The Cochlear Implant

The basic functioning of the CI is to bypass the workings of the outer and middle ear and use the surface of the cochlea to provide direct electrical stimulation to the auditory nerve. External to the body (normally worn on the ear) are a microphone and signal processor. The microphone converts the sound to an electrical waveform. The processor takes that waveform and develops appropriate electrical stimulus that needs to be transmitted to the auditory nerve. The signal is passed via wire to a transmitting coil, which sends a radio signal to the implant. Inside the body, a surgical procedure has implanted a receiving coil and a number of electrodes. These electrodes are connected from the receiver to various locations along the cochlea. Because the cochlea is location-sensitive to various frequencies, each of the electrodes can be associated with a frequency range. When the system is operating, the internal receiver takes the transmitted signal from the speech processor and sends the appropriate signal down each of the electrodes. The electrodes then can directly stimulate the nerve fibers by sending a small electrical current into the cochlea. These stimuli, transmitted to the brain, are interpreted as sound (see Figure 3.)

Figure 3. The Nucleus® cochlear implant system. (Left) External: (1) Microphone, (2) Speech processor (3) wire (4) Radio transmitting coil. (Right) Internal to body: (5) RF receiver (6) Electrodes (7) Auditory nerve - not part of implant.

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For animated movie providing descriptions of the function of normal hearing and cochlear implants, check out these links.