The evolution of color vision in tetrachromats

Many of us have a sense of color-mixing among trichromats. There’s this classic image of the primary and secondary colors of light achieved by overlapping monochromatic circles of primary colors.

We overlap the red, green, and blue in a triangle to produce magenta (red + blue), cyan (blue + green), yellow (green + red), and white (red + blue + green). We have three distinct cone receptors in our retinas, sensitive to red, green, and blue. So this all makes good sense.

But birds have four cones and can seen into the ultraviolet. Researchers say this gives them an additional dimension of color vision. Imagine red + ultraviolet. You can’t: we don’t have a name for that mixture, nor can we visualize it. A color mix square would be called for. Actually, that wouldn’t work.

Seems the number of possible color-mixing outcomes is 2^n – 1, where n is the number of primary colors. Three primary colors yields 2^3 – 1 = 7 outcomes (R, G, B, M, C, Y, W). Then four primary colors produces 15 outcomes. But the color mixing square can only accommodate 13. How unfortunate. Downright unlucky!

Here’s what I got when I tried to populate the cells of a color mixing square. D-oh! Now I’m getting why an extra dimension of color is called for here.

For many more details and implications, check out the Science Friday segment below.

When I say I’m a big fan of SciFri and appreciate the science communication work that host Ira Flatow does, you might suspect a “but” is sure to follow. Who am I to disappoint?

Listen again to the minute from 13:15 to 14:15. I cringed when I heard this over the air the first time through. Ladies, has this ever happened to you? Maybe it was the result of multitasking on Ira’s part, but I’m reticent to make excuses for him here. In any case: awkward. The guests maintained composure, so good for them. Still though… I hope I’m never that guy (but I probably have been).

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