 See this spiral-like shape? It's actually rings of concentric circles. And these lines look like they're leaning away from each other, but they're actually parallel. These lines look curved, but really they're straight. And the line that looks like it breaks as it goes underneath this rectangle? That's one continuous line. All of these are optical illusions, images that somehow trick our minds into perceiving something different from what exists in reality. These particular ones are something called geometrical illusions. A geometrical illusion arises when you put a geometrical figure, which could be a straight line or square or circle, onto a background of other, often parallel lines. And we call the background lines inducers, and the front figure we call the test. And typically you'll find that the inducers make the test tilt away from the inducers. People have been squinting and staring at optical illusions for centuries, but scientists still don't really understand why specific combinations of shapes and lines mess with our minds. What they do know is that these distortions probably come from how our brains interpret the images, rather than how our eyes see them. Each eye sees a slightly different view of the world that the brain then has to combine into a 3D perception. In the middle of the 20th century, neuroscientist Bella Uless invented a kind of optical illusion called the random dot stereogram. Those are similar to the magic eye illusions you might remember from the 1990s. These illusions are made up of two images of random dots. If you focus your eyes just right or use a stereoscope, they come together to form a picture. When Uless converted geometrical illusions like the Pogendorf illusion into random dot stereograms, people could still see the distortions. That means that the illusions occur in our brain once the images have been combined, and not in our eyes. One explanation has to do with a phenomenon called lateral inhibition. In the visual cortex, there are neurons that specifically respond to lines oriented in different directions. There are individual nerve cells that respond to the vertical, and a different set of nerve cells respond to the tilted ones. And these tend to inhibit or turn each other off, which means that the perceived direction of the two lines will diverge. So acute angle rules appear to get bigger. That can also explain the Herring illusion, where the long red lines seem to bend, but are actually parallel. The thing is, lateral inhibition can't explain every geometrical illusion. Like in the Pogendorf illusion, where a line passes behind a big vertical rectangle and it looks like the ends don't connect. But really they do. If this illusion occurs because lateral inhibition makes the acute angles look larger, then stripping the illusion down to just the obtuse angles should make it disappear. But you can still see it. And when you strip it down to just the acute angles, the illusion's gone. So another theory is that our brains are trying to process these 2D images like 3D objects, adding the illusion of depth and perspective where there is none. That especially seems to be the case with the Ponzo illusion, where even though the top line looks longer, it's actually the same length as the bottom one. That could be because of a process called size constancy. So to get a better sense for how this works, take your hand and put it out at arm's length, and then bring it to your nose. Even though your hand is taking up more and more of your visual field as it gets closer to your face, your brain doesn't think that your hand is getting bigger, it thinks that your hand is getting closer. The same thing could be going on in the image. Even though the two lines take up the same amount of space in our visual fields, because the top line looks farther away, our brains interpret it as larger. Another explanation could be that our minds interpret an object's size based on what's around it. So because the top line intersects with the lines next to it, it looks longer than the bottom line, which is surrounded by white space. What we do know for sure is that as the 3D world projects 2D information onto our retinas, which our brains then have to turn back into a 3D perception, sometimes our minds cut corners. So you might have noticed that the Virg's logo is an optical illusion too, and it's what's called an impossible triangle. It was first drawn in the 1930s by a Swedish artist as a series of cubes, but then in the 1950s, a British physicist was so inspired by the king of optical illusions, MC Escher, that he drew this version that we're familiar with today.
 See this spiral-like shape? It's actually rings of concentric circles.
 And these lines look like they're leaning away from each other, but they're actually parallel.
 These lines look curved, but really they're straight.
 And the line that looks like it breaks as it goes underneath this rectangle? That's one continuous line.
 All of these are optical illusions, images that somehow trick our minds into perceiving something different from what exists in reality.
 These particular ones are something called geometrical illusions.
 A geometrical illusion arises when you put a geometrical figure, which could be a straight line or square or circle,
 onto a background of other, often parallel lines.
 And we call the background lines inducers, and the front figure we call the test.
 And typically you'll find that the inducers make the test tilt away from the inducers.
 People have been squinting and staring at optical illusions for centuries, but scientists still don't really understand why specific combinations of shapes and lines mess with our minds.
 What they do know is that these distortions probably come from how our brains interpret the images, rather than how our eyes see them.
 Each eye sees a slightly different view of the world that the brain then has to combine into a 3D perception.
 In the middle of the 20th century, neuroscientist Bella Uless invented a kind of optical illusion called the random dot stereogram.
 Those are similar to the magic eye illusions you might remember from the 1990s.
 These illusions are made up of two images of random dots.
 If you focus your eyes just right or use a stereoscope, they come together to form a picture.
 When Uless converted geometrical illusions like the Pogendorf illusion into random dot stereograms, people could still see the distortions.
 That means that the illusions occur in our brain once the images have been combined, and not in our eyes.
 One explanation has to do with a phenomenon called lateral inhibition.
 In the visual cortex, there are neurons that specifically respond to lines oriented in different directions.
 There are individual nerve cells that respond to the vertical, and a different set of nerve cells respond to the tilted ones.
 And these tend to inhibit or turn each other off, which means that the perceived direction of the two lines will diverge.
 So acute angle rules appear to get bigger.
 That can also explain the Herring illusion, where the long red lines seem to bend, but are actually parallel.
 The thing is, lateral inhibition can't explain every geometrical illusion.
 Like in the Pogendorf illusion, where a line passes behind a big vertical rectangle and it looks like the ends don't connect.
 But really they do.
 If this illusion occurs because lateral inhibition makes the acute angles look larger,
 then stripping the illusion down to just the obtuse angles should make it disappear.
 But you can still see it.
 And when you strip it down to just the acute angles, the illusion's gone.
 So another theory is that our brains are trying to process these 2D images like 3D objects,
 adding the illusion of depth and perspective where there is none.
 That especially seems to be the case with the Ponzo illusion, where even though the top line looks longer,
 it's actually the same length as the bottom one.
 That could be because of a process called size constancy.
 So to get a better sense for how this works, take your hand and put it out at arm's length, and then bring it to your nose.
 Even though your hand is taking up more and more of your visual field as it gets closer to your face,
 your brain doesn't think that your hand is getting bigger, it thinks that your hand is getting closer.
 The same thing could be going on in the image.
 Even though the two lines take up the same amount of space in our visual fields, because the top line looks farther away,
 our brains interpret it as larger.
 Another explanation could be that our minds interpret an object's size based on what's around it.
 So because the top line intersects with the lines next to it, it looks longer than the bottom line, which is surrounded by white space.
 What we do know for sure is that as the 3D world projects 2D information onto our retinas,
 which our brains then have to turn back into a 3D perception, sometimes our minds cut corners.
 So you might have noticed that the Virg's logo is an optical illusion too, and it's what's called an impossible triangle.
 It was first drawn in the 1930s by a Swedish artist as a series of cubes, but then in the 1950s,
 a British physicist was so inspired by the king of optical illusions, MC Escher,
 that he drew this version that we're familiar with today.