Understanding the cadences and accents of a foreign language can take years. But getting used to a new pair of ears turns out to be much easier. People who wear fake ears that disrupt the ability to pinpoint a sound's origin--not that many people fit this description, mind you--can regain the ability to accurately locate a sound-emitting object in the dark after just a few weeks. The finding, reported in next month's issue of Nature Neuroscience, suggests that the human brain can hold several auditory "maps" at the same time.
The brain doesn't seem to need the outer ear, the part that juts from our heads, to tell whether a sound emanates from the left or the right--it simply measures the time difference between a sound's arrival at each ear. But distinguishing "up" from "down" is thought to depend on the brain's ability to sort out complex echoes and distortions in sound waves as they bounce off the outer ear's ridges and folds, called pinnae. To see how quickly the brain would adapt to a new set of ears, John Van Opstal, a biophysicist at the University of Nijmegen in the Netherlands, and two colleagues fashioned pairs of fake plastic ears with altered ridges. The three researchers and one volunteer wore the ears for up to 6 weeks. "It looked crazy," Van Opstal says. "People would see me at a party and say, 'Is there something wrong with him?' "
The wearer was otherwise largely unaware of the ears, Van Opstal says, because humans tend to rely on their eyes for locating things. To single out the effect of the ears, they had to take a hearing test in a dark room. The team rigged a robotic arm with a speaker that moved to random locations and emitted short bursts of white noise. ("Pffft" is how Van Opstal describes it.) During the first week, the researchers found they could tell right from left as well as they could with their real ears, but they could not accurately judge a sound's altitude. "You have a very good idea where you think it is," he says, "only its wrong. It's very wrong."
After a few weeks, they did as well with the plastic ears as they had with their flesh ones. And when they finally removed the ears, it took no time to readjust. "That's quite astounding," says Fred Wightman, a psychologist at the University of Wisconsin, Madison, pointing out that it can take as long to readjust from distorting eyeglasses as it takes to get used to them. The finding may mean that locating objects by sound is a more complicated cognitive function than by vision, he speculates.