Tetrachromats

By John Shearer March 2012 Shearer Painting

Women have a reputation for having a great eye for design and color. While this is not true of women everywhere, we can see that many interiors designers, artists, etc., are, in fact, women. How do women instinctually know what colors go well together? There is a scientific explanation for this called tetrachromacy. Tetrachromacy is “the condition of possessing four independent channels for conveying color information, or possessing four different types of cone cells in the eye.” People with tetrachromacy are known as tetrachromats – they’re extremely rare and have a super-human power. The first tetrachromat woman was discovered by researchers at Cambridge University in 1993. This is perhaps the most remarkable human mutation ever detected.

8% of the US male population is color blind – 95% of them with red or green receptor problems. : ”Color Vision Deficiency. Color vision deficiency is the inability to distinguish certain shades of color. The term “color blindness” is also used to describe this visual condition, but very few people are completely color blind.” – American Optometric

Being a tetrachromats is like being a super taster of color. Organisms that are tetrachromats see the sensory color space as four-dimensional; meaning, in order to match the sensory effect or arbitrarily chosen spectra of light within their visible spectrum, it requires the mixture of four (at minimum) different primary colors. So these people would see four clear ranges of color, instead of the three ranges that most of us possess.

A good example of this is, when looking at a rainbow, a woman with tetrachromacy can separate it into approximately 10 different colors; a trichromat (with three iodopsins) can only see seven (red, orange, yellow, green, blue, indigo and violet). For a tetrachromats woman, it was found that green was assigned as jade, emerald, olive, verdant, bottle, and 34 other shades.

The majority of birds are also tetrachromats; thus, we can surmise that several species of fish, reptiles, amphibians, insects, and arachnids are tetrachromats, too.

To understand tetrachromacy in layman’s terms, you must know that an organism’s retina has four types of higher-intensity light receptors; these are called cone cells in vertebrates, as opposed to rod cells, which have lower intensity light receptors, and have different absorption spectra. This means that the animal may be able to see wavelengths that a typical human being cannot; they also could distinguish colors that to a normal person may appear to be identical. There is obviously a small physiological advantage over competing species.

Certain animals, as previously stated, are tetrachromats as well. The zebrafish (Danio rerio) is a tetrachromats. They have cone cells that are sensitive for green, red, blue, and ultraviolet light. Species of birds, like the Columbidae and Zebra Finch use the ultraviolet wavelength (which is between 300 and 400 nm)that comes with tetrachromatic color vision as a ploy to lure mates and in foraging. When choosing a mate, ultraviolet plumage and skin coloration mean a higher level of selection.

Colors found in flowers are divided into two main wavelengths of light: 360 – 520 nm and 400 – 500 nm. Flowers can reflect four large domains of wavelength, including 300 – 400 nm, 400 – 500 nm, 500 – 600 nm, and 600 – 700 nm. These wavelengths have the colors ultraviolet (UV), blue, green, and red respectively within the color spectrum. Flowers will use said wavelengths to separate color patterns within a species. It has been found that these differences in color patterns are utilized for behavioral attractions in pollinator insects, which increases survival. Several trichromatic pollinators like honeybees use blue, ultraviolet, and green wavelengths. Increases in the wavelengths that flowers reflect happen when the color space within insects becomes more and more filled. Pollination is a mutualistic relationship between pollinators, like bees, and plants, like flowers; this leads to a very high competition level. This competition created a coevolution between foraging insects and plants, thus increasing the color variation in both orders, which then leads to directional selection.

Insects that are foragers can see all four color wavelengths. Plants can show an ever-increasing different amount of color variation, which extends into the ultraviolet color scale. Plants that have higher levels of color will attract higher levels of pollinators. A pollinator with a wider range of color can use tetrachromatic color vision to raise and keep a higher foraging success rate versus their trichromatic competitors. For tetrachromatic insects, background displays are important in terms of viewing flower color variation. Flowers that show pure color hues are more easily recognized by a pollinating insect. When a pollinating bug sees a flower, it can differentiate the flower from the background by noting the reflectance in the petals. This use of reflectance then draws the insect in closer towards the plant’s reproductive organs.

Humans and their primate relatives usually have three types of cone cells and thus are trichromats, or animals with three different cones. But at low light intensity, the rod cells could help color vision, which gives a small region of tetrachromacy in the color space.

For humans, two cone cell pigment genes are found on the sex X chromosome, the “classical type 2 opsin genes OPN1MW and OPN1MW2.” Women have two different X chromosomes in their cells; so, some women could carry some variant cone cell pigment. This makes being born as a full tetrachromat and retaining four different at-once functioning kinds of cone cells (each type with a specific pattern of responsiveness to different wavelengths of light in the range of the visible spectrum possible. There is a study that suggests that 2 – 3 % of the world’s females may have the type of fourth cone that is between the typical red and green cones, which theoretically means a significant increase in color differentiation. A similar study states that as many as 50% of women and 8% of men could have four photopigments.

In order to verify tetrachromacy in humans, we need to conduct more studies. We do know of two potential tetrachromats: “Mrs. M,” an English social worker, was found during a 1993 study, and an unidentified female physician close to Newcastle English, was found in 2006. Neither case is completely verified.

In cone pigment genes, variation is common in many if not most human populations, but the most common and obvious tetrachromacy comes from female carriers of major red-green pigment anomalies, usually classed as forms of “color blindness” (deuteranomaly or protanomaly). The biological reason for this is “X-inactivation of heterozygotic alleles for retinal pigment genes, which is the same mechanism that gives the majority of female new-world monkeys trichromatic vision.”

For humans, preliminary visual processing happens within the retina’s neurons. We don’t really know how these nerves respond to a new color channel, i.e., whether or not they could handle it separately or just put it in an already-existing channel. Visual information exits the eye through the optic nerve. It’s unknown whether the optic nerve has the extra capacity to handle a new color channel. Much of final image processing occurs in the brain. We don’t know how the different areas of the brain would respond if give a new color channel.

Typically, mice, who only have two cone pigments, can be “engineered to express a third cone pigment, and appear to demonstrate increased chromatic discrimination, arguing against some of these obstacles; however, the original publication’s claims about plasticity in the optic nerve have also been disputed.”

People who have four photopigments have been proven to have higher levels of chromatic discrimination compared to trichromats.

How do you tell if you’re a tetrachromats?

You’re more likely to be a tetrachromats if you meet the following criteria:

- You’re a woman

- You have a son, father, or other man in your family with red or green colorblindness.

But to truly verify whether or not you fit the definition of a tetrachromats, you need to take a genetic test. Also, Dr. Neitz, a well-known color vision researcher at the Medical College of Wisconsin, states that “only women have the potential for super color vision.” This is, again, because the genes for the pigments in green and red cones live on the X chromosome, and only women have two X chromosomes. Dr. Neitz estimates that 2 – 3 % of the world’s females have four types of color cones that lie right in between the typical red and green cones, which gives them a major range.

While scientists can do all the genetic testing they please, proving that a woman can see tens of millions of additional colors is, at this point, not possible – though through the utilization of further testing and technology, the future looks promising.

Read More Heavy Science on Tetrachromacy