Light is not made of different colors—we perceive color by detecting light of different wavelengths. Short wavelength light is perceived as blue, medium wavelengths as green and long wavelengths as red. By mixing these three primary wavelengths in different combinations we are able to match any color. This is referred to as metameric matching. The trichromatic theory of color vision states that we have three different types of cone, each of which is maximally sensitive to one of the three primary wavelengths. Any wavelength that we can see is coded by the relative activity of the three different cone types, which are commonly referred to as blue (short wavelength), green (medium wavelength), and red (long wavelength) cones (Figure 6.5).

Color Vision Deficiency

As we already mentioned, most people who are “color-blind” can see colors, but they confuse certain ones because they look similar to them. The psychological term for this condition is color vision deficiency. The circles in Figure 6.6 are part of a common test used to see whether people are able to tell red from green.

Despite much recent improvement in gene therapy for color blindness, there is currently no FDA approved treatment for any form of CVD (color vision deficiency) , and otherwise no cure for CVD currently exists. Management of the condition by using lenses to alleviate symptoms or smartphone apps to aid with daily tasks is possible.

The most common color vision deficiency is confusion between reds and greens due to the absence or malfunction of the green or red cone types. Deuteranopia is the term used when the green cones are not working at all (or are missing), whereas deuteranomaly is a milder form of green cone malfunction. Similarly, protanopia is the more severe form of red cone malfunction and protanomaly is the milder version. Deuteranomaly is the most common type of color vision deficiency. Red-green color deficiency affects males more than females (Birch, 2012). Among males, it affects approximately 8% of European Whites, 5% of Asians, 4% of Africans, and less than 2% of indigenous Americans, Australians, and Polynesians (Birch, 2012). Only about 0.4% in females of European Caucasian descent have red-green color deficiency (Birch, 2012). Males are affected more than females because this is a sex-linked recessive genetic trait. Males inherit this gene from their mothers via the X chromosome. The Y chromosome in males is small and so lacks many of the genes that are present on the X chromosome (like the ones associated with color vision). So, if the recessive gene is present on the X chromosome in a male, it will be expressed—giving rise to red-green color deficiency. Typically, females are not affected because they are likely to also have a dominant normal color vision gene on their other X chromosome.

Other, much rarer, forms of color vision deficiency include tritanopia and tritanomaly, which affect the blue cones. Figure 6.7 shows how individuals who are missing their red, green, or blue cones perceive red, yellow, green and blue.