You don’t look like you feel well. How often has this statement been a precursor to some medical intervention? What exactly are people seeing that triggers this question? How physicians, especially, discern differential skin pigmentation in patients casts an intriguing glimpse into the physical development of color perception in the human eye as well as possible avenues for advanced medical training.
Over millions of years, we had become a trichromatic species, meaning we now possess 3 separate photoreceptors detecting blue (Short), green (Medium), and red (Long) wavelengths of visible light (between 400-700nm). We were a nocturnal species and had large pupils to aid in scotopic vision but had poor vision in the daytime. Fast forward 90 million years, and we had evolved into a diurnal species. Now we had a new environment, different predators of which to be aware, and a new perception of the world around us. The evolution of the human eye improved how we interpret the environment, interactions with other humans, and potential predators around us.
Aside from environmental changes, there are a few theories as to “why” this visual and evolutionary process happened. One theory is the “Opponent Process Theory” first proposed in 1892, by German Psychologist Ewald Hering. This theory explains that our visual system interprets colors in an antagonistic manner, for example, green vs. red, yellow vs. blue, or black vs. white. Activation of one member of the pair inhibits activity in the other. There are other theories that include how our photoreceptors developed for the purposes of foraging to find berries and edible leaves.
However, another theory that piqued my interest was proposed by Mark Changizi in 2009 with his book “The Vision Revolution.” Looking at the photo below, list the colors you find in this photo.
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| Photo credit Crystal Anthony, ABOC, of CrysOptics Photography. |
You most likely saw colors on clothing such as green, red, blue, but probably didn’t notice the skin color. It’s difficult to say white, porcelain, or even peach would be accurate. According to Changizi’s evolutionary color theory, our skin, or the skin of our community is a ‘baseline’ (along the lines of homeostasis). He proposes “Ask yourself, how many other natural objects can dynamically achieve every hue, and also appear to have no color?” While we all have lighter or darker skin relative to others, once we develop a baseline for our community, we will notice subtle changes in the skin; he calls this “color signaling” which is an involuntary gesture. It will be evident on bare spots, such as our cheeks, one of a few areas of skin that will generally remain hairless and visible to others.
As blood changes in concentration and oxygenation, skin can spectrally change in color in the same ways. This is useful for recognizing when someone is sick or in distress. If there is an excess concentration of blood, our skin will turn bluer or darker, while a low concentration can appear yellower and lighter. Deoxygenated skin appears greener, and highly oxygenated skin can appear red, as when we are angry or blushing. Should the physician be color deficient, this can be quite challenging to properly diagnose. Changizi quotes an optometrist who had a Red/Green color deficiency. He states “As a child I could never understand what people meant when they said they were blushing… His embarrassment when a patient complained of a red eye, but the offending side was not specified… His most severe problem was in differentiating between blood and pigment in the retina.” All of these color signals are important to interpret, especially if you’re a physician. When a patient walks into a clinic for treatment there are visual assessments that are done even prior to physical ones that are important to note while treating.
Though we as opticians do not treat or diagnose patients, it's important to understand these color deficiencies and how it can affect a person's lifestyle. We can educate ourselves on knowing the products we can offer to improve their quality of life. I highly suggest reading Lori Passarelli’s continuing education article “Achromatopsia,” as it gives an in-depth explanation of color deficiencies, and how we as eye care professionals can properly fit these individuals and improve their quality of life.
