We can recognize a friend instantly—full-face, in profile, or even by the back of his head. We can distinguish millions of shades of color, as well as 10,000 smells. We can feel a feather as it brushes our skin, hear the faint rustle of a leaf. It all seems so effortless: we open our eyes or ears and let the world stream in.

Yet anything we see, hear, feel, smell, or taste requires billions of nerve cells to flash urgent messages along cross-linked pathways and feedback loops in our brains, performing intricate calculations that scientists have only begun to decipher.

"You can think of sensory systems as little scientists that generate hypotheses about the world," says Anthony Movshon, an HHMI investigator at New York University. Where did that sound come from? What color is this, really? The brain makes an educated guess, based on the information at hand and on some simple assumptions.

When you look at the illustration to the right, for instance, you see an X made of spheres surrounded by cavities. But if you turn the image upside down, all the cavities become spheres, and vice versa. In each case, the shapes seem real because "your brain assumes there is a single light source—and that this light comes from above," says Vilayanur Ramachandran, a professor of neuroscience at the University of California, San Diego. As he points out, this is a good rule of thumb in our sunlit world.

To resolve ambiguities and make sense of the world, the brain also creates shapes from incomplete data, Ramachandran says. He likes to show an apparent triangle that was developed by the Italian psychologist Gaetano Kanizsa (see right). If you hide part of this picture, depriving the brain of certain clues it uses to form conclusions, the large white triangle disappears.

We construct such images unconsciously and very rapidly. Our brains are just as fertile when we use our other senses. In moments of anxiety, for instance, we sometimes "hear things" that are not really there. But suppose a leopard approached, half-hidden in the jungle—then our ability to make patterns out of incomplete sights, sounds, or smells could save our lives.

— Maya Pines

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The black line in the back seems much longer than the one in the front because your brain assumes it is seeing the effects of perspective. When the background is removed, the lines are seen to be equal.

The shaded circles seem to form an X made of spheres. But if you rotate the image 180°, the same circles form an X made of cavities, since the brain assumes that light comes from above.

Are these triangles real? They appear to be, because the brain automatically fills in lines that are missing. But if you block out parts of the picture, the white triangle vanishes.

	A Language the Brain Can Understand

Illustrations: Mark R. Holmes, ©National Geographic Society

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