The Science Behind One Of The Craziest Optical Illusions We've Ever Seen

This totally freaking crazy optical illusion has been making the rounds online over the last little while (even George Takei posted it last week). It's very simple: stare at the red dot in the full photo below for 30 seconds and then quickly shift your gaze away to any plain, flat surface. (It helps a bit if you then blink a lot.)

Craaaaazy, right?

Now, it just so happens that a little red friend of ours, Matthew Ivanowich, is working on his PhD at the University of Western Ontario about the way our brain perceives colour. So we figured we'd ask him to explain how this illusion works. Here's what he told us:

First off, you need a little background on the way your visual system works. You've got these neurons called "opponent cells". There are three different kinds you need to know about. One produces the difference in sensation between red and green. Another between yellow and blue. The third between black and white. Together, they're responsible for all the colours you see.

So how do these opponent cell neurons express the different colours? By changing the rate at which they fire. Each one has a base rate — if the neurons fire more or less often than that, you see the corresponding colour. So, for instance, if you look at a red firetruck, the red-green neuron begins to fire at a different rate, and you see red. If you look at a green plant, it fires at a different rate, and you see green. (Whether red or green corresponds to a faster or slower rate depends on the particular neuron and its location in your visual system.)

That brings us back to the optical illusion.

When you look at a negative image like this one, your opponent cell neurons adjust their firing rate just like they usually do. In this case, since the image of the woman is composed of shades of blue, the yellow-blue neurons fire at a faster or slower rate, and you see blue. But it takes energy to change the firing rate — and these neurons are lazy little bastards. When you stare at an image for a while, they run out of energy and go back to their base rate. So now, essentially, they've re-calibrated; the blue woman has become the new normal. When you suddenly look away, the neurons react to the sudden lack of blueness. They change their firing rate again and you see the woman in yellow.

This is the same reason that when you stare at something red for a while and then look away, you see green. And vice versa.

This isn't even the craziest optical illusion we've seen based on this principle. Here's another one. Stare at the castle during the countdown and you'll see the black and white image in colour. (If you then move your eyes, the effect will disappear, but if you look back at the spot again, it reappears.)

Here's another one in video form (with ridiculous music), which you can make fullscreen:


Ramkiran Putrevu said...

Thank you very much for this informative piece! But in the castle picture, which count down are you referring to?

Adam Bunch said...

Huh, it WAS an animated GIF, but it seems to have stopped working... So I've replaced it with a video version instead.

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