You've been seeing optical illusions probably since kindergarten. They're fun little party tricks that you look at on the Internet and go "Eh, that's weird" before immediately forgetting about them. And that's too bad, because these images are actually exposing glaring gaps in our brain's fragile sense of reality.
For example ...
#5. Your Brain Changes What Your Eyes See to Force It to Make Sense
This illusion is so bizarre that you'll probably call bullshit on it even after you see it explained. Take a look at squares A and B in the above picture. The squares are the exact same shade of gray. Not the letters -- the squares themselves.
We know you don't believe us. Take it into Photoshop -- cut out the "B" square and move it up:
Adrian Pingstone / Edward Adelson via Wikipedia
Now scroll back up and look at the original again. Bullshit, right?
What the Hell?
Every human should have this tattooed onto their arm, just so they don't forget it:
You see with your brain, not your eyes.
Dynamic Graphics Group/Dynamic Graphics
Of course, your brain could just alter what's written on your arm.
When you observe the world, your eyes aren't just lenses that coldly record whatever is in front of you. Instead, your brain adjusts the image for context from moment to moment. And in this case, it's filtering what you see based on a built-in knowledge of what shadows are.
Your brain understands that the floor is a checkerboard pattern of dark and light squares. It sees that there is an object casting a shadow over it. It knows that square B is still one of the "light" squares, even though it's the same color in the shadow as the dark squares are in the light, so in order to avoid confusion, it automatically adjusts the lightness of square B for you before that shit reaches your consciousness. It's like you've got an entire Hollywood VFX crew living in your skull, fixing the world in post-production before you actually get to see it. Keep that in mind the next time you hear some eyewitness testimony.
We could give you endless examples of this trickery. Take these two yellow lines, which are exactly the same length:
Don't believe us? Go try it on an actual railroad track!
Most people see the top line as being bigger, obviously because it thinks that one is farther away due to the "tracks." And even beyond the visual, you're unconsciously adjusting the world for context all the time. For example, if you hold a large and small box of equal weight, you will perceive the large box as lighter. Or, if you drink soda after eating cake, it will taste less sweet.
What's amazing is that you cannot unsee or unfeel these illusions despite all of the information to the contrary.
#4. Your Brain Predicts the Future So You Can Live in the Present
These two lines are actually perfectly straight and parallel, but they look like they're bulging outward in the middle. Stare at it for a while -- is that a touch of motion sickness you feel, or did you just eat a bad taco?
What the Hell?
The blue lines are tricking your brain into believing that you're moving toward the image, in much the same way you use lines in a doodle to show which way something is moving. Your brain interprets lines as motion, like in Star Wars, where we knew they were going fast because the stars turned into a vortex of white lines in their windshield:
Lucasfilm/20th Century Fox
"Sir, the vertical lines aren't actu-"
"Shut him up or shut him down!"
So why are the red lines bulging? Here's where it gets really weird -- that's the way your brain expects them to look in the future.
As we've already mentioned, there's a fair bit of calculation going on in your head from the time the light hits your eyes to the time you actually perceive something. This calculation is super quick, but it still takes time -- about one-tenth of a second. That means that you're actually living one-tenth of a second in the past at any one time. It's like a really shitty superpower.
Agri Press/Lifesize/Getty Images
Somewhere between being able to detach your own limbs and the ability to grow nipple hair at will.
So why are we still able to dodge fast-moving objects and not getting brained by flying baseballs? Because your brain has that shit covered. It actually predicts the future and adjusts the world accordingly. The world you're seeing is actually your brain's prediction about what the world will look like one-tenth of a second after you actually perceive it. That is to say, it brings you back to the present.
Because of perspective, two parallel poles will appear to bulge out a tiny bit as you pass between them. Your brain knows this, so it automatically puts that bulge in when it thinks you're hurtling toward the red lines. That's right: Precognition is real, but it only allows you to see into the present.
#3. You Interpret What You See Piece by Piece
Here's a picture of President Obama upside-down. You recognize him right away and nothing looks off about him, other than the fact that the image is flipped. Turn him over, though, and you realize you've been lied to. It's actually a terrifying alien parody of a human being.
It's the Manchurian Candidate from the Uncanny Valley!
What the Hell?
Your brain processes visual stimuli in pieces before it makes a whole image. When you look at a face, you see a face. But what your brain sees are pieces of a face -- eyes, nose, and mouth. Each piece of the puzzle is recognized separately, so even if we flip Obama's head over, we don't see anything weird about him as long as his eyes and mouth are the right way up. It doesn't work the other way -- your brain sees an upside-down face on a right-side-up head and just throws its arms up in confusion. Its brain-arms.
"Fucked if I know."
This is due to a flaw in what's called vestibular correction -- it's your eyesight trying to adjust for anything that's weird about how your head is oriented (in this case, seeing it as if you were upside-down). Think of it this way: Why are movies like Cloverfield, with their shaky camera, so headache-inducing? Aren't our eyes whipping around just as much as that camera when we're running, or even just looking quickly around the room? Yes, but under normal circumstances, your brain steadies all of that shaky, unsteady input and gives you one smooth, coherent picture.
And your innate sense of balance tells your subconscious which way is up, so even if you tilt your head to the side, you still see a diamond as a diamond and a square as a square. Try it:
Put down the monitor, please. Just tilt your head.
That team of graphic designers living in your head picks out every individual object in your visual field and adjusts them separately so that what you're looking at makes sense, and it does all of this in a fraction of a fraction of a second. But it does it without a ton of attention to detail, which is why a horrible monstrosity can pose as the president just by standing on his head.