Helping the deaf hear

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Efforts to bring sound into the silent world of the deaf once focused on amplifying the intensity of sounds. It was hoped that causing an increase in the vibrations of hear cells would translate into increased stimulation of the auditory nerve. But some researchers have tackled the problem of deafness by bypassing the damaged or destroyed hear cells and transmitting electrical impulses directly to the nerve fibers. The result has been the cochlear implant, in which an array of electrodes is threaded into the cochlea and connected to a small receiver-stimulator implanted behind the ear. Sounds picked up by the receiver are processed by a pocket-sized computer, which, relying on the frequency theory of pitch, separates sound information into its components and applies them directly to specific points on the basilar membrane. The implant delivers information about tone, tempo, and intensity.

Profoundly deaf adults whose auditory systems still function beyond the cochlea seem most likely to benefit from these devices. After lengthy training, patients with implants find that they can detect most environmental sounds, such as a knock on the door, footsteps, running water, barking dogs, ringing telephones, whistling tea kettles, and crumpling paper. They can also hear environmental sounds that warn them of danger, such as a car horn or a shout. A substantial minority regain enough hearing to use the telephone, although the majority still can not decipher the sounds of human speech without reading lips. But even for these patients, lip-reading ability improves.

People who have been deaf from birth cannot use cochlear implants successfully, possibly because the structures in temporal cortex that usually process speech have been coopted for other functions in the congenitally deaf. Their best hope seems to be some kind of device based on the sense of touch. One such device is the “tickle belt”, developed by psychologists C. Sherman and B. Franklin. Small rectangular transducers that respond to various sound frequencies are mounted on a belt worn next to the skin. The transducers change sounds into brief busts of electricity, which the wearer senses as vibrations. High frequencies are felt at one end of the belt and low frequencies at the other; a work is felt as a pattern of stimulation moving across the belt. Of course, this sort of device requires the wearer to associate speech sounds with tactile sensations, which entails a great deal of training.

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