Human Tetrachromacy is Real. Here’s What We Know
After decades of exhaustive study, scientists have concluded that human tetrachromacy is real. Some people have a truly superhuman range of color vision. In fact, there are two distinct types of tetrachromacy. In some cases, it’s genetic. But in some rare cases, it can also be an acquired trait. While it’s difficult to test, enough tetrachromats have stepped forward that scientists now have visual and genetic tests for the condition.
One percent of the world’s population is thought to be tetrachromatic. These lucky folks may be able to see a thousand times as many colors as the rest of us trichromats. In order to test that idea, researcher Gabriele Johnson devised an experiment. She used precise amounts of pigment to create shades of paint that could only be distinguished by a machine — or a tetrachromat. In 2010, Johnson found a subject who was able to tell each subtle shade apart, every time — just as fast as trichromats could identify the colors they saw. “When you ask them to discriminate between the two mixtures, a tetrachromat can do it very quickly,” she said. “They don’t hesitate.”
Concetta Antico is a painter and art teacher with genetic tetrachromacy. Growing up in Sydney, she says, she was always “a little bit out of the box,” alone in her own visual dreamland. She always preferred the kaleidoscope of colors she saw when she looked at the natural world. But nobody else seemed to see it quite like she did. So she decided to paint what she saw. “I’m sure people just think I’m high on something all the time,” she said, “but I’m really just high on life and the beauty that’s around us.”
Antico leaned into her impressionist euphoria. She opened a teaching studio in California. Then in 2012, genetic tests revealed an explanation for Antico’s preferences and the way she saw her world. She’s a genetic tetrachromat.
Genetic tetrachromacy means that a person has two different genes for their photopigments, both active at the same time. Up to 12% of people with two X chromosomes may have the condition. With a copy of OPN1MW on one X chromosome and a copy of OPNMW2 on the other, it’s possible for a person to have four separate and functional cones in their retina, instead of the usual three. But the cones’ response ranges overlap, so the same wavelength of visible light can saturate more than one receptor. This may explain the visual differences characteristic of this type of tetrachromacy. People with the condition can make finer distinctions between shades, because they have more combinations of color receptors to do it with.
Having the right hardware is essential for tetrachromacy, but it’s not sufficient. A person has to recognize the perceptual effects, and then learn to pay attention to them. Dr. Kimberly Jameson, who has studied Antico, said of the artist that “In Concetta’s case … one thing we believe is that because she’s been painting sort of continuously since the age of seven years old, she has really enlisted this extra potential and used it. This is how genetics works: it gives you the potential to do things and if the environment demands that you do that thing, then the genes kick in.”
Paint With All the Colors of the Wind
Rainbows are a delight to the eye as they gleam from raindrops or a crystal prism. If Antico’s art is any guide, a person with genetic tetrachromacy sees their entire world illuminated with that deep, luminous palette. Many of her works strive to capture a certain slant of light. Some, like this painting of the water’s edge at La Jolla, also show her unique perception of color.
While the bright morning light throws harsh shadows with hard edges, the muted colors in the background create depth of field. Combined, they show how mist over the water catches and holds the light. In the foaming water, greens and teals and Caribbean turquoise swirl around reflections of the amber cliffside. The nearest cliff casts a deep shadow over the water. In the shadow, Antico sees an iridescent purple shimmer, like an abalone shell.
Where the painting of La Jolla shows sunlight on a clear, hot day, the sky in this painting of Mission Hills looks like light breaking through after a thunderstorm. In the flowers and foliage, richly saturated colors leap out of the painting as though they can’t contain themselves. After a good rain, sometimes the plants are just very green, and the flowers very bright.
Overlapping activation of cones could explain the colors that seem to pop up in unexpected places. “It’s not just an affectation and it’s not artistic licence,” Antico says. “I’m actually painting exactly what I see. If it’s a pink flower and then all of a sudden you see a bit of lilac or blue, I actually saw that.”
Where some people have an unusual variety of cones that respond to the visual spectrum, there’s another kind of tetrachromacy. Some people with tetrachromatic vision can see into the UV band, perceiving a bright purple glow where others perceive nothing at all. This is the kind of tetrachromacy that can be an acquired trait.
Normally, the lens of the human eye blocks most light below 400 nm, which is where the UV band “starts.” Cones that respond to the deepest violets can actually be sensitive to near UV. However, because they don’t receive that light, they never have a chance to fire in response to it. This is why UV lasers are so dangerous. Even though too much UV can damage the eye, we don’t see it, so we don’t know to look away.
Most people don’t perceive ultraviolet light at all. But all of that can change if a person doesn’t have a lens (a condition called aphakia). Aphakia is mostly caused by surgical removal of the lens in order to treat cataracts. Without a lens, some UV can reach the retina and light up those deep-violet cones, which is remarkable all by itself. But aphakia can be treated by implanting an intra-ocular lens (IOL). In rare cases, recipients of a crystalline IOL called the Crystalens report a newfound ability to see into the near UV. The Crystalens permits some near UV, above 340nm.
Engineer and former Air Force officer Alek Komar has a website detailing how his color vision changed following major cataract surgery. In Komar’s case, however, he didn’t just get his normal color vision back with the Crystalens implant. The lens allows some near UV light to hit Komar’s short-wavelength cones. As a result, he can now see wavelengths of light that are invisible to most humans. Komar did A/B testing with a black light and a UV flashlight. It seemed that he could see the UV as a purple glow.
Still skeptical, Komar secured the help of another engineer, this one from HP. To test Komar’s vision, they used a Monochromator, a device capable of projecting light in 10nm wavelength increments. The results confirmed his perception. Komar can see near-UV light, down to 340-350nm.
Subsequent reports indicate that he’s not the only Crystalens patient to see ultraviolet wavelengths following the procedure. On his site he details anecdotal reports from people with a Crystalens IOL in only one eye, who describe a startling difference in what their left and right eyes see.
Birds and the Bees
While it’s uncommon in humans, UV tetrachromacy is widespread elsewhere in the animal kingdom. It goes way beyond mantis shrimp. Numerous species of bird have a fourth cone that allows them to see well into the ultraviolet. As with the cryptochrome that enables them to see the Earth’s magnetic field, UV light may help birds navigate.
Bees also use ultraviolet cues to navigate. Under UV light, some flowers look very different from what we see in the visible spectrum. Bees use these spectral differences to choose flowers — and tell them apart. For instance, check out this picture of a flower from Alek Komar’s backyard:
Taken in the visible spectrum, this simple snap shows a sunny yellow-and-orange flower as most humans see it. But the same flower looks very different when it’s photographed in the UV band.
Komar plans to continue his UV experimentation. He’s working on a test of spatial resolution, which would require an eye chart only visible in UV. And for her part, Antico is teaching less and painting more these days.”My gift allows me to see the true colors of the beauty that surrounds me, my life long passion and dedication to art allows me to paint it,” she says. “Painting provides a medium through which I can show those colors to others too.”
Personally, I’m holding out for a gene therapy that lets me hot-swap my vision with a mantis shrimp’s. But what kind of eye chart could we possibly use to test for that?
Feature image by Thorsten Hartmann, CC BY 2.0. Image was cropped from original format.