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The best illusions of the year

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Phil Plait

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Every year, the Neural Correlate Society holds a contest for the best illusions. Here's some of the most mind-bending.

I love optical illusions for a lot of reasons. As a scientist, they fascinate me, showing us how our visual and interpretation systems work. As a skeptic they embolden me, because they show that seeing should very, very rarely lead to believing; we are all too easily fooled in ways we don’t even notice.

But as a human being they just delight me. They’re so freaking cool!

Every year, the Neural Correlate Society (which promotes scientific research into how sensory perception affects the brain) holds a contest for the best illusions of the year, and it's announced its top 10 such brain-benders for 2014. To qualify, the illusions had to be published no earlier than 2013, or be previously unpublished.

They really are fun to look at; it’s amazing how your brain can be tricked. The top pick was a variation on the classic Ebbinghaus effect, where you perceive the size of an object compared with objects around it. It turns out that animating the objects makes the effect much stronger:

Cool, eh? I stopped the video several times to measure the circle, and it does stay constant in size. The effect really is all in your head.

The other illusions are also a lot of fun, but I have to say, there was one I liked best of all, even better than the Ebbinghaus one. My favorite kind of illusion is one that’s so convincing that you’d swear it’s a trick, that someone’s pulling something over on you. Colour illusions are great for this, because they’re utterly convincing, but they can also be checked to make sure they’re not cheating (by replacing colours, for example).

This one’s honest, and completely overwhelming. Pick a spot in the grid (near the middle is best) and just keep watching it as the animation runs:

Graphic by Mark Vergeer, Stuart Anstis, and Rob van Lier

<insert evil laugh here>

Oh, how I love stuff like this!

Did you see the illusion? When the black stripes run vertically, the colours between them look like they alternate pale green and pale blue. When the stripes are horizontal, the colours look a slightly deeper green and red (your perception of the colours may vary a bit from my description).

But here’s the thing: The background grid of colours doesn’t change! Honestly, it’s the same background. To prove the colours aren’t changing, just the direction of the bars, I picked a random spot in the image—in this case, the red spot just below the centre to the left—and sampled the colour of the same pixel in both frames. I created two squares, each one showing the colour I sampled:

Squares of colour from the two frames of the animation: Left is from the frame with horizontal stripes, and right is from the frame with vertical stripes. Image by Phil Plait, used with permission. Squares by Pythagoras.

The square on the left is from the frame with horizontal stripes, and the square on the right is from the frame with vertical stripes. They are virtually the same (for those interested in details, the RGB colours of the two squares are (189, 158, 147) and (189, 158, 149), which are extremely close; the one on the right looks ever so slightly darker to me, but clearly very nearly the same colour).

So why do the two frames look so different as they alternate? The exact mechanism is a bit complicated (and explained in a paper written by the illusion’s creators), but very basically our perception of colour is based in part on contrasting it with colors around it. The colour of, say, a red square may look very different depending on what colours are around it, and how bright they are.

In the illusion, the black stripes are forcing your eyes to pair colours one way in one frame, and another way in the other frame.

The grid of colours (left) and the two frames of the animation (middle and right) show just how your eyes are being fooled. Graphic by Mark Vergeer, Stuart Anstis, and Rob van Lier

This is best shown by example. The illustration above is from the paper; it shows the grid of colours by itself on the left, with the two frames from the animation in the middle and on the right. Look at the unstriped grid on the left. Pick a square, let’s say a red one. Above and below it are greenish squares, but the squares to the left and right are bluish. When the stripes are vertical, your eyes and brain naturally compare the red square to the greens ones, and when the stripes are horizontal you compare them to the blue ones. This totally changes the way you perceive the colour of the red squares.

If you move your eyes around a little bit as you watch the animation, you can kinda sorta see the colours as they “really” are, but let your gaze remain fixed, and boom! The colours appear to change. It’s amazing.

But don’t take my word for it. If you have some image editing software like Gimp or Photoshop, grab the animation and analyse it yourself. You’ll see the colours really are the same.

This reminds me of my favourite illusion of all time:

A spiral of blue and green... or is it? Graphic by Akiyoshi Kitaoka, used by permission

Incredibly, the blue and green spirals are the same colour. Seriously. I can prove it.

Even if you don’t get the kick out of these illusions that I do, I hope they make you realise that just because you see something doesn’t make it so. Whenever I read a UFO report, or some other such claim where the witness says, “I know what I saw,” I always think of how fallible our brains are.

You know what you saw? Maybe. But it’s not the way to bet.

Slate