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The Neuroscience of Illusion
How tricking the eye reveals the inner workings of the brain
The Neuroscience of Illusion
3-D ILLUSIONS The cupola of the St. Ignatius's church in Rome is a great example of Baroque illusionism. The architect of the church, Horace 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, the Jesuit artist Andrea Pozzo (1642-1709) 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 even then, the results he accomplished could hardly be believed. Even today, many visitors of St. Ignatius's are amazed to find out that the spectacular cupola is not real, but an illusion.
AMBIGUOUS FIGURES This bunch of violets contains the faces of Napoleon, his wife, and their child. Can you find them among the flowers? During Napoleon's exile, his supporters used to distribute reproductions of this 1815 engravings. In such illusions, the brain interprets same picture in two different ways, with each interpretation mutually exclusive of the other. You can see one of two possible images, but never both of them 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.
ILLUSORY MOTION Some stationary patterns generate the illusory perception of motion. The illusory effect is usually stronger if you move your eyes around the figure. For instance, in this illusion, invented by the scientist Akiyoshi Kitaoka, 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 on the center of each "snake," the motion will slow down or even stop. Because holding the eyes still stops the illusory motion, we speculate that eye movements are required to see it. Vision scientists have shown that illusory motion activates brain areas that are similar to those activated by real motion.
SHAPE DISTORTION ILLUSION This illusion is known as the Café Wall illusion, and it was first discovered by Richard Gregory's laboratory in a café in Bristol, in the U.K. The black and white tiles are perfectly straight, but look tilted. It is a shape distortion illusion: an object will appear to take on shapes that are different from its actual shape. Like brightness and color illusions, shape distortion effects are also 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.
BRIGHTNESS AND COLOR ILLUSIONS In this illusion, created by Edward Adelson at MIT, squares A and B are the same shade of gray. (If you don't believe it, print it out and then cut out the two squares and place them side by side.) This illusion 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
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!
Edward H. Adelson