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Hank: This video is an illusion. It might seem like I’m moving around, but I’m not. I’m just a series of completely still images flashing quickly before your eyes. Your brain is stitching those frames together to create the perception that I’m moving around like I would be if I were right there in front of you.

Your brain is always interpreting and trying to understand the world on the fly, and it doesn’t always get all the information it needs from your senses to figure out what’s actually happening. So a lot of the time, your brain has to guess based on what you’ve experienced in the past and how humans have evolved. And it usually does a very good job of this, but once you know how to exploit the shortcuts that brains like to take, it’s easy to fool of them.

And that is exactly what optical illusions do. And they’re more than just fun party tricks. So here are eight illusions that have taught scientists a lot about how eyes and brains process and interpret different kinds of visual information.

Let’s start with the illusion you’re looking at pretty much whenever you are on YouTube: videos like this one. It works because it takes some time for your brain to process whatever you’re looking at. So if you suddenly look somewhere else, there’s about a fifteenth of a second where your brain is still holding onto what you were just looking at — just in case it might come in handy for processing whatever you’re looking at now.

This effect is called the persistence of vision, and it’s why videos fool your brain. In videos, the next frame is already on the screen before your brain lets go of the one before it, and so you naturally meld them together into a continuous single thing. That’s how a bunch of stationary images create the illusion of motion.

There are also other ways old images can stick around. Stare at a bright green screen for about a minute without blinking and then quickly shut your eyes or look at a white wall, and you’ll probably see a magenta rectangle right where the green used to be. That magenta rectangle is an afterimage, and it’s caused by what’s known as retinal fatigue.

When certain cells in your eyes have been active for a long enough time, they can basically run out of energy. When you stare at the green screen, the green cone cells in your eyes are constantly firing to tell your brain that there’s something green in front of you while the red and blue ones are just sort of sittin' there. Then, when you look at something else, those green cones desperately need a break.

If you’re looking at a white wall — which should be an equal mixture of red, green, and blue — the greens just aren’t able to keep going. So instead, your eyes tell your brain that you’re looking at something that’s equal parts red and blue, but not green — also known as magenta.

Other kinds of cells can also get overstimulated, and that’s what’s going on with a spiral that’s sometimes called the waterfall illusion or the motion aftereffect. Stare at the very center of this spiral and try not to blink. Keep your eyes locked on the spot. You are getting sleepier... Very sleepy... getting sleep... ok, it’s actually, that’s a different thing.

Alright, now, how do I look? A little bloated? Maybe a little spinny? The motion aftereffect is like the motion version of that magenta rectangle from before. Your eyes have special cells that detect movement and rotation in different directions. They’re really important to help you navigate the world, but they can get overused just like your color cones.

Scientists think that’s what happens when you watch the spiral: The cells that detect rotation in that direction fire constantly while the ones for rotation in the other direction aren’t doing much at all. The firing cells were exhausted by the time I showed back up on screen, so they probably couldn’t respond well enough to match their neighbors and tell your brain that I wasn’t rotating. So your brain assumed I started rotating the opposite way the spiral was, which is why I seemed like I was spinning in the opposite direction from the spiral. It’s called the waterfall illusion because rocks can start to look like they’re moving up into the sky after you’ve been staring at a waterfall for long enough

Shepard’s tables are named after the cognitive scientist Roger Shepard, who published and explained the illusion back in 1990. These two tables might seem like they’re completely different shapes: One is long and slender, and the other’s short and squat. But they’re actually the exact same shape and size, just rotated so that one is horizontal and the other is more vertical.

The trick has to do with perspective, like the kind in a photo of train tracks. Train tracks look like nearly vertical lines, but your brain knows that they’re only like that because they’re getting farther from you. So you don’t see 2D lines; you see 3D tracks. Your brain tries to interpret Shepard’s vertical table in the same way. It assumes that the long, nearly vertical lines mean that it’s receding into the distance, which would mean that the table on the left is much longer than it looks.

This illusion actually dates all the way back to the 1850s, when a German psychologist named Adolf Fick noticed that people usually think that vertical lines are longer than horizontal lines. But the science behind it wasn’t fully explained until Shepard came along a century later.

The spinning dancer illusion was created by Japanese web designer Nobuyuki Kayahara back in 2003. About two-thirds of people think the dancer is spinning clockwise on her left foot, but everyone else sees her spinning counter-clockwise on her right foot. And some people are able to switch between the two if they try hard enough.

This is called a bistable illusion, and it works the same way as the famous rabbit-duck illusion and the Necker cube. In a bistable illusion, there just isn’t enough information to tell you exactly what you’re seeing, so different people’s brains make different kinds of assumptions — and settle on different conclusions.

A lot of people have said that illusions like the spinning dancer show which side of your brain is dominant... that’s not actually true. It just has to do with the way you interpret the lighting and camera angle.

Most people automatically assume that they’re seeing the dancer from above, so they follow her hand, which seems to go clockwise from that perspective. Others assume they’re looking at her from below and focus on her foot, which moves counterclockwise from that angle. So the dancer can move in either direction depending on how your brain interprets the images.

There are lots of illusions where all we know that the brain is being tricked because it’s making some kind of assumption. Brains are complicated. Others, we do know a lot about. Take this image.

The boxes marked “A” and “B” might look like different shades of gray, but they’re actually the exact same color. It’s called the checkered square illusion. Your brain is making an assumption here — about shadows — but there’s a lot more to the illusion than that. It has to do with something called lateral inhibition.

When you look at a big region that’s all the same color, your brain doesn’t want to be receiving all those identical signals. They’d be clogging it up when it could be focusing on something more important. So the nerves in the back of your eyes sort of summarize what you’re seeing. They send your brain signals that say things like, “This entire area is gray, and then it gets brighter here along this line.”

In the checkered square illusion, your brain gets signals saying that “A” is surrounded by lighter areas while “B” is surrounded by darker areas. So your brain assumes that “A” must be darker and “B” must be lighter. And that’s why “A” looks darker than “B.”

There’s another illusion that happens because of the way your brain interprets light and dark. And you might’ve noticed it in this very video. People point it out all the time in the comments every time I wear this shirt. It’s called the Hermann-grid illusion, where you tend to see dark patches on light lines separating darker squares into a kind of checkerboard pattern. But you don’t see the patches when you look directly at the lines.

There are two kinds of white areas in the checkerboard grid illusion: intersections — that is, the areas between four squares — and stretches, or the lines between squares. Like with the checkered square illusion, the illusion on my shirt probably happens because of the way your brain interprets different regions based on their surroundings. Between squares, white is mostly surrounded by dark, so your brain assumes these little white strips must be even lighter— as if the darker squares nearby meant the white stretches were in shadow.

But at the intersections, the opposite happens. The white in the center of the intersection is surrounded by white on all sides, so your brain assumes that it’s probably darker than it appears. Your brain puts in dark spots to compensate, which is why it looks like there are little dots all over my shirt, but not actually when you are looking directly at them.

All the illusions we’ve talked about so far happen because of neuroscience and psychology, but some have to do more with the laws of physics. Like mirages. Mirages happen because of temperature inversions, where a layer of hot air is trapped right above the ground by a layer of cooler air above it.

Light bends in air just like it does in water, but the amount that it’s bent depends on the temperature of the air. If the hot air on the ground is thick and warm enough, it can act almost like a mirror, bending incoming light back away from the ground and into your eyes, creating a mirage. Since air is pretty turbulent, mirages tend to shimmer, as if the light is reflecting off of water instead of a mirror. When you hear about somebody in the desert seeing a mirage of a body of water, this is why.

But mirages might also be an explanation for a famous myth: the ghost ship called the Flying Dutchman. Legend has it that the Flying Dutchman is doomed to sail the seas forever, and sailors would sometimes even see the ghostly-looking ship on the water. But the ship was just a mirage.

It happened when the sun warmed the air above the ocean, but the water kept the lowest layer of air cool. This can create a mirage that’s the opposite of what happens in the desert. Instead of looking like it’s reflected off the ground, light can bend and make things look like they’re floating in mid-air. A ship just over the horizon, for example, might look like it’s floating above the water like some sort of ghost ship.

So, sometimes you might see a nonexistent magenta rectangle, a bunch of dots on my shirt, or even the Flying Dutchman shimmering above the horizon. But they’re just optical illusions — what happens when your brain, or the laws of physics play tricks on you.

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