Color Perception, Photoreceptors and the Different Types of Colorblindness

Contrary to what the name suggests, colorblindness isn’t an absolute blindness to color. The perception of color is a complex process involving the eyes and the brain, and even two people who consider themselves “color vision normal” may disagree on certain hues – is that sunset more pink or purple? In a person with a diagnosed color vision deficiency (a more accurate term), they usually will experience what is considered a “shortened spectrum”, meaning they are able to distinguish fewer colors compared to a color-normal person and may confuse a few different shades as the same color. In Caucasian populations, the overall prevalence of colorblindness is typically quoted as 8% in males and 0.5% in females. A study conducted in Californian preschool boys found the lowest rate of colorblindness in African American at 1.4% and the second lowest in Hispanic boys at 2.6%.

Color Perception
The colors in the world are determined by the reflectance and absorption of the different wavelengths of the visible light spectrum (think of the colors of the rainbow). These wavelengths are picked up by the light-sensitive cells of the retina, called photoreceptors. The cone photoreceptors, six to seven million of them crowded in the part of the retina called the macula, are organized into three types depending on which wavelength they respond to – short (blue), medium (green), and long (red) wavelength cones. The intensity of the signal received by each cone photoreceptor is combined and compared with the others to determine colors. Rod photoreceptors are arranged in the peripheral retina and pick up light but do not distinguish color.

Types of Colorblindness
The most common category of color vision deficiency is known as red-green blindness, which can either be a type of dichromacy (that is, only two types of working cones instead of the usual three) or an anomalous trichomacy (there are still three functional types of cones but one is just a little off). Under the category of red-green colorblindness, there are four types:

  • Protanopia: the protanope entirely lacks long wavelength cones, which means their ability to detect colors at the red end of the spectrum is inhibited. Shades of red, orange, yellow all appear as black, browns or yellows. The violet section of the rainbow, which contains some degree of red, may be confused with blues.
  • Protanomaly: the protanomal still has long wavelength cones but containing and abnormal pigment. Because of the weak input from red cones is unable to compete with input from the other types of cones, the red end of the spectrum appears greener, and the brightness of such colors more subdued.
  • Deuteranopia: deuteranopes lack the green (mid) wavelength cones. The long wavelength colors become brown and yellow while green shades are perceived as a cream or beige (is that poor seasick man looking a little green or beige around the gills?). Blues are still blue.
  • Deuteranomaly: as you’ve correctly guessed, deuteranomals have abnormal green pigment in their middle wavelength cones. The resultant color spectrum means middle wavelength colors such as yellow to green are perceived as closer to red, and purples and blues may be mixed up.

Red-green color vision deficiencies are typically inherited in a recessive X-linked pattern, meaning a mother who carries an X chromosome with the defective color vision gene has a 50% chance of passing it on to her sons and daughters. If she passes this X chromosome to her son then he will be red-green colorblind. A father passes on either a Y chromosome to his son or another X chromosome to make a daughter. If the father is colorblind it means he has the X chromosome containing the colorblindness gene and so his daughter will at least be a carrier of the gene, or (if unlucky) will be colorblind herself if she also receives a “colorblind” X chromosome from her mother. Because the gene is recessive, women need two colorblind X chromosomes to display the trait while men only need one (and as it stands, men only get one anyway). This explains the significantly higher prevalence of color vision deficiencies in males compared to females.

The third dichromatic colorblindness category is known as tritanopia and is extremely rare. Tritanopes are missing short (blue) wavelength cones and commonly mix orange and red hues, greens and blues, and blues with greys. Because tritanopia is inherited in an autosomal recessive manner rather than being associated with a sex chromosome, males and females are equally affected.
While most cases of color vision deficiency are congenital, meaning present at birth, certain eye diseases can cause an acquired color vision deficiency. Cataracts are a common condition in all aging humans and because of the clouding of the lens inside the eye, may act as a filter to certain wavelengths, subsequently altering color perception. This is generally easily addressed by undergoing cataract surgery.

Treatment
Currently there is no “cure” for color vision deficiency. Gene replacement therapy research is underway and has found good success in animal models of dichromacy but is not yet ready for humans (since the technique involves injecting the cone photopigment gene straight into the eye, perhaps it is more pertinent to ask – are humans ready for this therapy? Yikes!). In the meantime, there are optical devices available on the market that profess to improve color vision discernment in red-green dichromats but these have been found to provide very limited success.

While we wait for a safe, commercially available cure that hopefully doesn’t involve a needle into the eye, colorblind people must find a way to navigate a colorful world. This may involve remembering that the green go light is at the bottom of the row and the red stop light is at the top, or asking a trusted friend or relative to check that your top isn’t horribly clashing with your shorts. Children at school may need to use written labels rather than color-coding for various objects in the classroom and various professions may need to find a work-around to determine if that wound is red and inflamed or that chicken fillet is too pink. Oh, and by the way, that sunset was definitely purple.

 

 

References
Colour Blindness. http://www.colourblindawareness.org/colour-blindness/
Color Blindness. https://www.allaboutvision.com/conditions/colordeficiency.htm
Facts about color blindness. https://nei.nih.gov/health/color_blindness/facts_about
Caucasian boys show highest prevalence of color blindness among preschoolers. https://www.sciencedaily.com/releases/2014/04/140403084243.htm
Can specialized glasses correct color blindness? https://nuscimag.com/can-specialized-glasses-correct-color-blindness-c7e139ee73a7

 

 


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