, seeing only in black-and-white throughout life. This is called achromatopsia, or rod-monochromatic colorblindness.
There are many other versions of colorblindness, but by far the most common is red-green colorblindness, which affects as many as one out of 25 people. These people either do not have red cones (protanopia) or green cones (deuteranopia). They are unable to distinguish between green and red, but with their remaining two types of cones are able to see all of the other colors. The absence of blue cones is extremely rare.
Colorblindness is usually tested for at children’s four-year physicals. The doctor asks them to identify a red and a green line on the eye chart. Some doctors have a series of dotted pictures in which children are asked to pick out numbers or letters hidden inside the dots. Colorblind children have difficulty finding those letters and numbers. If any question remains, more precise visual testing can determine the exact nature of the problem.
Colorblindness is almost always a hereditary condition. Red-green colorblindness is a recessive condition passed on the X chromosome. Only one healthy color vision gene is necessary to provide color vision. Since boys have only one X chromosome, it is much easier for them to be colorblind. If their mothers are carriers (having one normal X chromosome and one colorblind X chromosome), the sons have a 50% chance of having the condition. Red-green colorblindness occurs in about 8 per cent of American males. These men cannot pass the condition on to their sons (since they give their sons a Y, not an X, chromosome), but they will pass the gene to their daughters.
All girls whose fathers are colorblind will at least carry the gene for colorblindness. In order for a girl to actually be red-green colorblind, she must have a mother who is a carrier or colorblind AND a father who is colorblind. This happens in only about 0.64 percent of American girls (although these numbers vary considerably in other population groups).
There is no known way to restore color vision in those who have hereditary colorblindness. By being aware of their condition, we can help our children learn other ways to distinguish between red and green — the position of traffic lights, for instance. And we can decorate their worlds, and wrap their presents, in the millions of nuances of color that are still available to them.