Colour #4: Colour Blindness

Why do we need 3?

M and L cones are relatively close together; it turns out that they are actually genetic mutations of a common ancestor cone that drifted apart over time. Mollon (1989) suggests that M and L peak where they do as it evolutionarily helps in finding red fruit in a green forest.

But are both really necessary? It might seem like the S and L cones are enough to see the world, as the two of them cover the whole visible spectrum. M is just an overlapping cone in the middle, so what value does it add?

Overlap is good

People without M cones exist; they have a form of colour blindness called deuteranopia.

The lack of M collapses the rainbow quite a bit. Many colors become completely indistinguishable. Look for example at 600nm onwards — only the L cone is active in varying amount, which won’t be enough to produce distinct color perceptions. Higher intensity light would also stimulate the L cones more, so the brain has no way to tell the difference between a dim 600nm light and a bright 700nm light.

The M cone is necessary to give the brain a second signal to compare against.

Maybe Mollon is right

The forest above loses quite a bit of detail when no M cone is present.

Less overlap would be better

You can also imagine how in normal vision, if the M and L cones were even closer together, we would be approaching a deuteranopia situation, because the 2 signals would be too similar to distinguish.

And even in normal vision, we have a bit of a color blindness situation going on on the red end of the spectrum, where the reds all start to look the same.

It would be better to have just enough overlap to distinguish colors, and move the L cones further into the longer wavelengths to give us more range.

What if L were further out?

Here we’ve moved the L cone to be a mirror image of the S cone about the M cone’s peak. This would give us 50% more distinguishable colors, and a much wider range of reds.

There really isn’t a way to visualize this, as I can hardly ask you to “imagine new kinds of red”, much like a deuteranope cannot imagine a distinct red and green.

As a result I’ve marked the “extra colors” in the rainbow with a hatching pattern.

Our eyes are still pretty good

Even with our imperfect cone setup, humans can distinguish around a million different colors.

The fact that the EM spectrum is a one dimensional line, and our eyes detect across three dimensions (S, M and L), means that we can see a lot more colors than there are wavelengths of light.

Seems counterintuitive, but it is true!