It is a fact of neuroscience that everything we experience is actually a figment of our imagination. Although our sensations feel accurate and truthful, they do not necessarily reproduce the physical reality of the outside world. Of course, many experiences in daily life reflect the physical stimuli that send signals to the brain. But the same neural machinery that interprets inputs from our eyes, ears and other sensory organs is also responsible for our dreams, delusions and failings of memory. In other words, the real and the imagined share a physical source in the brain. So take a lesson from Socrates: “All I know is that I know nothing.”

One of the most important tools used by neuroscientists to understand how the brain creates its sense of reality is the visual illusion. Historically, artists as well as illusionists have used illusions to gain insights into the inner workings of the visual system. Long before scientists were studying the properties of neurons, artists had devised a series of techniques to deceive the brain into thinking that a flat canvas was three-dimensional or that a series of brushstrokes was indeed a still life.

Visual illusions are defined by the dissociation between the physical reality and the subjective perception of an object or event. When we experience a visual illusion, we may see something that is not there or fail to see something that is there. Because of this disconnect between perception and reality, visual illusions demonstrate the ways in which the brain can fail to re-create the physical world. By studying these failings, we can learn about the computational methods used by the brain to construct visual experience.

Brightness, color, shading, eye movement and other factors can have powerful effects on what we “see.” In this series of images, we showcase several basic categories of visual illusions and what they can teach us about perception in the brain.

In this illusion, created by vision scientist Edward H. Adelson of the Massachusetts Institute of Technology, squares a and b are the same shade of gray. (If you don't believe it, cut out the two squares and place them side by side.) This trick of the eye occurs because our brain does not directly perceive the true colors and brightness of objects in the world but instead compares the color and brightness of a given item with others in its vicinity. For instance, the same gray square will look lighter when surrounded by black than when it is surrounded by white.

Another example: when you read printed text on a page under indoor lighting, the amount of light reflected by the white space on the page is lower than the amount of light that would be reflected by the black letters in direct sunlight. Your brain doesn't really care about actual light levels, though, and instead interprets the letters as black because they remain darker than the rest of the page, no matter the lighting conditions. In other words, every newspaper is also a visual illusion!

Some stationary patterns generate the illusory perception of motion. This unsettling effect is usually stronger if you move your eyes around the figure. For instance, in this illusion created by Akiyoshi Kitaoka, a professor of psychology at Ritsumeikan University in Japan, the “snakes” appear to rotate. But nothing is really moving other than your eyes!

If you hold your gaze steady on one of the black dots in the center of each “snake,” the motion will slow down or even stop. Because holding the eyes still stops the false sense of motion, eye movements must be required to see it. Vision scientists have shown that illusory motion activates brain areas that are similar to those activated by real motion.

This illusion, created by Beau Lotto and Dale Purves of Duke University, is another example of how the brain can perceive the same color differently when viewed in a different context. The central brown square on the top of the cube is exactly the same color as the central orange square on the side of the cube facing the viewer. The latter square looks orange because the lighting and surrounding squares make it appear brighter than the brown square in the mind's eye.

This bunch of violets contains the faces of Napoleon Bonaparte, Marie Louise of Austria and their son. Can you find them among the flowers? Napoleon's admiring troops gave him the name of “Petit Caporal,” or “Little Corporal”: their leader's short stature had not prevented him from defeating four armies larger than his own during his very first campaign. Years later, when Bonaparte was banished to the isle of Elba, he told his friends he would return with the violets, thus earning the nickname of “Corporal Violet, the little flower that returns with spring.” When he broke his imposed exile to return to France, women supporters assembled to sell violets. They would ask passersby, “Do you like violets?” Answering “oui” indicated that the person was not a confederate; “eh bien” signaled that the respondent adhered to Napoleon's cause. Napoleon's supporters distributed reproductions of this 1815 engraving.

In ambiguous illusions such as this one, the brain interprets the same picture in two different ways, with the two interpretations being mutually exclusive. You can see one of two possible images, but not both at the same time.

These so-called ambiguous figures are especially powerful tools to dissociate the subjective perception from the physical world. The physical object never changes, yet our perception alternates between two (or more) possible interpretations. For this reason, ambiguous illusions are used by many laboratories in the search for the neural correlates of consciousness.

The visual oddity above, known as the café wall illusion, was discovered on the exterior of a small restaurant near Richard Gregory's psychology laboratory in Bristol, England. (The photograph, taken a few months ago, shows Gregory outside the café.) Steve Simpson, a member of Gregory's lab at the time, noticed that the parallel grout lines between the green and white tiles on the wall appeared to be tilted, even though the tiles were actually straight.

Scientists use a simplified black-and-white version of the café wall illusion (above, center) to demonstrate how objects or patterns can appear to take on shapes that are different from their true physical form. The illusion works only when the contrasting black and white “tiles” are offset and when every tile is surrounded by a border of gray “grout.” Because different types of neurons in the brain react to the dark and light shades of the tiles, the grout appears to be dimmer in some places and brighter in others—and the brain interprets this contrast as a sloping line.

As with brightness and color illusions, shape distortion effects are produced by the interaction between the actual shape of the object and the shapes of nearby figures. For the brain, perception is very often dependent on context.

In another illusion, created by Kitaoka, a circular section of black-and-white tiled “floor” appears to bulge out toward the viewer, even though the image contains nothing but perfect squares—and all the floor “tiles” are of equal size (above, right). As with the café wall, this geometric illusion is an example of shape distortion. The smaller, contrasting squares provide context that deceives the brain.

Visual artists often try to imitate reality closely. Painters convey the illusion of reality, volume or distance by making intuitive use of perspective, color, lighting and shadow. When they are successful, the artwork is sometimes difficult to distinguish from the subject itself.

Pliny the Elder, in his Natural History encyclopedia, narrated the legendary competition between two renowned painters in ancient Greece: Zeuxis and Parrhasius. Each of the artists brought a covered painting to the contest. Zeuxis uncovered his work: he had painted grapes so realistic that birds flew from the sky to peck at them. Convinced of his victory, Zeuxis tried to uncover Parrhasius's painting to confirm the superiority of his work. He was defeated, however, because the curtain he tried to pull back was Parrhasius's painting itself.

Such techniques were carried to the limit in trompe l'oeil, a French term that means “to trick the eye.” This style of photographic realism first appeared in the Renaissance and flourished in the 17th century in the Netherlands. The lifelike pictures sometimes appeared to literally jump from the frame. In The Attributes of the Painter, a 17th-century work by Cornelius N. Gysbrechts, a painting appears to curl off the artist's easel (above, left).

The cupola of the St. Ignatius of Loyola church in Rome (above, right) is a great example of Baroque illusionism. The architect of the church, Orazio Grassi, had originally planned to build a cupola but died before finishing the church, and the money was used for something else. Thirty years later, in 1685, Jesuit artist Andrea Pozzo was asked to paint a fake dome on the ceiling over the altar. Although Pozzo was already considered a master in the art of perspective, the results he accomplished could hardly be believed. Even today many visitors to the church are amazed to find out that the spectacular cupola is not real but an illusion.

Architects soon realized that they could manipulate reality by warping perspective and depth cues to create illusory structures that defied perception. Need a big room in a small space? No problem. Francesco Borromini accomplished just that at the Palazzo Spada, a palace in Rome (below, right). Borromini created this spectacular trompe l'oeil illusion of a 121-foot-long courtyard gallery in a 28-foot-long space. There is even a life-size sculpture at the end of the archway. Not really. The sculpture looks life-size but is actually just two feet tall.