2.11 What is Color Blindness? – Causes & Overview
Color Blindness: Definition
Did you know that 8% of males and 1% of females are unable to detect or discriminate between one or more colors? This condition is known as color blindness, and it comes in a few different forms. Sometimes, it affects an individual’s ability to see certain colors, like red or green. Other times, it makes it difficult for individuals to discern between colors at all. But what causes color blindness in the first place?
How Do We See Colors?
As human beings, we have the incredible capacity to detect electromagnetic energy (light) and transform it into meaningful visual images. This formidable task, which cannot yet be replicated by any computer in the world, depends on the correct functioning of our eyes as well as several brain structures devoted to visual information processing.
In our eyes, we have specialized cells called photoreceptors that function as light-detectors. Of these, rods are the most abundant and most sensitive to light, whereas cones are specialized for detecting specific wavelengths of light (which we perceive as colors), but can only function at relatively high illumination levels. If we had no cones, we would perceive the world to be made of different shades of gray (Figure 1). In fact, if you look at a color image in a very weakly illuminated room, the image will look grayscale to you! This is because only the rods would be active at such low illumination levels. (Try it out and see!)
Once the rods and cones detect light, they send the information to the brain, where it is distributed to specialized areas. One of these areas, called V4, specializes in the processing of colors and shapes, while a different area, called MT, is devoted to the processing of motion information. In case you are wondering, there are dozens of these visual brain areas.
Thanks to rods, cones and all of the visual areas of the brain, most of us are able to detect a wide range of colors and discriminate even the smallest color nuances (just think of the large number of paint colors you can find at a paint store). However, a small, but significant, percentage of the population has deficiencies in one or more of these elements, resulting in color blindness. The rest of this lesson is devoted to describing several forms of color blindness and their causes.
Different Forms of Color Blindness
The most common forms of color blindness are due to alterations in the light-detecting molecules present in cones (photopigments) or the cones themselves. Most people are trichromats, which means they have three normal photopigments. However, people suffering from these forms of color blindness are either dichromats (sensitive to only two primary colors) or monochromats (unable to discriminate colors). There are several types of dichromacy, the most common of which is deuteranomaly, which is the inability to discriminate between reds and greens. Deuteranopes actually see both of these colors as a yellowish brown color (see Figure 2). Deuteranomaly is also commonly called: red-green color blindness.
Causes of Color Blindness
Color blindness can be caused, as we just mentioned, by deficiencies in the photoreceptors in the eye or by alterations in the visual pathways or color-processing cortical areas.
Congenital forms of color blindness usually involve mutations in the genes that encode the photopigments. The most common form, red-green color blindness, is an X-linked disorder. The red and green photopigments are located in the X chromosome, while the blue photopigment is located in a somatic chromosome. As a result, men only have one copy of the red and green pigments and two of the blue one. Thus, mutations in one of the red or green pigments will invariably result in red-green color blindness in men, while women, who have two copies of the X chromosomes, will continue to express all three pigments and see colors normally. For women to suffer red-green color blindness, they will need to carry mutations in both X chromosomes. Thus, deuteranomaly is much more prevalent in men than women (8% vs. 1%).
Acquired forms of color blindness usually result from illness or injury to the brain caused by an accident, tumor or stroke. Color input from the eyes into the brain converges onto the ventral occipito-temporal cortex in humans. Lesions to this area result in total color blindness, which is also commonly referred to as color agnosia or cerebral achromatopsia. People can lose color vision throughout their whole visual field (if both hemispheres have been damaged) or only to a quarter of the visual field (if only one hemisphere was damaged). This condition is permanent.
After you’ve completed this lesson, you should have the ability to:
- Define color blindness
- Describe the mechanisms through which the body sees colors
- Identify the different forms of color blindness
- Explain what causes color blindness, both congenital forms and acquired forms