Color me Bad

I once asked a colleague of mine whether he was color blind, and he replied with great agitation that he was not color blind, but color deficient!  Defensive, are we?  Color perception is one of the great wonders of the human eye. Our retinas house a high-tech built-in imaging system that consists of 90 million rod and 4.5 million cone sensors.  Rods are in charge of night vision, peripheral vision, and large movements.  The cones, being more refined, take care of daylight vision, fine details, central vision, and color.

As you may remember from your physics days of yore, color is a wavelength of light that is reflected by an object.  And, like old technicolor televisions, we have three cone types capable of perceiving three color wavelengths: green, blue, and red.  This makes us trichromats.  Allegedly, there are individuals who have a fourth cone type, and they are called tetrachromats. (Although a tetrachromatic retina is exceedingly rare amongst humans, people immediately think they are tetras when they first read about it.)  Because the human lens filters out most ultraviolet light, we do not perceive it, but there have been reports of aphakic patients (patients without a lens) who see into the ultraviolet spectrum (!)

Many different forms of hereditary color deficiency and color blindness exist, and they usually have to do with either complete absence of one (or more) type of cone, or a reduced sensitivity of one cone type.  The most common type of color deficiency (red-green) is due to a reduced sensitivity of the green cones in affected individuals.  Because most of the genes that code for vision are on the X chromosome, males are much more likely to suffer from color deficiency and females are much more likely to be tetrachromats.  Seven to ten percent of males are color deficient (which explains why some dress the way they do).  Other reasons for deficient color perception are acquired causes related to optic nerve and retinal health.

But, it's not all bad news for color deficient folks.  Studies conducted by the military have shown an evolutionary advantage to color deficiency: the ability to see through camouflage.  Was GI Joe color deficient?  Maybe so.