Musicians hear--and see--in tune.

• Position: Binocular Rivalry

Musicians do not just hear in tune; they also see in tune. That is the conclusion of an experiment designed to puzzle out how the brain creates an apparently seamless view of the external world based on the information it receives from the eyes.

"Our brain is remarkably efficient at putting us in touch with objects and events in our visual environment, indeed so good that the process seems automatic and effortless," says Randolph Blake, professor of psychology at Vanderbilt University, Nashville, Tenn. "In fact, the brain is continually operating like a clever detective, using clues to figure out what in the world we are looking at."

Scientists have known for some time that the brain exploits clues from sources outside of vision to figure out what we are seeing. For instance, we tend to see what we expect to see based on past experience. Moreover, we tend to see what our other senses tell us might be present in the world, including what we hear. A remarkable example of this kind of bisensory influence is a beguiling visual illusion created by sound: when a person views a single flash of light accompanied by a pair of beeps presented in close succession, the individual incorrectly perceives two flashes, not just one.

"In our study we asked just how abstract can this supplementary information be?" Blake relates. "Melodic sound enhances visual awareness of congruent musical notes, but only if...