Optical Illusions: Sights for Sore Eyes
Moving Illusions and More
They've been called trompes de l'oeils - triumphs over the eye. They invite, or rather, force you to take a second, third, or fourth look at them. Some have become famous works of art. Learn about the different types of illusions that can trick your eyes into seeing things beyond the range of normal perception.
This image, for example, isn't moving at all. Don't believe it? Close one eye and focus in on any single colored area. Now what do you see instead?
Illusions were first noticed centuries ago, and have contributed to our entertainment ever since. Keep reading to learn how illusions have changed the world. Afterward, be sure to check out the page that highlights illusions that work because of the physical structure of our eyes and the one featuring perceptual & psychological ones.
First Optical Illusion
Optical illusions have existed since the dawn of time. A distant branch may look like a bird. The moon may look bigger at moonrise and smaller as night plunges on. By definition, an optical illusion, sometimes called a visual illusion, means we see something that differs from reality. This can be achieved using a variety of techniques.
Perhaps the earliest example of a human created example of an optical illusion was an image stamped onto Greek coinage. Believed to date from 550-450 BC, this coin depicts two boars facing each other in battle. Together, they create a ferocious boar's head facing the viewer. (Special thanks to ArchimedesLab.org for this terrific find - it's the only illusion site I found featuring this piece. Click on the picture at right to view their extensive collection there.)
By the 19th century, optical illusions flourished and laid the groundwork for popular inventions and scientific knowledge we enjoy today. Nonetheless, there isn't a universal system for classifying different types of illusions. Should the be described by their impact (double images, camouflage, psychedelic, geometric, and so on) or by what contributes to their effects (visual, perception, or combination?) For the purpose of simplicity, this article will use simple classifications used by Terry Jennings in his Jennings uses three basic categories: Sight illusions occur because of the way our eyes work, perception illusions occur because of the way our brains interpret information, and Movement illusions that may involve they way our eyes and brain interact. introductory book.
How Eye Illusions Work
One of the earliest and most simple illusions was illustrated by a line drawing of a cube. Louis Necker, a Swiss crystallographer, first published the transparent cube in 1832, asking people which cube face was closest to them. Because there were no clues enabling their depth perception to guide them, viewers saw the lower left square as being nearest, or the top right one. Some people perceived the two squares competing for frontal position.
Today, scientists believe that Necker cubes can reveal information about how we store memories and demonstrate our ability to direct attention effectively. They measure how often subjects see the frontal face change, something that should be tested after head injuries or when patients are experiencing severe stress or major disease according to the American Journal of Nursing.
Directed Attention Demonstration
- Necker Cube Directed Attention
Thorough discussion of Necker cubes and a free directed attention demonstration. (No registration required.)
This scintillating grid, based on work published by Ludimar Hermann, in the 1870s, has white dots placed where the lines intersect. Hermann grids don't have dots added, and a viewer would see dark circles appear where the lines crossed. In the scintillating grid, the white dots contribute to a sense that white and black dots are both present and in motion.
This effect is due to the way photoreceptors in the retina work. Some of these cells activate when exposed to light, and others are designed to respond to an absence of it. Black color lacks light-reflecting pigment, so it activates the dark-oriented photoreceptors. When both types of cells activate at the same time, they're forced to compete. A process known as lateral inhibition takes place as one tries to dominate the other, which can make our eyes misinterpret what we see.
Illusions for Your Walls
Moving Pictures Books and Toys
During the Victorian era, thaumatropes were a popular optical illusion toy. Invented in 1824, these simple devices consisted of two cards with two completely different (but related) images. When the cards are spun rapidly, the pictures merge to create another image.
The image shown here illustrates a thaumatrope with an empty birdcage on one side of the card, and a bird on another. When an elastic band is threaded between the cards to form handles for spinning, the little boy sees a bird in a cage.
Thaumatropes demonstrated the idea of persistence of vision. It was believed that the eye's retina retained an afterimage of what it saw briefly, though the theory was debunked in 1912. Nonetheless, thaumatropes led to other persistence of vision toys and laid the groundwork for cartoons and cinematography. Fantascopes, daedalums, and phasmatropes are just a few of the persistence of motion toys that developed after the invention of thaumatropes and contributed to the development of cameras that could capture images in sequential frames to reproduce movement.
Making Movies Without Pictures
These simple illusions are based on the same principles as thaumatropes. Can you think of one you could create?
More on How Illusions Contributed to Toys & Film
Persistance of Vision
When the persistence of vision theory was disproved, science sought to explain how our eyes and brain work together to create an illusion of persistent movement rather than perceiving a sequence of separate images. Known as the "phi phenomenon," the principles that underlie movie-making continues to be called persistence of vision, even though the original meaning of that phrase (that our retinas capture and retain an image after we've stopped seeing it) has been shown to be false.
First described by Max Wertheimer in 1912, the term phi phenomenon actually includes two closely related types of movement perception: phi and beta. Beta perception is what we see when we read moving letters on a light emitting diode (LED) display on our bank's marquee. The letters appear to move across the surface, even though we're simply seeing a series of flashes of lights. The actual position of the lights never move, even though different bulbs light up in sequence to create the illusion of movement. Phi phenomena are similar, and even experts have trouble classifying the difference.
Phi results from images that have no contour, always take on the color of their background, and are presented in faster sequences than is visible with the beta phenomenon. Notice the image here of lilac colored dots against a gray background. Twelve of these images, when flashed quickly, make it look like a gray dot is moving across each of the lilac points. (Click on the image to view it in motion.)
Though poorly understood a century after it was first described, the "apparent movement" created launched the Gestalt movement in psychology. One group of researchers at Purdue University have put together an explanation of the difference (with examples) that can be viewed by clicking the link below.
Phi vs. Beta - Still confusing after all these years
- Purdue University Discussion of the Phi Phenomenon
On this page, a link called "Phi is Not Beta" gives a scientific explanation of the difference between the phi and beta effects with examples.
Optical Illusions with Color
Our brains were probably intended for tracking movement, as the successful hunter must do if he wants to feed his tribe. This ancient characteristic is deeply ingrained, and makes us vulnerable to a variety of illusions. Cognitive illusions may be based on distortions, ambiguity (illusions that can be one thing or another), paradoxes, or fictions.
Distortions rely on our depth and color perceptions, combined with the way our brains work. In this image, for example, the squares marked A and B are exactly the same shade of gray.
Don't believe it? Our brains tell us that squares that are the same color should look the same. In fact, because it's shadowed, square B should appear darker than square A. By introducing a contrasting color and shadow, our mind is fooled.
Mind Teaser Illusions
Make Optical Illusions with Photoshop
By using a program like Adobe Photoshop or Corel Draw, it's possible to match the color of either square and connect them using the matched color.
What do you see now?
Essentially, our brains see what they anticipate. The same process is at work in distortion illusions.
Schizophrenics may be immune to this effect and see both shades accurately. To learn how illusions may detect undiagnosed schizophrenia, visit thy psychological illusions page I mentioned earlier.
Optical Illusions Drawings
Optical Illusion Art Relies on Colors and Shapes
Mark Changizi, a researcher at Rensselaer Polytechnical Institute, believes a perceptual lag is responsible for the way our brains misinterpret straight lines as bent in this illusion. Science has long recognized that our brains don't interpret what the eye sees until about a tenth of a second after an image hits our retina. Changizi proposes that our brains compensate for the lag. "Illusions occur when our brains attempt to perceive the future, and those perceptions don't match reality."
Changizi believes that our brains anticipate what the image will be when we move toward it. The effect can be seen with illusions like this one, known as the "cafe wall" because it was first noticed on... yep, the wall of a cafe in Bristol, England. The wall wasn't really poorly built - it just looked that way.
Many illusions are successful because of the way our visual system is structured. To see many fascinating examples and learn more, visit Sight Based Illusions.
Optical Illusions for the Brain
So far, we've looked at illusions that are the result of how our visual receptors can be fooled, and the way our brains and eyes interact to see something clearly and yet not see or understand it. The third basic category of illusions involves those clever images occur when our eyes accurately see and transmit data to our brains, but our brain believes it's seeing something else.
M.C. Escher, Salvador Dali, Bev Doolittle, and other well-known artists create works that tantalize our brains by showing us one thing while our mind sees another. Here, fans may see a man's profile or a couple pausing in the lane. (Look closely to see more illusions.)
This summary about the types of optical illusions describes how our eyes and brain work by themselves and together to process visions that are something other than what they appear to be. For more examples of illusions like these and how they're being used by marketers to influence our thinking, check out this page.
Expand Your Vision Every Day
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