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hollow, fluid-filled structure shaped like a snail shell that resides in the inner ear. Uncoiled, it would stretch well over an inch, but in its natural state it is the size of a pea. Its nearly three full turns contain the nerve endings that make human hearing possible.The process begins when the sound vibrations detected by the eardrum enter the co- chlea, where they set internal structures into corresponding vibration. This in turn affects bundles of hairs growing out of sensing units called hair cells.Through a complex mechanical and electrochemical process, the motion of the hairs is converted into electrical signals that travel through the cochlear nerves to the auditory cortex, the part of the brain that interprets the signals as sound.

To perform the artificial equivalent of this natural process, a co- chlear implant is surgically embedded in the skull just behind the ear. An external microphone worn behind the ear picks up sound and sends it to a processor,also external.The processor amplifies the sound, filters out extraneous noises, and converts the result into digital elec- tronic impulses that go to a wireless transmitter worn behind the ear, which sends the pulses to a receiver implanted under the skin. The receiver picks up the signals and sends them along wiresup to 24 of thembundled into a narrow tube that has been woven into the cochlea. There, the digital signals stimulate the auditory nerves to produce neural impulses that are interpreted by the brain as specific sounds.The entire affair is operated by a small battery.

The cochlear implant restores a greater or lesser level of hearing in many deaf implantees.While the sensitivity of the device is too low to allow the listener to hear the very softest sounds, medium- to high- level sounds can be heard. Almost one-third of cochlear implantees hear spoken words clearly enough to use the telephone, and about half of implanted adults who knew how to speak before they lost their hearing can understand at least some words. Even those who do not hear speech clearly can benefit by combining sound cues from the implant with lipreading and other cues to improve their ability to communicate. In many cases, however, these enhancements require brief or sometimes extended training for the benefits to be realized.

Although cochlear implantation is the state of the art in neural

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