Science Sunday: Color blindness
What exactly does it mean to be color blind? People have varying misconceptions about this–the most common forms of color blindness doesn’t mean that you can’t perceive any color, or that you see the entire world in grayscale. What it really means that their total color space is skewed.
That is, the color blind are incapable of seeing the entire rainbow as everybody else sees the rainbow. I’ve given a couple examples of what this set of color pencils might look like if you were color blind. What’s the biology behind this?
Well, we have three types of cells in our eyes that respond to different wavelengths of the light spectrum. The most common types of colorblindness involve a defect or absence of one or more of these cones. Since they still have other cones intact, they can perceive some colors–just not all of them.
In this figure, you can see the response curves for each type of cone cell, which have been dubbed S (short), M (medium), and L (long) cells, for the wavelengths that they prefer. These cone cells, like many other sensory cells, respond maximally when they are hit with the wavelength that they prefer. That is, the response of an S cell peaks somewhere in the blue part of the color spectrum, and the response of an L cell peaks somewhere in the red part of the color spectrum.
Why are there only three cones that, roughly, perceived just blue, green, and red? Well, that’s because you can create the entire spectrum of visible colors from just these few. For example, purple is created from mixing blue and red together. Someone with a defect in their S or blue cone cell would also have trouble seeing purples because they involve seeing blue, as well.
There are a few different types of defects that might lead to altered color vision, or defects in color vision. For a person who has trouble seeing blue, it might be the case that they have fewer blue cones, or that the response curves of your blue cone cells is shifted to produce a great overlap in response between the blue and green cones. It might also be the case that they have no blue cones at all. However, their red and green cones are still intact and normally responsive.
You might be able to see now that “color blind” seems like a misnomer. Most people with color vision defects are simply “color challenged.”
So are there color blind people who see the world in grayscale? Certainly–they are just much more rare. This condition is known as achromatopsia, and is more accurately described as “total color blindness.” They may not have any cones at all. However, this is rarer than the other types of colorblindness and is generally not what we mean when we say “color blind.”
Did I miss anything? Do you have follow-up questions? I have plenty of vision scientists to consult if you come up with a real stumper!
Color pencils, Colour Blind Awareness
Simplified cone response curves, based on Stockman et al.
Ishihara plate, featured image, source unknown