Have you ever seen a picture like the one at the top of this page? It looks like it’s just a repeating pattern from left to right, with some small flaws every time the pattern repeats, but it has a special property, and a special name. These pictures are called autostereograms, and if you stare at one long enough it might show you a 3D picture that seems to come right out of the page, even though it’s only a flat, 2 dimensional picture! If you’ve seen these before, then you probably know how to make the 3D image appear, but I’m going to let you know how to see the 3D picture if you can’t, and WHY you see something 3D from only a flat picture.
First, I want you to hold your finger up in front of your face and look at it. Now, look at something behind your finger, and you should now see two, slightly fuzzy, slightly see-through images of your finger side by side. There’s another way to do this, if you can, and that is to cross your eyes, and you should again see two, slightly blurry, slightly see-through images of your finger side by side.
Well, now, that’s fun and all, but not much of an optical illusion is it? But let’s take another look at the top picture with that in mind. This time when you look at the picture, either cross your eyes, or unfocus them like you’re looking past the picture, so that you get that double vision again like you did with your finger. You should see two images of the picture and they should be overlapping in the middle. When this happens, your brain will try to match up the repeating pattern bits, since they look so much alike. When your brain matches up the overlapping parts that look the same, it thinks that it’s only looking at one image, not two overlapping images, so you’ve tricked your brain into forming one image out of two slightly different ones.
Since the pattern in the picture doesn’t repeat perfectly, there are some flaws each time it repeats, and your brain thinks that these small differences are closer or farther away! So, if we control where these flaws show up, we can trick our brain into seeing something come out of (or sink into) the picture when we unfocus our eyes! If you did it correctly (be patient, it often takes some time and practice to see the 3D picture), you should see the words “Science AL!VE” come out of, or sink into, the picture (depending on whether you crossed or unfocused your eyes).
You’ve probably seen plenty of other optical illusions, and they weren’t all autostereograms like the picture at the top of this page, so how do we explain how all the other optical illusions out there work? Is there a different explanation for each optical illusion? Well, since each optical illusion is different, they all have slightly different explanations, but they can all be generally explained by one of two reasons: you’re either tricking your brain or you’re tricking your eyes.
Optical illusions where you trick your brain rely on the fact that your brain expects to see things a certain way, they way it sees most things. Some pictures, like the 3D image picture above, confuse our brain into thinking they’re something that they’re not. When we trick our eyes, however, we are abusing a physical weakness that our eyes have, whether it’s exhausting our eyes by looking at something for too long, or trying to look at something our eyes just can’t see.
Tricking our Eyes:
To know how to trick our eyes, we need to know how our eyes work. Basically, our eyes are round balls with a hole in them to let light in (that hole would be our pupil, the black spot in the middle of our eyes). The light that gets into our eyes hits the back of our eyeballs, which is covered in tons of tiny little light detectors called rods and cones. Rods detect black and white, and cones detect colors like red, blue, green, or any color you can see. When these light detectors detect light, they send that information to the brain to be translated into an image in our heads so we know what we’re looking at. This sounds pretty complex, but our eyes have certain weaknesses that allow us to see some neat optical illusions.
The most obvious weakness that our eyes have is that the light detectors in our eyes get tired if we use them too much. This happens when we look at something too bright, or stare at something for too long. If you’ve ever looked at something bright and then looked away, you might have noticed that you see a black spot floating in front of you that disappears after a while. This happens because the bright light really wears out our light detectors, so when we look away, those light detectors that got hit with the really bright light aren’t working as well any more, so that part of our vision is darker.
(Image courtesy of Google Image Search, http://www.qualitytrading.com/illusions/queen.html)
We can see a similar effect if we look at something not very bright, but for a long time. Take a look at the picture of the Queen above. It’s the Queen, but black and white have been switched. Stare at one point in the center of the picture of the Queen for about one minute, then look at the blank white square just to her right. In the blank square you now see a picture of the Queen, but with black and white they way they should be. When we look at the black and white picture of the Queen, the black parts don’t wear out the light receptors in our eyes, because they’re not very bright, but the white parts of the picture do start to wear out the light detectors in our eyes. Then, when we look away, we see black where our light receptors have been worn out, and the white background where the light receptors were not worn out. But don’t forget, we have black and white light receptors, as well as color light receptors, too, so this optical illusion should work just as well with colors.
(Image created by Jason Kirkby)
Our eyes can also get tired of motion. Stare right at the center of the image above with the inward moving rings for about one minute, then look at your hand, or some nearby object. You should notice that whatever you look at seems to expand from the point you’re looking at for a while, then your vision goes back to normal. Just as our eyes got tired of the colors we were looking at in the above pictures, the parts of our eyes and brain that were watching the inward moving rings got tired when staring at the image above. Then, when we looked away, since that part of our eyes and brain were worn out, we started to see the reverse motion and whatever we looked at seemed to expand until those parts of our eyes and brain recovered. So whenever we look at something for too long, steady color OR steady motion, it tires our eyes out so they start to see the reverse when we look away. That’s not too big of a weakness for our eyes, but there’s another one that’s even worse: there’s a part of our vision that our eyes just can’t see!
(Image created by Jason Kirkby)
Take a look at the moving image just above. There’s a cross on the left, and a dot moving back and forth on the right. Now, cover your LEFT eye and stare at middle of the cross on the left, with your face about 6 inches from the screen. You should be able to see the dot moving out of the corner of your eye (when I say that you see the dot moving out of the corner of your eye, I mean that you aren’t actually looking right at it, you’re still looking at the center of the cross, but you don’t need to look right at the dot to see it move). At some point, while you’re still staring at the center of the cross, you should notice that the moving black dot seems to disappear! But if you look away from the cross, it’s still there (if the dot doesn’t disappear, make sure that you are looking right at the center of the cross, not the dot, and if that doesn’t help, try moving your face closer to or farther from the screen). You’ve just found your eye’s blind spot!
That’s right, there’s a part of your eye that is actually blind, because the back of you eyeball isn’t entirely covered in light receptors. There’s one spot on the back of your eye where a nerve attaches the back of your eye to your brain. This is your optic nerve, and it’s the part that carries all the light information that your eye gets and sends it to the brain. But right where this nerve is attached to the back of your eye, there are no light receptors, so any light that hits it is not seen.
The reason we don't notice the blind spot often is because our brain blends in all the surrounding colors to cover up the blind spot, so that there isn’t a big, black dot there all the time. However, when it’s a black dot that gets covered up by the surrounding white background, it’s really easy to notice. Also, since we have two eyes, our right eye can see what’s in our left eye’s blind spot, and our left eye can see what’s in our right eye’s blind spot, so each eye takes care of what the other misses. Try it out, look at the cross with both eyes and the black dot doesn’t disappear out of the corner of our eyes, because our left eye can see it the whole time, even if our right eye can’t.
An optical illusion that relies on tricking your eyes is usually pretty obvious because it will require us to stare at something for a while to wear out our eyes, and the effect fades after a few seconds (or, in the case of our blind spot, the effect doesn’t fade, but it’s pretty obvious when it’s a blind spot trick). But even if our eyes get everything right and don’t wear out, we will see that our brain can still mess up the information the eyes give it.
Tricking our Brain:
Your brain is constantly trying to make sense of the world around you. To do this, when your brain gets visual information from your eyes, it tries to compare it to other familiar things that you remember, things you’ve seen before. But what if it’s very much like two different things at once? What if it’s not like anything you’ve ever seen before? What if your brain thinks there’s missing information, and adds false details to what you see? When that happens, it’s very easy for our brains to be tricked into thinking it sees something that it really doesn’t.
The most obvious way to trick the brain is to show it a picture that can easily be seen multiple ways. Take the above pictures for instance, is the top picture showing a rabbit looking right, or a duck looking left? Is the bottom picture showing two black, silhouette faces on a white background, or a white vase on a black background? Your brain can easily see two different things from the same picture, and will probably flip back and forth between one and the other, trying to decide what it really is. So, what’s the answer? Which is it? Is it a vase or some faces? Well, the answer is that it’s neither. Both pictures are just a collection of white and black markings, nothing more. But when our brain looks at these markings, it tries to compare it to similar objects, like a vase, or a face. Normally, your brain has no problem doing this, but with these pictures, it could easily be one or the other, and our brains can’t accept the fact that it’s neither since it is our brain’s job to make sense of the world that as we see it. That’s not so bad, but what about when our brain tries to make sense of something and ends up something that can’t possibly exist?
(Image created by Jason Kirkby)
(Image created by Jason Kirkby)
What do you see when you look at the two pictures above? Probably two 3D objects, the one on the right is a three ended fork-shape, and the one on the right is a 3D triangle. But look a little closer and you’ll see that neither shape could possibly exist in a 3D world. On the left, look at the top of the shape and you see two square pieces coming down, but look at the bottom and you see three round pieces going up, and in the middle they somehow connect without combining or splitting up. How does that work? The answer is that it doesn’t, the top and bottom of the figure don’t match up to create a realistic 3D shape. And look at the 3D triangle shape on the right, if the bottom is flat, then the left side must be going into of the page, and the right side must be coming out of the page, yet they somehow connect at the top when they were going in opposite directions to begin with. How does that work? Again, it doesn’t work, the triangle could never exist since one of the corners wouldn’t match up if you tried to build it. So why does our brain see these shapes as 3D when they can’t really exist? Once again, your brain is just trying to relate what it sees to what it’s used to, and your brain is used to seeing a 3D world. These drawings are only black, white and grey markings, but when our brain sees them it thinks they look like 3D shapes it’s seen before, even though the shapes are impossible in 3D.
(Images hosted on Wikipedia, http://en.wikipedia.org/wiki/Image:Ponzo_illusion.gif)
With certain pictures, our brain will do more than just think there are 3D shapes when there aren’t. In some pictures, our brain relates them to similar situations that aren’t really taking place, and can end up thinking two things are different when they’re actually the same. For example, look at the above picture on the left. Are the two yellow lines the same length? It probably looks like the top one is longer, but look again at the picture on the right, which is the same picture but with red lines added. In the pictures above, the yellow lines are the same length (as you can see when they’re matched up by the two red lines), so why does you brain think that the top one is longer? Because of the train tracks in the picture, your brain thinks that the top line is farther away than the bottom line, so it expects the top line to be larger than the bottom one since it only looks smaller because it’s farther away. So even though it doesn’t really even look like the top line is farther away, you brain takes this into account and decides that the top line is larger.
(Image hosted on Wikipedia, http://en.wikipedia.org/wiki/Image:Grey_square_optical_illusion.PNG)
Even weirder is the picture with the checkerboard and the shadow. Are the squares A and B the same shade of grey? At first glance, square A looks darker than square B, since there are light and dark alternating squares, where A is one of the dark squares and B is one of the lighter squares, but B is in the shade so it’s actually darker, making it the same shade of grey as A! Take a look in the top left corner of the picture, where A and B have been removed from the checkerboard. It’s obvious that they’re the same color, but when we see them in the situation of the checkerboard and the shadow, our brain thinks “No, B is still lighter than A since it’s one of the lighter squares on the board, it just happens to be in the shade right now.” So taking this into account, our brain decides that B is lighter than A, when it isn’t. Wow, way to mess things up, brain. But that’s not all, our brain can also see motion when there’s none at all.
(Image created by Jason Kirkby)
Take a look at the ring of smiley faces above. Follow the motion of one of the smiley faces with your eyes. Do you see the smiley faces circling around clockwise or counter-clockwise? Actually, they’re not circling around at all, the image is just switching between two different pictures of the smiley faces in a circle. It might be possible to look at the smiley faces and just see two pictures switching, but more likely you’ll think that they are moving around in a circle because your brain is used to piecing together pictures to create motion (that’s basically what a movie is, a bunch of pictures shown one after the other very quickly, and if our brain didn’t try to connect one picture to the next, we couldn’t watch movies!). And what’s more, depending on whether you’re thinking about the ring moving clockwise or counter-clockwise, the ring will change direction, because it’s not really moving so your brain can think it’s going either way.
(Images hosted on Wikipedia, http://en.wikipedia.org/wiki/Image:Revolving_circles.svg)
Now check out the above picture. There don’t appear to be any changing or moving parts to it, but stare at the dot in the center and move your head towards and away from the screen. Whoa, did that just move? No, it couldn’t have, it’s just a picture, but that doesn’t mean we didn’t THINK it moved. See, the squares that make up the two rings are slanted, and when we move our head closer or farther from the picture the rings get bigger or smaller, so the slanted squares seem like they’re spinning when seen out of the corner of our eyes. This would work with only one ring, but having two slanted in opposite directions makes the effect more noticeable. So our brain can think there’s motion even when something is sitting perfectly still, huh? That might make you wonder next time you think you see something move out of the corner of your eye.
