Ever heard about rods and cones from a science class or your eye doctor? They’re the components in your eyes that help you see light and colors. They’re located inside the retina. That’s a layer of thin tissue at the back of your eyeball near your optic nerve.
Rods and cones are crucial to sight. Rods are sensitive to light and are important for allowing you to see in the dark. Cones are responsible for allowing you to see colors.
Most people, as well as other primates like gorillas, orangutans, and chimpanzees and even some marsupials, only see color through three different types of cones. This color visualization system is known as trichromacy (“three colors”).
But some evidence exists that there are people who have four distinct color perception channels. This is known as tetrachromacy.
Tetrachromacy is thought to be rare among human beings. Research shows that it’s more common in women than in men. A 2010 study suggests that nearly 12 percent of women may have this fourth color perception channel.
Men aren’t as likely to be tetrachromats. Men are actually more likely to be color blind or unable to perceive as many colors as women. This is due to inherited abnormalities in their cones.
Let’s learn more about how tetrachromacy stacks up against typical trichromatic vision, what causes tetrachromacy, and how you can find out if you have it.
The typical human has three types of cones near the retina that allow you to see various colors on the spectrum:
- short-wave (S) cones: sensitive to colors with short wavelengths, such as purple and blue
- middle-wave (M) cones: sensitive to colors with medium wavelengths, such as yellow and green
- long-wave (L) cones: sensitive to colors with long wavelengths, such as red and orange
This is known as the theory of trichromacy. Photopigments in these three types of cones give you your ability to perceive the full spectrum of color.
Photopigments are made of a protein called opsin and a molecule that’s sensitive to light. This molecule is known as 11-cis retinal. Different types of photopigments react to certain color wavelengths that they’re sensitive to. This results in your ability to perceive those colors.
Tetrachromats have a fourth type of cone featuring a photopigment that allows perception of more colors that aren’t on the typically visible spectrum. The spectrum is better known as ROY G. BIV (Red, Orange, Yellow, Green, Blue, Indigo, and Violet).
The existence of this extra photopigment may allow a tetrachromat to see more detail or variety within the visible spectrum. This is called the theory of tetrachromacy.
While trichromats can see about 1 million colors, tetrachromats may be able to see an incredible 100 million colors, according to Jay Neitz, PhD, an ophthalmology professor at the University of Washington, who has studied color vision extensively.
Here’s how your color perception typically works:
- The retina is takes in light from your pupil. This is the opening at the front of your eye.
- Light and color travel through the lens of your eye and become part of a focused image.
- Cones turn light and color information into three separate signals: red, green, and blue.
- These three types of signals are sent to the brain and processed into a mental awareness of what you’re seeing.
The typical human being has three different types of cones that divide up visual color information into red, green, and blue signals. These signals can then be combined in the brain into a total visual message.
Tetrachromats have one extra type of cone that allows them to see a fourth dimensionality of colors. It results from a genetic mutation. And there’s indeed a good genetic reason why tetrachromats are more likely to be women. The tetrachromacy mutation is only passed through the X chromosome.
Women get two X chromosomes, one from their mother (XX) and one from their father (XY). They’re more likely to inherit the necessary gene mutation from both X chromosomes. Men only get one X chromosome. Their mutations usually result in anomalous trichromacy or color blindness. This means that either their M or L cones don’t perceive the right colors.
A mother or daughter of someone with anomalous trichromacy is most likely to be a tetrachromat. One of her X chromosomes may carry normal M and L genes. The other likely carries regular L genes as well as mutated L gene passed through a father or son with anomalous trichromacy.
One of these two X chromosomes is ultimately activated for the development of cone cells in the retina. This causes the retina to develop four types of cones cells because of the variety of different X genes passed on from both mother and father.
Some species, including humans, simply don’t need tetrachromacy for any evolutionary purpose. They’ve almost lost the ability altogether. In some species, tetrachromacy is all about survival.
Several bird species, such as the zebra finch, need tetrachromacy to find food or choose a mate. And the mutual pollination relationship between certain insects and flowers have caused plants to develop more complex colors. This, in turn, has caused insects to evolve to see these colors. That way, they know exactly which plants to choose for pollination.
It may be challenging to know if you’re a tetrachromat if you’ve never been tested. You may just take your ability to see extra colors for granted because you have no other visual system to compare yours to.
The first way to find out your status is by undergoing genetic testing. A full profile of your personal genome can find the mutations on your genes that may have resulted in your fourth cones. A genetic test of your parents can also find the mutated genes that were passed on to you.
But how do you know if you’re actually able to distinguish the extra colors from that extra cone?
That’s where research comes in handy. There are several ways that you can find out if you’re a tetrachromat.
The color matching test is the most significant test for tetrachromacy. It goes like this in the context of a research study:
- Researchers present study participants with a set of two mixtures of colors that will look the same to trichromats but different to tetrachromats.
- Participants rate from 1 to 10 how closely these mixtures resemble each other.
- Participants are given the same sets of color mixtures at a different time, without being told that they’re the same combinations, to see if their answers change or stay the same.
True tetrachromats will rate these colors the same way every time, meaning that they can actually differentiate between the colors presented in the two pairs.
Trichromats may rate the same color mixtures differently at different times, meaning that they’re just choosing random numbers.
Warning about online testsNote that any online tests that claim to be able to identify tetrachromacy should be approached with extreme skepticism. According to Newcastle University researchers, the limitations of displaying color on computer screens make online testing impossible.
Tetrachromats are rare, but they sometimes make big media waves.
A subject in the 2010 Journal of Vision study, known only as cDa29, had perfect tetrachromatic vision. She made no errors in her color matching tests, and her responses were incredibly quick.
She’s the first person to have been proven by science to have tetrachromacy. Her story was later picked up by numerous science media outlets, such as Discover magazine.
In 2014, artist and tetrachromat Concetta Antico shared her art and her experiences with the British Broadcasting Corporation (BBC). In her own words, tetrachromacy allows her to see, for example, “dull grey...[as] oranges, yellows, greens, blues, and pinks.”
While your own chances of being a tetrachromat might be slim, these stories show how much this rarity continues to fascinate those of us who possess standard three-cone vision.