It is a fact of neuroscience that everything we experience is 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 researchers 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 a still life.

Illusions are defined by the dissociation between the physical reality and our 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, 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 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.

ILLUSORY MOTION

 
COURTESY OF AKIYOSHI KITAOKA Ritsumeikan University

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.

3-D ILLUSIONS

 
ANDREA JEMOLO Corbis (left); COURTESY OF DARRIENNE THOBAVEN (center); RMN-Grand Palais/ART RESOURCE, NY (right)

Visual artists often try to imitate reality. Painters convey the illusion of 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.

Artistic replication was 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 (right).

The cupola of the St. Ignatius of Loyola church in Rome (center) 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, too, 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 (left).

FIGURES

 
© THE TRUSTEES OF THE BRITISH MUSEUM

This detail of a 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 also 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.

COLOR IN CONTEXT

 
COURTESY OF R. BEAU LOTTO University College London AND DALE PURVES Duke University

This image, created by R. Beau Lotto of University College London with 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.