The folds of the outer ear--the pinna--seem gratuitous, a waste of cartilage and skin. But they are, some Dutch researchers have found, crucial to our ability to pinpoint the vertical location of sounds--whether they come from above us, below, in front, or behind. When a sound wave enters the outer ear, it bounces around the pinna's ridges. Some frequencies are enhanced, others diminished, with the pattern of those changes determined by the particular shape of the pinna. Since everyone's pinnae are different, so is the acoustic stamp placed on sound entering the brain.
Biophysicist John van Opstal, who works at the University of Nijmegen in The Netherlands, and his colleagues placed plastic molds inside their own ears (and those of a student) to alter the shape of their pinnae and wore them day and night for six weeks.
The new "earprint," Van Opstal surmised, would have to be learned by the brain if he and his colleagues were to retain their normal hearing. (The brain determines horizontal distance to a sound's source by measuring the differences between the two ears in the arrival time and loudness of the sound waves.) Initially, in tests in a darkened laboratory, none of the subjects could determine the vertical source of a sound. "If you position the source of a sound at many random locations--up and to the right, down and to the left--the subject thinks that the sound is moving only in the horizontal direction and not at all in the vertical," says Van Opstal. But after four or five days, three of the subjects began to regain their normal hearing. (The fourth took almost three weeks.)
Eventually, the hearing of all four returned to normal, and remained so when they took out the molds, which suggests that the brain can learn and retain many auditory maps. Interestingly, the subjects noticed no difference in their hearing during the experiment while they were outside the lab, even at the beginning of the trial. "We think the reason is that our visual system tells us where a sound is coming from, although our auditory system tells us something different," Van Opstal says. "So the visual system is the driving force that trains the auditory system, by correlating what is seen with what is heard."
