Understanding Color Blindness

An Overview

If you wear clothes that are not color matched, people often tease you as color blind.  It may be a matter of taste but there are people who are indeed medically color blind. They can see colors but they have trouble distinguishing between certain colors like green and red.  But that is often not a hindrance to normal living as they learn to adapt to their handicap.  For instance, they may not see a green light at the intersection but they learn that it is the light at the bottom and respond correctly.

Color blindness, medically known as color vision deficiency, is considered a mild disability. But some studies show that color blind people can see through color camouflages many people with normal vision could not, appearing more like an evolutionary compensation for the disability. The condition was first recognized in when a scientific paper by English chemist John Dalton first appeared in 1798.  It was for this reason that the condition was initially referred to as Daltonism which now refers to a deuteranopia, a special kind of color blindness.  

How Color is Perceived

The normal human retina has two types of photoreceptors or light cells: the low light rod cells and the normal daylight cone cells. The cone cells have three types each with a different spectral sensitivity in absorbing light.  The S cone is sensitive to short wavelengths that peak in the blue spectrum, the M cone ito medium wavelengths that peak in the green spectrum, and the L cone to long wavelengths that peak in the yellow spectrum. The red color stimulates the L cones more than the rest.  These are the color cell receptors that get stimulated as different color wavelengths reach the retina and the brain interprets them accordingly. 

Causes of Color Blindness

A retinal aberration that adversely alters the way these three cones act to absorb color information results in color deficiency syndrome.  And it can be either acquired or inherited.

Most color blindness cases are inherited. It has been shown that mutations on the X chromosome have causal link to red-green photoreceptor dysfunction or color blindness. There are at least 19 different chromosomes and 56 genes that could lead to the condition, according to the Online Mendelian Inheritance in Man database of the John Hopkins University. And because the genes associated with color sensitivity is X-linked, it is more prevalent in males than in females.

Color blindness can also be acquired.  They can be caused by damage to the retina in childhood often resulting from inordinate exposure to ultraviolet light. This is considered the leading cause of non-hereditary or acquired color blindness which often presents itself during the adult years.  Other causative factors include retinal damage caused by accidental trauma that causes inflammation of the occipital lobe of the brain.  These are the visual area V4 area of the visual cortex and the parvocellular pathway in the lateral geniculate nucleus of the thalamus. 

Certain ailments or degenerative conditions can also trigger color blindness such as macular degeneration associated with age and retinal damage due to diabetes.

Types

The various types of inherited color blindness result from total or partial loss of color cones or photoreceptors in the retina.  It only takes one cone system to malfunction to result in dichromacy where the remaining two color cones create an inadequacy in the perceiving the full visual color spectrum, and you become color blind. (Normal people are trichomats who can see three primary colors.) The most common type is the loss of either the middle or long wavelength cones that makes it difficult to distinguish among reds, greens and yellows. The condition is simplified to refer to red-green color blindness. 

• Protanopia is a dichromacy where the L cones are missing making it difficult to distinguish colors in the red and yellow spectra.
• Deuteranopia lacks the M cones which also disables discernment in the yellow to red color spectrum.
• Tritanopia is the loss of short wavelength color S cone, making it difficult to distinguish between blue and yellow. This is the rarest of all types afflicting about 1% of the male population.

There are other types that are less common such as monochromacy where you have only one channel for taking color information resulting in total color blindness so you see the world in grayscale. Monochromats suffer complete inability to perceive colors and come in two forms:  Rod monochromacy or achromatopsia, where the retina contains no cone cells and Cone monochromacy where only one cone cell exists such as in Blue cone monochromacy marked by the absence of L and M cones.