The distribution of cone cells in the fovea of an individual with normal color vision (left), and a color blind (protanopic) retina, by Mark Fairchild.
Tetrachromacy means that you have four types of cone cells in the retina. Tetrachromats exist among birds, fish, amphibians, reptiles and insects, but in most mammals there are two kinds of cones, and in humans and some of our primate relatives, there are three kinds.
As you remember from high school, there are two types of photoreceptor cells in your retina. (Actually, they now say there are three, but the photosensitive ganglion cells aren’t there for vision). The rods work in low light; the cones work in normal light. The three kinds of cones respond, more or less, to short, medium, and long wavelengths.
The four pigments in a bird's cones extend the range into ultraviolet. That would do you no good, since your eyes are designed to block ultraviolet rays.
In humans, those three cone types give us the capacity to distinguish a million color variations. Some men suffer from having two kinds of normal cones and a third, mutant, cone that is less sensitive to green and red. We call that color blindness. It’s a sex-specific trait.
In 1948 a scientist named H.L .de Vries studied the daughters of color-blind men to see if they might be carrying the mutant cone type along with their three normal ones. He did notice that the daughters of one of his test subjects responded to reds and greens differently than most women. Since then, a lot of people have searched for the so-called ‘four-coned woman’.
Turns out a significant portion of woman have dud ‘fourth cones.’ In June 2012, after 20 years of studying them, neuroscientist Dr. Gabriele Jordan identified a woman who could detect a greater variety of colors than trichromats could, meaning that she is a ‘true tetrochromat.’
|To be a true tetrochromat, you'd have to start by being a carrier female in a family with genetic colorblindness.|
One woman thus far. Yes, there probably are several more, but I doubt that includes you. For one thing, you’d need color-blindness to run in your family.
Visual information has to be collected and processed with retinal neurons and the resulting information sent via the optic nerve to the brain. It is processed and refined all along the way. What would the existing neural structure of the brain do with the information it got from a fourth set of cones if the infrastructure to interpret it wasn’t in place?
The cones in your eyes are part of a complex system for distinguishing color. From Anatomy & Physiology, Connexions Web site.
Human vision has plasticity that we don’t even begin to explore. When I first see painting students, they are puzzled about the color temperatures of white and grey. A year later, they’re manipulating color temperature like old pros. That isn’t because they sprouted new hardware; it’s because they’ve started to use the hardware with which they were born.
Want to see more color? Take up painting.