Normal Vision VS Red/Green Colourblind

But what if I told you that some animals–known as tetrachromats–can see four colors? Goldfish have long been shown to be tetrachromatic, and some birds, such as the zebra finch, are as well. In fact, there are even some humans who can perceive four colors! So, how does this occur? What’s the fourth color they’re seeing? Wait, what do we even mean when we talk about particular “colors”

To explain this, we must first understand the mechanisms of the perception of color in vertebrates. When light reaches the eye, it is focused onto the retina at the back of the eyeball, which contains two types of cells that aid in vision: rod-shaped “rods” and… well… cone-shaped “cones.” The former are more sensitive and are what allow us to see in dim environments, and the latter plays a major role in the perception of color. 

In “regular” trichromat humans, there are three types of cone cells, each containing a different light-sensitive protein, also known as an opsin. Due to the different opsins, the three cones are sensitive to different wavelengths of visible light: S or short wavelength, M or medium wavelength, and L or long wavelength. This is what is usually referred to when people talk about there being three perceivable “colors”–three initial detectable wavelengths of visible light that the brain then processes to form a spectrum of visible colors. 

The genes of two of the opsin pigments are located on the X chromosome, and mutations in them may result in one opsin’s active wavelength to be shifted so that it overlaps one of the others’. This is why colorblindness is more common in males; they have only one X chromosome, and if that one happens to contain the “colorblind” version of the gene, the individual will only have two distinct cones and only perceive two “colors.” For example, the M opsins are anomalous in the type of red-green colorblindness known as deuteranopia (yes, types of colorblindness have more subtypes). 

Theoretically, human tetrachromacy can occur when the opposite of this happens. Females have two X chromosomes, so it is possible for mutations to shift the opsins’ absorbing wavelengths about such that there are four distinct cones. However, even though studies show that this phenomenon is almost common–one suggests that 15 out of 100 women might have the fourth cone–“true” tetrachromacy, disappointingly, seems extremely rare; all these four-coned individuals, except for one, can’t actually see that fourth color. One theory that may explain this is the “opponent process”, which hypothesizes that the human visual system interprets color in an “antagonistic” manner by comparing them. This theory suggests that there are three “opponent channels,” each comprising an opposing color pair. Therefore, if an individual possesses a fourth cone but no fourth color channel, they will not be able to utilize the information from the fourth cone and are hence not a true tetrachromat. 
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Those of us who are considered to have “normal” vision might think the world seen by colorblind people seems dull and lackluster, but tetrachromats might think the same about trichromat vision!