Color Vision

Color is a visual sensation. Light illuminates an object and the light that is reflected into the eye, stimulates the eye-brain and the response is color. The retina, located behind the eyeball, contains two types of light sensitive cells, rods and cones. Rods perform best in dim lighting, allowing us to see in dark lighting but without much perception of color. Cones allow us to see color in areas with high levels of light. Most humans are trichromats, which means that they have three types of cones, the S, M, and L cones. When the S cone is mostly stimulated the color we see is blue, when the M cone is mostly stimulated we see the color green and when the L cone is mostly stimulated we see the color red.

Each cone can perceive about 100 shades of a color, so the typical trichromat can see 1 million different colors. Tetrachromats, people who have four cones, can see 100 million colors. They believe that the additional cone is due to the mutation that causes red-green color blindness. Typically, red-green color blind men have two cones that function normally and one mutated cone that is less sensitive to the color red or green. The carriers of this gene, mothers and daughters of red-green color blind men, have three normally functioning cones and one mutated cone. It is believed that 12% of women have four cones. However, most women that have four cones, usually have a non-functioning mutated cone and so they are not true tetrachromats.  There are a few true tetrachromat with additional color vision due to their additional working cone. Tetrachromacy is also present in a few species of fish, birds, and insects. These animals have red, green, blue and UV light sensitive photoreceptors.

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In addition to humans, apes and most old world monkeys are trichromatic. Bees are also trichromatic, but they have blue, green and UV light sensitive photoreceptors, unlike humans, apes, and old world monkeys, who have red, green, and blue sensitive photoreceptors. New world monkeys color vision is sex-linked, so female and male monkeys of the same species can be either trichromatic or dichromatic. Most often, the females are trichromatic and the males are dichromatic.

Dichromatic animals include dogs, cats, mice, rabbits and rats. Animals that are dichromatic only have two cones.  They typically have blue and green photoreceptors. Most marine animals, including seals, seal ions, walruses, dolphins, whales, and sharks are monochromatic. They are only able to see black, white and grey.

Mice are dichromatic, their color vision consists of two cones. Scientists have performed studies to mutate the genes of mice, with the intention of making them trichromatic. They tested the color vision of the mice by rewarding them with soymilk if they were able to correctly identify which set of colors varied from one another. The control group, normal mice, were not able to distinguish between red and yellow light, whereas the mutated mice were able to identify the correct set 80% of the times. The results from this study could potentially help to correct color blindness in humans.

The history of the discovery of color blindness begins with John Dalton. In 1793 he wrote a paper titled, “Extraordinary Facts Relating to the Vision of Colours,” where he explained his theory on the cause of color blindness. He believed that color blindness occurred because the liquid around the eyeball had a bluish tint, which blocked out the other colors. Dalton himself suffered from color blindness. When he died his eyes were examined and it was revealed that he had clear liquid surrounding his eye, just like those with normal vision, thus his theory was wrong. The first people to correctly describe trichromatic color vision was Thomas Young and Hermann von Helmholtz, and from there it was not long until it was applied to correctly determine the cause of color blindness.

In trichromatics each cone is sensitive to a certain wavelength of light, the S cone is stimulated by short wavelengths, the M cone is stimulated by medium wavelengths, and the L cone is stimulated by large wavelengths. Our perception of color is the amount of stimulation of each cone.  Those who are color blind are unable to see color in a normal way due to cones that are missing, nonfunctioning, or detect a different color than normal.

Red-green color blindness is a recessive trait that is on the X sex chromosome, so the only way for a woman to be red-green color blind is if both of her parents have the recessive trait. A father cannot pass his red-green color blindness to his sons, but if a woman is red-green color blind all of her sons will be affected. Women can be carriers and pass the gene on. Typically very few women are red-green color blind. It affects about 8% of men and 0.5% of women. This mild form of color blindness is the most common.

There are four types of red-green color blindness, protanomaly, protanopia, deuteranomaly, and deuteranopia. Protanomaly is when the red cone is affected, causing red, orange, and yellow to appear greener and other colors to be not as bright.  Protanopia is when the red cone is non-functional, causing red to appear black and certain shades of orange and green to appear as yellow. Deuteranomaly is when the green cone is affected, causing yellow and green to appear redder and it is difficult to distinguish violet from blue. Deuteranopia is when the green cone in non-functional, causing reds to appear as brownish-yellow and greens to appear as beige. Affecting 5% of men, deuteranomaly is the most common red-green color blindness. Protanomaly, protanopia, and deuteranopia each affect 1% of men. Protanomaly and Deuteranomaly are milder forms of red-green color blindness.

Blue-yellow color blindness is rarer than red-green colorblindness. Red-green color blindness is a sex-linked disorder, but blue-yellow color blindness is not a sex-linked disorder, so it affects the same amount of males and females. There are two types of blue-yellow color blindness, tritanomaly and tritanopia. Tritanomaly is when the blue cone has limited function, causing blue to appear greener and it can be difficult to differentiate yellow and red from pink. Tritanopia is when the blue cone is non-functioning, causing blue to appear green and yellow to appear violet or light grey.

Monochromacy is when someone does not experience color at all. Someone with monochromacy suffers from complete colorblindness and the clearness of their vision may also be affected. There are two types of monochromacy, cone monochromacy and rod monochromacy. Cone monochromacy is when two cones are non-functioning. There are then three types of cone monochromacy depending on which cone is functioning. There is red cone monochromacy, green cone monochromacy, and blue cone monochromacy. Someone with cone monochromacy cannot see color because the brain needs to compare signals from different types of cones in order to see color. Rod monochromacy is the most severe form of color blindness. Someone with monochromacy has three non-functioning cones, causing them to only see black, white and grey.

Currently, there is no cure for color blindness, however there have been innovations to help those who suffer from color blindness. The company Enchroma produces glasses that improves color vision, but they do not give back complete normal color vision. The glasses are for improving color discrimination for those with red-green color blindness. Another option to help correcting color vision is, the X Chrom lens. It is a monocular contact lens that was developed by Dr. Harry Zeltzer. The lens is only worn in the non-dominant eye.  There is a magenta circle on the lens that intensifies the colors red and green, enhancing the users’ perception of color.