100 More Things #103: SOME PEOPLE HAVE AN EXTRA COLOR CONE

In 1948, H. L. de Vries was studying the eyes of men who were color blind. He made an amazing discovery that he mentioned only in passing, on the last page of the paper he wrote about his research. His discovery went virtually unnoticed for more than three decades.

Before I tell you about the discovery, here’s some background on color vision:

• People see color with special cells in their eyes called cones. Most people have three types of cones, each of which is triggered by certain wavelengths of light. The cones send signals to the brain, and the brain interprets those signals as blue, or turquoise, or pink, or any of the other colors.
• Each cone allows the eye to see approximately 100 shades, so all three cones combined result in 100 to the third power, or about 1 million, different colors that most people can see.
• For some people, one or more cones don’t activate in the same way—these people have one of several forms of color confusion or color blindness. They may have trouble distinguishing between certain colors, for example, red and green. People who have only two color cones working properly can see approximately 100 to the second power, or 10,000, colors. People who have only one color cone working properly can see approximately 100 colors.
• Color vision is determined by the X chromosome. Men have only one X chromosome, and women have two X chromosomes. This is why more men than women are color blind.

Back To The Amazing Discovery

To test the men with color blindness, de Vries had them turn dials on an instrument to mix red light and green light until they saw yellow. Because the men were color blind, they added more red or green than someone without color blindness would add.

Out of curiosity, de Vries tested the daughters of one subject and observed that even though they were not color blind—they seemed to distinguish red and green as well as anyone—they still used more red in their test light than normal people to make the match precise. If the women weren’t color blind, why were they adding more red?
De Vries hypothesized that since color blindness runs in families, the mothers and daughters of the color-blind men would have four color cones, not three. They would have the three normal cones, plus the abnormal cone that the men in the family have. De Vries’s idea was that having four cones enabled them to see more colors than most people, and that was why their test results were unusual. He put this idea about four cones at the end of his paper, and didn’t mention it in any of his work after that.

It wasn’t until many years later that de Vries’s ideas were rediscovered by Gabriele Jordan and John Mollon (2019), who were studying color vision in monkeys. Since color blindness is fairly common in men (9 percent of men are color blind), Mollon and Jordan realized that as many as 12 percent of the women in the world may have four cones. The name for someone who has four cones is a “tetrachromat.” These women would be able to see 100 to the fourth power, or 100 million, colors.

Functional Tetrachromats Are Rare

Much to her surprise, Jordan has had a difficult time finding women who are tetrachromats and can correctly do the matching tests for tetrachromacy. Why is this? It turns out that although a woman may be a tetrachromat, she may not be able to distinguish all the colors. She may report colors as though she only had three cones. The theory here is that tetrachromats are living in a world of trichromats. The objects they interact with were created by and for people who see 1 million colors, not 100 million colors, which means that tetrachromats haven’t had much opportunity to learn how to distinguish between the extra colors they see.

There is some evidence to support this theory. Artist Concetta Antico was tested and found to have the DNA of a tetrachromat. She also is a “functional” tetrachromat. Her early training and continued immersion in art may have taught her how to use her fourth cone.

Testing for tetrachromacy

The best way to test for tetrachromacy is with a DNA test. Watch out for fake tests. One bogus test that went viral suggested that any viewer who could see 33 or more colors in the test image was a tetrachromat. In fact, computer displays don’t display enough colors to test for tetrachromacy.

Takeaways

• If you’re a woman with color-blind men in your family, you might be a tetrachromat. If you are, you might need special training to see the extra colors.
• With advances in technology, color displays are likely to show more colors in the near future. As a designer, you might be asked to—or you might want to—create designs that use the extra colors that tetrachromats can see. There might be unique designs using pictures and graphics with extra colors for those who can see them.