Optical illusions reveal the mechanics of our mind.
Back in 1952, curious moviegoers filled theaters across the nation to see a new action-adventure movie, Bwana Devil, which was being touted as the harbinger of a revolution in Hollywood entertainment. The film utilized a new process, designed to trick viewers’ brains into thinking that they were seeing a three-dimensional vision of reality, rather than a flat two-dimensional movie. The movie had been shot with two cameras, and in theaters, double projectors put both of those images on the screen—one projected through a red filter, the other through a green one. The audiences then were given special glasses, in which one lens had a red-tinted lens and the other a green one. The result was that movie goers saw a
slightly different view of the same movie scene in each eye, just as they would when looking at their actual surroundings. Though a critic for Variety denounced Bwana Devil’s “banal dialog, stilted sequences and impossibly-directed players,” audiences didn’t seem to care. The movie became such a big hit that Polaroid, which made the special 3D glasses, couldn’t keep up with the demand.
The funny thing about the hoopla over Bwana Devil—and that surrounding more recent 3D hits, such as 2009’s Avatar—is that moviegoers were wowed by a gimmick that was crude and unsophisticated compared to the complicated perceptual tricks that their own brains routinely played upon them.
While you may assume that you see the world around us in 3D, the odd truth is that your retinas—the screens inside our eyes upon which visual information is projected—can only perceive two dimensions, height and width. But that’s okay,
because your brain is pretty clever when it comes to creating illusions. Your visual processing regions are able to combine the data from both of your eyes and to utilize cues such as size and shading, and then perform complicated calculations to figure out where objects are positioned in space. Additionally, to make up for the brain’s slight delay in processing visual information, it makes educated guesses about where moving objects are located at any given moment.
In short, your brain continuously is creating and watching a 3D movie with far more convincing special effects than any Hollywood computer jockey could create. Pretty slick, huh? But there's a catch. Just as your brain is adept at creating the illusion of 3D, perception researchers and magicians sometimes can trick your brain into thinking that it sees things that aren't there.
What Your Eyes See Is Different From What Your Brain Sees
"In reality, we don't see truly in 3D," explains theoretical neurobiologist Mark Changizi. "It's a construct that our brain makes for us.
"The reason that we see at all is so that we can interact with the world around us—not bump into it. But the only way we could do those things is if we had perception of 3D."
What we think of as reality starts with light that’s reflected off people, places and objects in the environment around us. That reflected light enters the human eye through the cornea, the lens on the outside, which then focuses it so that it strikes the retina, a small patch of tissue about quarter of a millimeter thick at the back of the eyeball. The retina contains cells called photoreceptors, which are sensitive to light. They transmit biochemical and electrical messages to the retina’s neurons, which then send them on a relay path down the optic nerve to the brain’s image-processing areas.
But your brain doesn’t see the same world that your eyes see. Your corneas actually flip light as they focus it, so that the images of objects, people and the environment that they project onto your retinas are actually upside down. The brain flips that image over, so that you see the world right-side up.
More importantly—and this is a hard concept to get your
head around—the picture of the world that’s projected onto your retina only has two dimensions, height and width. When light reflected off an object—a pine tree, for example—is projected onto your retina, the size of the image is determined by a combination of the object’s actual size and its distance from you. If you’re looking at two pine trees that are both six feet tall, the closer one will have a bigger image than the
more distant one. Your eye can’t really tell that they are identical in size, or where they are related in relation to each other or to other objects in your sight.
Now, if you perceived the world literally as your retinas see it, life would be pretty difficult. You’d have a lot of trouble reaching for a glass of water, let alone driving a car or hitting a tennis ball. Fortunately, though, the world you see actually is created by your brain, which manipulates information from the eyes to add depth to height and width. But to accomplish that, your brain has to resort to some pretty ingenious tricks and educated guesses.
How Your Brain Turns What You See into 3D
Your brain transforms 2D images into a 3D world by looking for cues that help it to figure out where things are in relation to other things. Here are the main types of cues.
Monocular cues: You’ve probably heard someone say that 3D perception is created by using your two eyes together. But that’s not completely the case. If you close one eye and look at the objects around you, you’ll find that you still have some sense of depth, though it’s not as good as when you use both your eyes. That’s because some of the depth cues that your brain utilizes are monocular cues, which are a lot like the tricks that artists use to make a drawing on a flat piece of paper look three-dimensional. For example, your brain utilizes linear perspective—that is, how parallel lines that are pointing away from us will appear to converge in the distance. It also notices the relative texture density on the surface of an object. If you look at a patterned carpet, for example, the details of the design are less clear toward the far end, and sharper toward the near end. There’s also interposition, in which a nearer object hides part of a more distant object, and shading, in which the position of an object creates a pattern of light and dark. Another important
cue is aerial perspective; light is scattered as it travels through the atmosphere, so that more distant objects have less color contrast and seem hazier. Finally, there’s the motion parallax, a phenomenon in which objects that are closer to your retina will appear to move faster if they move across your field of vision, even if they’re actually moving at the same speed as a more distant object. Your brain somehow knows all these things, and uses them to interpret visual information.
This is the part of 3D perception in which your brain makes use of the fact that most of us have two working eyes to provide information. Because your eyes are a few centimeters apart, each of your retinas gets a slightly different picture of any given object. To see the difference, take one of your fingers and align it with a vertical edge in the room where you’re sitting, such as the window edge or the door frame.
Then alternately open and close each eye. You’ll see that your finger appears to jump back and forth, even though it’s not moving. The closer that you bring your finger to your face, the more drastically your finger will appear to jump when you switch eyes. This effect is called binocular disparity, and it’s the one utilized by 3D movies to create the illusion of depth. But your brain also utilizes the fact that there are corresponding points on your retinas as well. These points fall along an arc called the horopter in your central, high-resolution vision, where your retinal images essentially overlap. As E. Bruce Goldstein explains it in the textbook Sensation and Perception, imagine that you’re a lifeguard, looking at three swimmers—Susan, Frieda and Harry—who are treading water in a pool in an arc around you. Your brain senses that they’re all the same distance away from you, because the images match up. But say that there’s a fourth swimmer, Carole, who is closer to you. Susan’s image falls on non-corresponding points on your two retinas. Your brain
notices that, and calculates the angle of disparity between where Carole appears on your right and your left retinas. That helps you to figure out how far away Carole is.
These cues don’t come from the images on your retina. Instead, your brain picks up the sensations of the muscles around your eyes as they contract. For example, your eyes tend to turn inwards more to focus upon an object when it’s close and outward when it’s distant, a phenomenon called convergence. Similarly, the ciliary muscles in your eyes contracts, which allows the lens to take on a more spherical shape. Conversely, those muscles relax when you look at a more distant object. That effect is called accommodation. You don’t really notice any of that, but your brain picks up the feedback, and uses it to help estimate where things you’re looking at are located in space. But oculomotor cues are only really useful when the object is within three feet—roughly arm’s length—away from you.
How Your Tricky Brain Can Be Fooled, Too
Your brain has a lot of different tools that it uses to help turn 2D images into 3D, and it’s pretty good at doing that. So good, in fact, that your brain itself can be tricked into making incorrect assumptions about the world around you.
Because your brain assumes that things that are further away are smaller, for example, if you look at a 2D drawing of train tracks with two yellow lines of equal length superimposed upon it, you’ll probably assume that the line that’s toward the top of the picture is bigger. That’s because your brain automatically converts the 2D into a 3D picture. If you measure the two lines with a ruler, though, you’ll see that they actually are the same length.
Another famous illusion of this sort is the Kanizsa square. You’re shown a picture in which four dark solid circles,
presented in evenly spaced rows of two, each have arcs cut out, you’ll perceive a white box that’s in front of the black circles. That trick exploits the phenomenon of interposition, something we mentioned previously, in which an object that’s in front of other objects will block a portion of the background objects. When your brain automatically transforms the 2D image into 3D, it creates the white box in the foreground.
Long before neuroscientists discovered how the brain processes visual perception, artists were exploiting it to create vivid illusions on canvases. In ancient Greece, for example, Pliny the Elder recounts how two legendary painters, Zeuxis and Parrhausius, had a contest to decide who was the more accomplished artist. They each brought covered canvasses to show each other. Zeuxis pulled off his cover, and showed a picture of a bunch of grapes so lifelike that, as the story goes, birds swooped down to peck as the artwork. Confident of victory, he then leaned over and tried to pull the cover off Parrhasius’s painting—only to discover that there was none. Instead, Parrhasius had painted what appeared to be a cloth with three-dimensional wrinkles and creases. In Renaissance Europe, the painting techniques used to create 3D illusions eventually became known as trompe l’oeil, French for “trick of the eye.” But it really is your brain that is being fooled.
The red or the blue?
- The Pendragons Metamorphosis
- Penn and Teller Bullet Catch
- David Copperfield’s Death Saw
- David Copperfield Vanishing Statue of Liberty
- 1David Copperfield’s Death Saw
- 2Penn and Teller Bullet Catch
- 3The Pendragons Metamorphosis
- 4David Copperfield Vanishing Statue of Liberty
- Disappearing hand trick
- Flashed face distortion effect
- Color wagon wheel
- Colored dot / peripheral vs. central vision
- 1Disappearing hand trick
- 2Flashed face distortion effect
- 3Color wagon wheel
- 4Colored dot / peripheral vs. central vision
- Toy Story 3
- The Avengers
- Harry Potter and the Deathly Hollows Part 2
- 2The Avengers
- 3Toy Story 3
- 4Harry Potter and the Deathly Hollows Part 2
|Magic Tricks||Best Illusions||3-D Movies|
Three ways to get your sight into tip-top shape.
Our eyes are pretty amazing in their versatility. They’re able to see distant objects, people and places, and also things that are close to us as well. But in our modern civilization, in which we spend a lot more time staring at computer screens than, say, stalking mastodons on the horizon, our eyes and the muscles around them are subjected to a lot of unnatural strain. That’s why some vision-care professionals prescribe eye exercises for their patients. While the Harvard Medical School Family Health Guide cautions that visual training isn’t a panacea for all eye problems, it notes that they may help delay the need for eyeglasses or contact lenses in some people. “If your eyes are tired from excessive
close-up work—such as staring at the computer—visual breaks to focus on objects at longer distances are a good idea,” the publication explains. “And it’s important to encourage your visual system to do its best.”
Here are a few exercises that may help your sight:
This exercise is often prescribed by ophthalmologists to help correct binocular vision disorders, in which the eyes don’t work together correctly. Hold a pencil in front of you at arms’ length. Pull the pencil in slowly toward your nose, and follow it along the way with your eyes. Try to maintain a clear focus. When the pencil starts to morph into a double image, draw it away from the nose again. Do this several times in succession, several times a day.
Range of Motion
If you spend a lot of time looking in a particular direction—such as at a computer screen—it’s important to also spend a little time using your eyes over a fuller range of motion. Sit upright, while facing and looking forward. Without moving the your head, look up and down, and then look left and right. Repeat this exercise several times a day.
If you spend a lot of time staring at a close-up screen, you need to also spend some time looking at distant objects. Eye-care professionals often suggest the 20/20/20 rule. After 20 minutes of computer use, look at something at least 20 feet away for 20 seconds. Repeat this throughout your workday.