Do You See What I See?

While driving to work the other day, I heard that men only see one shade of red; therefore, women shouldn't spend so much effort picking the "right" shade of lipstick. Now, I don't believe everything I hear on the radio any more than what I read on the Internet. But the tidbit did get me thinking.

I taught biology for many years. Every year when the genetics unit rolled around, we looked at sex-linked traits. These get their name from the genes being physically located on the X chromosome (one of the chromosomes involved in determining gender). Generally speaking, human males have one X and one Y chromosome...and females have two X chromosomes. Because men end up with only one X, any bad code on it will be expressed. Women have a "spare," so a mutated gene doesn't matter so much---the good copy will always pump out the right stuff. Color-blindness is a sex-linked trait, appearing far more often in men because the genes that support color perception are on the X chromosome. If you're male and get a bad copy from mom, you're doomed. This is the incredibly oversimplified way we teach about (a) sex determination (b) being able to see a full range of colors, and (c) sex-linked traits.

The reality, of course, is far more complex. I think it's worth exploring in a bit more depth because of the impact this has on how we design our data displays.

I should say at the outset here, that this is really about color perception, not vision. There are people who have perfectly formed retinas, capable of capturing the full range of wavelengths, but who are color blind because of dain bramage...er, brain damage. There are a few different parts of the brain involved with interpreting the wavelengths that activate the retina and assigning a color/name to them. (For a friendly, but in-depth analysis, I recommend Island of the Colorblind by Oliver Sacks.) But perception happens in the eye.

Remember this from your high school bio textbook?

from here

Light comes in the pupil, hits the back of the eye where the retina is, and those little red, green, and blue "cones" in the back (far right of above diagram). If the cones are stimulated, a nerve signal travels to the brain for further processing. Again, I'm oversimplifying...but the part to really pay attention to are those three cartoonish-looking cones in the diagram. Each one contains a protein that responds to different wavelengths of light...and each of those proteins is determined by a specific gene. And those genes? Somewhere on a chromosome.

Only the genes for the pigments used in red and green color perception are on the X chromosome. They sit right next to one another at the end of the long arm of chromosome.

You are here! (image from here)

Captain K gets a lecture re: Green Jeans (source)

You might remember from your high school biology days that chromosomes engage in something called "crossing over." This has nothing to do with death or getting to the other side, but a way to make new combinations of genes. Variety really is the spice of life, especially at the molecular level. During crossing over, matching parts of chromosomes swap genes. Sometimes, this leads to pieces getting lost, put in the wrong place, or inserted backward. This crossing-over-gone-wild (not the technical term) is the source of most red-green color perception problems. But over the generations, it has also resulted in an extraordinary variety of "red" genes---about 3 times the number of variations in most other human traits (source). Meanwhile, most of us have multiple copies of the green gene, all lined up in a row, but only the one next to the red gene gets used. So, even if there are lots of dittos for green sitting around, if the first one is messed up, your color perception is hosed (source).

So what?

Remember, men only get one copy of this chromosome...women get two. The variations mean that women are more likely to see a wider range of colors, especially in the red part of the spectrum. It may be that 60% of women actually have four different types of cones in their retinas, but research has not yet identified successful ways to test what this means in terms of what these women see. And fellas? It doesn't mean that they only see one shade of red, but the range of wavelengths perceived may differ from man to man. With ~85 different versions of the gene, there are a lot of possibilities.

When developing data displays, you will never really know if someone can "see" them the same way you do. There's no way to know which gene variant is present and which wavelengths will be activated. But perhaps there are some lessons to learn about using color. Here is a diagram of the wavelengths our cones respond to (the dotted line is a curve for the rhodopsin used in night vision...just ignore it):

If you only have genes for S&M, do you see 50 shades of grey? (image from here)

Notice that there is some overlap between the S (for short wavelength; blue), M (for medium wavelength; green), and L (for long wavelength; red) curves. You might not be able to guarantee that your viewer will see the same color as you, but you will probably stimulate some activity in the cones if you need to differentiate your data if you stick within a particular range. So ladies, go ahead and choose any color of lip rouge you like---there will be some guy out there who can see it just like you.

Does this mean that you should go to extremes when developing your design? No. Most people (~92% of men and 99% of women) have fully functional cones. They might not be activated at exactly the same wavelength as yours, but it's a pretty good bet that it will be close. But it is important to remember that if you have a critical point to make, you should choose a color that is highly visible to anyone who needs the information. (Oddly enough, medical illustrations and charts about color vision seem to rely on red/green. Hmmm.)

All of this reminds me of the Color Survey done by xkcd, where people were asked to assign names to colors...and the ensuing interpretations, like this one from Doghouse Diaries:

from here

Perhaps the variety of genes and combinations in women mean that they see more subtle variations in color. No way to prove that's the explanation for the difference in results, but another idea to ponder.

Bonus Round
What about ole blue? The pigment that responds to blue wavelengths resides on a regular chromosome (one not involved with sex determination). Because everyone inherits two copies of the gene, this variation of color blindness is not as common (i.e., everybody's got a spare). Blue-yellow colorblindness affects 1 in 10,000 people, its effects distributed equally between men and women.