Resource: Gallery of Visual Illusions

Humans and other sighted animals rely heavily on their sense of sight to make their way in the world. Our eyes enable us to run, jump, find food, and otherwise move around, hopefully without incident, in an environment filled with potential dangers. In fact, our eyes perform reliably so much of the time that optical illusions of any kind tend to puzzle us, whether they are distortions of size, shape, direction, motion, or perspective. Most people, especially children, assume that illusions are the result of sophisticated trickery, rather than simple misperception.

The eye is responsible for gathering visual information from an infinitely complex world. The three-dimensional environment of moving images, colors, and variations in perspective poses a daunting challenge for our visual system. As complex as this task is, the eye collects visual information in a strikingly simple way. Light reflected off of objects enters the eye and stimulates cells on the two-dimensional surface of the retina. These cells then send electrical signals to the brain in patterns that reflect the intensity, placement, and timing of the light received. It is the role of the brain to create images from these signals based on past experiences.

Although optical illusions sometimes result from slight imperfections in parts of the eye, especially the retina, it is more likely that the brain has misinterpreted visual signals. The interpretation of an infinite variety of visual information is an exceedingly complex process—so complex, according to vision experts, that the brain could not possibly rely on a single formula for processing such varied information. Instead, it is thought that the brain creates a number of possible scenarios based on signals from the eyes, and then chooses the most probable of these possibilities based on past experience. Given this explanation, optical illusions occur when the chosen interpretation—the one that we "see"—differs from the objects and light patterns that actually exist in the real world.

To learn more about how the eye and brain work together to enable animals to see, check out More Than an Image.

To learn more about the anatomy of the eye, check out Cow's Eye Dissection.

To learn more about the function of various regions of the brain, check out Brain Geography.

To learn more about the complexity of the human eye and how it likely evolved, check out Evolution of the Eye and Life's Grand Design.

• What is an optical, or visual, illusion? How would you explain the difference between perception and reality?
• When the moon appears just over the horizon, it looks larger than it does when we see it higher in the sky. Why do you think this might be so? How could you convince yourself that the moon is the same size no matter where we see it?
• Try to create an optical illusion similar to one of the ones in this interactive. Why do you think your optical illusion will work?