Sounds of Deception

SEATTLE--Everyone's familiar with optical illusions--those tricks that tell us not all seeing is believing. Now scientists are beginning to learn that our ears can be fooled in much the same way, as they demonstrated at the annual meeting of the American Association for the Advancement of Science (which publishes ScienceNOW). And just as optical illusions provide a window into the workings of the visual system, musical illusions are fast becoming a tool for investigating how our auditory system works.

At a session here, psychologist Diana Deutsch of the University of California, San Diego, showed how these illusions work by playing a tape of a repetitive message: "high, low, high, low, high, low." Within minutes, the message had metamorphosed into something different for many listeners in the room. For some, the words became "buy low, buy low," while others heard "boatman, boatman." Yet the words on the tape remained unaltered.

Another Deutsch-crafted illusion involves a steady oboe tone played in concert with a glissando, in this case a synthesized sine wave that glides up and down in pitch. The two sounds are repeatedly switched from ear to ear, such that when the oboe tone is heard in the right ear, the glissando is in the left, and vice versa. But that's not what listeners hear. Instead, the oboe tone appears to jump from ear to ear, while the glissando appears to rise and fall seamlessly in both ears. Moreover, righties and lefties hear this illusion differently. Righties generally hear the glissando moving through the scale from the left side to the right side of the head, while anecdotal evidence suggests that southpaws hear the reverse.

"I'd heard of the experiments before, but this is the first time I'd listened to them," says neuroscientist Patricia Kuhl of the University of Washington, who wants to see the illusions studied for their ability to shed light on the relationship between language learning and hearing. "This has piqued my interest enormously."

Deutsch adds that because handedness clearly correlates with how people hear the stereo illusions, speech and music may be based on "overlapping perceptual and cognitive structures." But most of all, she says, "the illusions show us that there aren't hard-and-fast rules for how we hear things, as much as we would like there to be."