In the last column, we looked at the way in which light perception occurs and how this process can go awry. But what about color?
Though related to light sensation, color sensation relies on a slightly more specific subset of our optical machinery. After light enters the eye, it interacts with two different classes of photoreceptors located in the retina, called rods and cones, that then relay this optic signal into the brain. While rods are sensitive to many wavelengths of the visible light spectrum and allow for better vision at night, they are not responsible for any sensation of color. Cones, on the other hand, respond to a narrower range of the spectrum that is associated with three hues: red, green, and blue. You can read more about rods, cones and vision generally here.
In general, humans have three different types of cone cells, called S, M, and L cells after the relative length (short, medium, and long) of the waves they most strongly detect. S cells detect mostly blue light, M cells mostly green light, and L cells mostly red light. Since three of these cells are responsible for sensing a particular color, it is said that our vision is tri-chromatic. But not all animals see color in this way. Other so-called trichromats include humans and insects, but dogs, for example, are dichromats: they have only two different kinds of color detecting cone cells. Most marine mammals, on the other hand, only possess one, whereas birds and some insects have four.
…the cells in our retinas filter wavelengths of light from the world around us to paint our realities, infinitely various and vibrant.
So what is the effect of having more than one kind of cone cell? Though it may seem somewhat counterintuitive, much of our knowledge about this topic comes from studying colorblindness. Since cone cells, like all cells in our bodies, are vulnerable to disease and subject to genetic influence, some people have difficulty differentiating certain hues. After examining the genetic makeup of colorblind individuals, it was found that while some people have mutations that affect the function of some classes of cone cells, others lack the pigments they use altogether (1). These people, who in fact have monochromatic vision, are capable of differentiating around a hundred colors, much as one would be able to discern on a black and white television set. But because of the ways in which hues are detected, each additional cone cell class multiplies the number of colors a person can distinguish by a factor of one hundred, which is how we arrive at the estimate that the average person sees somewhere around a million colors. In the same way that a computer makes images using combinations of red, green, and blue light, the cells in our retinas filter wavelengths of light from the world around us to paint our realities, infinitely various and vibrant.
But for those whose eyes cannot properly make sense of these signals, these worlds are a bit less colorful. For milder cases of colorblindness — usually where a patient has difficulty differentiating certain hues, as opposed to being entirely mono- or dichromatic — treatments do exist. Some companies make corrective eyewear for the condition, but since eyewear does not alter the structure of the eye it can only help those that have all of the necessary pigmentation and cell classes (2). Enter gene therapy. This past summer, clinical trials (3) were conducted for a drug called CNTF (ciliary neurotrophic factor), which was found to restore cone function in dogs (4). Though the trial was unsuccessful and we will not be seeing CNTF prescribed to treat colorblindness, it does represent a significant shift in scientific thought about how to search for a cure.
Matthew Aguirre is a staff writer on Brevia. He can be reached at email@example.com
This piece is part of a series of short, biweekly(ish) columns on topics of general interest, including common-sense explanations of complex scientific phenomena. Stay tuned for more!
Sharpe, Lindsay T., et al. “Opsin genes, cone photopigments, color vision, and color blindness.” Color vision: From genes to perception (1999): 3-51.
Pogue, David. “Glasses That Solve Colorblindness, for a Big Price Tag.” The New York Times. 15 Aug. 2013. Web. Accessed 25 Nov. 2014.
Zein, Wadih M., et al. “CNGB3-achromatopsia clinical trial with CNTF: diminished rod pathway responses with no evidence of improvement in cone function.” Investigative ophthalmology & visual science 55.10 (2014): 6301-6308.
Komáromy, András M., et al. “Transient photoreceptor deconstruction by CNTF enhances rAAV-mediated cone functional rescue in late stage CNGB3-achromatopsia.” Molecular Therapy 21.6 (2013): 1131-1141.