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See Inside October/November 2008

A Perspective on 3-D Illusions

Paint and architectural illusions provide clues to how your brain reconstructs 3-D images


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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 enter the brain. But the same neural machinery that interprets actual sensory inputs 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 neuroscientists use to understand how the brain creates its sense of reality is the illusion. Historically, artists as well as illusionists have used illusions to develop deep 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 “trick” the brain into thinking that a flat canvas was three-dimensional or that a series of brushstrokes was actually a still life. Applied to architecture, their work continues to astound.

Visual illusions are defined by the dissociation between physical reality and subjective perception of an object or event. When we experience such an illusion, we may see something that is not there, or fail to see something that is there, or even see something different from what is there. Because of this disconnect between perception and reality, these optical tricks 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 the brain uses to construct visual experience.

Your Lying Eyes
Visual artists often try to imitate reality closely. Realistic painters convey the illusion of reality, volume or distance by making good intuitive use of perspective, color, lighting and shadow. When they are successful, the creation is sometimes difficult to distinguish from the model. Pliny the Elder, in his Natural History, narrates the legendary competition between two renowned painters in ancient Greece: Zeuxis and Parrhasios. 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 Parrhasios’s painting to confirm the superiority of his work. He was defeated, however, because the curtain he tried to pull back was Parrhasios’s painting itself.

Realism in paintings did not start in ancient Greece. Even prehistoric painters used tricks to make their works appear more realistic. For instance, the Altamira bison are strategically painted over bulges of the rock, which enhances the impression of the beasts’ volume (a).

Such techniques were carried to the limit in trompe l’oeil. Trompe l’oeil, sometimes called illusionism, is 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.

The cupola of the church of St. Ignatius of Loyola in Rome is a great example of Baroque illusionism (b). 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. Pozzo was already considered a master in the art of perspective, but the results he accomplished still could hardly be believed. Even today many visitors to St. Ignatius are amazed to find out that the magnificent cupola is not real, but an illusion.

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