Not Just Black & White
This article appeared in the Pontiac Daily Leader and Fairbury Blade in October 2014. By Sasha L Radford, OD
What does it mean to be colorblind? Do some people really see the world in black and white? Before I even begin to answer those questions, I must first answer this: What is color?
Color is a brain construct, a perception of different wavelengths of light which reflect from the surfaces of everything around us. We call this range the visible spectrum. What we perceive as color is this spectrum of reflecting light which our brains interpret as red, orange, yellow, green, blue, indigo, and violet. However, not everyone perceives color in this way. Discrimination of colors can differ for a variety of reasons.
There are two types of cells in the retina that gather light: photoreceptors known as rods and cones. Rods are responsible for night vision, peripheral vision, and detection of movement; cones are responsible for sharp central vision and color perception.
In humans, there are typically three types of cones which absorb certain wavelengths of light and our brain then interprets them as different colors. To simplify, we can say that there are red, green, and blue cones, although each cone actually absorbs a range of wavelengths. The combined input of these three types of cones is what allows us to perceive the entire visible spectrum.
When people talk about “color blindness” they are usually referring to the most common type, affecting the perception of reds and greens. In these cases, there is an absence of either red or green cones making it difficult to distinguish these colors. There are varying degrees of deficiency; some may still be able to identify reds and greens, just not as well as someone with all three cones functioning.
Red-green color deficiency occurs in about 8% of males and rarely in females, yet it is mothers who pass the color-deficient gene to their sons rather than fathers. This is because the gene for this condition is located on the X chromosome and, since women have two X chromosomes, a defective gene on one X will be dominated by the normal gene on the other X. However, a woman may still pass the defective X to her son, which will make him color deficient because males have only one X chromosome.
Another form of color deficiency results from faulty blue cones and affects the ability to see blues and yellows; instead, the world is seen in shades of red, gray, and turquoise. Only about 6% of males and 0.3% of females have this form.
True color blindness, in which the world is seen only in white, grays, and black, is quite rare. This occurs when there is a complete absence of cones – only rods are present to detect light – or there is only one type of cone in addition to rods. People with true color blindness also have very poor central vision since cones normally reside in the macula, which is responsible for sharp central vision.
There are also acquired forms of color deficiency, usually as a result of ocular disease. The most common is nuclear sclerosis, a type of cataract in which the lens inside the eye becomes cloudy and yellowish, which impairs the ability to see shades of blue. It occurs so gradually that most people don’t even notice until after the cataracts are removed. I’ve had more than a few patients remark how vibrant and brilliant colors appear after their cataract surgery. Other causes of acquired color deficiency include damage to the optic nerve or retina as well as certain medications.
On the other side of the spectrum, recent research in color perception has uncovered that some women may have four different types of cones, so probably see many more shades of color than those of us with only three. This may explain the phenomenon when a woman names a certain color “coral” while her husband insists it is orange!
It’s difficult to imagine colors outside our personal visible spectrum, but it’s possible to simulate what things look like for those with different forms of color deficiency. The website vischeck.com does just that. Several examples are given, and you can also upload your own photo to see through the eyes of someone with only two functioning cones. You’ll find that “color blindness” isn’t really what the name implies; some people just see colors differently.