Train Yourself To See Impossible Colours

Hiding in the shadows between the colours we see everyday are weird, impossible shades, colours that you shouldn't be able to see and generally don't... unless you know how. Here's a simple guide to seeing impossible and imaginary colours.

Image by Cody James.

Understanding a little about how humans perceive colour is crucial to seeing impossible colours. Our eyes use something called opponent process to work more efficiently. This plays upon the fact that the eye's primary light receptors, the cones, have certain overlaps in what light wavelengths they can perceive. To save energy, our eyes measure the differences between the responses of various cones rather than figuring out each cone's individual response.

We long ago found out that there are three opponent channels: red vs green, blue vs yellow, and black vs white. (Technically, black and white aren't colours, and their opponent process has more to do with brightness than anything else.) Now, let's say you stare right at the bluest object you've ever seen. Your cones that primary perceive the blue wavelengths are going to be excited, while the cones responsible for yellow will be inhibited. If you then switched to looking at the yellowest thing you've ever seen, the exact opposite would happen.

It probably isn't all that shocking to point out the cones can't be excited and inhibited at the same time. That means that it's impossible to see an object that's simultaneously blue and yellow or red and green. I'm not talking about what happens when you mix those colours and then look at them - obviously, you'd get green and a sort of murky brown if you did that. No, what I'm talking about here are colours that are equal parts blue and yellow at the exact same time. Can you imagine that? Well, you shouldn't be able to, because that's an impossible colour.

This might all seem a bit abstract, but there's some evidence backing up the existence of such colours. A 1983 experiment featured a special machine which separated the fields of vision of the test subject's eyes. One eye would see a red screen, while the other would see a green screen. Given time, the colours would mix together, but the mixing only occurred in the brain. Without the eye there to mediate the mixing, red and green didn't become brown - they became a new colour, a reddish-green colour that none of the test subjects had ever seen before, and that includes an artist with an extensive knowledge of different hues and shades.

Admittedly, the methodology of that experiment has since been criticised, and many vision researchers say impossible colours are called that for a reason – they really are impossible. There are, to be sure, a lot of alternative explanations for the colours the people saw: they were just intermediate colours between the two, the experimenters hadn't properly controlled for luminance and that threw off the results, or the test subjects were really just see red, then green, then red, and so on, and never actually viewing them simultaneously.

These are all fair points. However, if I may make a counterpoint, you're ruining all the fun, vision experts. Sure, impossible colours might actually be impossible, but that doesn't change the fact that test subjects saw colours they had never seen before. Impossible colours might not exist, but if it's possible to fool our brains into thinking they do, then I'd say that's still pretty awesome.

This is one of the least scientific viewpoints I've ever put forward, and I'm not exactly proud of it, but hey... impossible colours are cool. Now relax each eye on these two plus signs and see if you can't make some impossible colours appear. Let your eyes cross so that the two pluses are right on top of each other. I'll say right now that not everyone is going to be able to see these weird colours - I'm almost certain that I can't - but I'd still say it's worth a try.

I'd be remiss if I didn't also mention imaginary colours. These are colours that cannot be produced in the physical light spectrum, and yet it's possible to derive them mathematically. The easiest way to understand what an imaginary colour is would be to think about the three wavelengths of cones - short, medium and long. Like I said when talking about the imaginary colours, there's an overlap in the responses of these different wavelengths.

But what if you had a colour that only created a response in the medium wavelengths? In real life, this can't happen, as anything that excites the medium wavelengths is going to excite one or both of the other wavelengths. But if you did have a colour that only excited the medium, green wavelengths while leave the other two types alone, then you'd be able to see a colour greener than any real green.

So that's the theory - here's how you do it. Again, you've to be smart about your opponent processes. If you want to see an imaginary green, you need to find an example of heavily saturated red and one of a heavily saturated green. Stare at the red colour for as long as you can, then switch to looking at the green. The red receptors have become too fatigued to do their job and be inhibited by the green colour. That means your green receptors are getting excited with nothing to counterbalance them. The result is the greenest colour you've ever seen, one that can't exist in the physical world.

Again, this might all seem a bit out there, but America's most lovable evil geniuses have known about this for years. Walt Disney World took advantage of this effect in their design of the EPCOT park, making the pavements a particular shade of pink that tires out the red receptors and forces the park's grass to look greener than it really is. On second thought, I'm not sure that makes this seem any less out there.

For more, check out "Impossible" Colors: See Hues That Can't Exist (Scientific American).

Republished from io9

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