How Do Optical Illusions Work?

(Inside Science TV) These are two identical, straight lines. Kind of boring, right? But what happens when you change what’s going on around them? All of a sudden, the lines look like they’ve changed size in relation to each other; they could shrink and grow; they might change to different shades; they may stretch and compress; and they could bend, buckle, or even change directions. But even though it would seem like the lines themselves are changing, the only thing that’s actually bending or buckling is your mind -- the truly mesmerizing part of optical illusions.

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Your brain can morph a mere pair of identical lines in all sorts of ways, as illustrated by these optical illusions: the Ponzo illusion (1), the Chub illusion (2), the Muller Lyer illusion (3), and the Café Wall illusion (4). See original illusions here. (credit Inside Science)

When you look at something, what you’re really seeing is the light that bounced off of it and entered your eye, which converts the light into electrical impulses that your brain can turn into an image you can use. The process that takes about a tenth of a second but your eyes receive a constant stream of light, an incredible amount of information, so it’s really difficult for your brain to try to focus on everything at once. It would be like trying to take a sip of water from a firehose. So your brain takes shortcuts, simplifying what you see to help you concentrate on what’s important, which helps compensate for your brain’s tenth-of-a-second processing lag. This trait helped early humans survive encounters with fast predators – or at the very least avoid running into obstacles like trees.

Optical illusions fool our brains by taking advantage of these kinds of shortcuts.

Take the Hering illusion, for example. If you put a bike-spokes radial pattern behind two identical, straight horizontal lines, the lines will look warped, even though they are actually straight. When your brain sees the radial pattern, it focuses on the point in the middle, as if you’re traveling towards it. Your brain then thinks the two parallel lines are approaching you, so it makes them look larger as they approach the center of the radial pattern, making the lines look bent.

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The Hering illusion. (credit Inside Science)

Not all optical illusions trick our brain into seeing motion. Some can also trick our brains into perceiving colors or shades that aren’t visibly present. Take the Mach bands illusion, as illustrated in the image below. A given band appears to be lighter in color on the top and darker on bottom, but if we separated the bands, we’d see that each band is a solid color. The illusory gradient arises from a process called lateral inhibition, which is mediated by light-sensitive cells in our eyes called rods.

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The Mach Bands illusion (credit: Inside Science)

Another type of eye cell collects from lots of different photoreceptors about light and dark values, but it has a way of “leaking” this light information to neighboring cells, creating a visible halo when you’re looking at certain high contrast images. The effect can be seen in this famous illusion, the Hermann grid, where you can see gray circles at the intersection points of each square.

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The Hermann grid. (credit Inside Science)

What about those illusions that make it seem like a still image is moving? The psychedelic peripheral drift illusion is an impressive brain fart. Take a look at the center of this figure. Looks like a cool pattern, nothing more. But when you look outside of it, it starts to move. This is due, in part, to how we perceive light and dark.

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The peripheral drift illusion. (credit Inside Science)

Luminance, our sense of light and dark, is kind of unreliable. Our brains are able to perceive lighter values much more quickly than dark values. This explains why the discs seems to rotate in the direction of the lighter shades. There are also key points where your perception of motion is reset: blinking, shifting your eyes, and looking away and back fuels the illusion of motion.

So don’t believe the cliché. What you see isn’t always what you get.

Kirk Zamieroski is a science filmmaker and contributor to the American Chemical Society’s video blog, Bytesize Science. His award-winning videos have shown in film festivals around the world. He has a BFA in Kinetic Imaging from Virginia Commonwealth University.