How Does a Cochlear Implant Work

Speech and other sounds are picked up by the microphone and sent to the speech processor. The processor codes the sounds into an electrical signal which is sent via a cable to the transmitting coil. The coil passes the signal through the skin to the implant which transforms the signal to electrical pulses. The pulses pass from the electrode array and stimulate hearing nerve fibres within the cochlea.

The speech processor does not just make sounds louder as does a hearing aid. Instead, it selects out some of the important information in the speech signal and then produces a pattern of electrical pulses in the patient's ear. This pattern is selected to sound as close as possible to the original speech sound. It is not possible to make sounds completely natural, because there are only 22 electrodes that are replacing the function of tens of thousands of hair cells in a normally hearing ear.

The electrical patterns are different for each person and need to be programmed into the speech processor by a trained clinician. The differences arise because the electrodes are not always in the same position relative to the surviving nerves and the nerves vary in sensitivity to electrical currents. The clinician must measure the lowest and greatest current for every electrode to determine the softest and loudest sounds that will be heard. The different electrodes produce sounds with different pitch. The speech processor combines sounds on different electrodes with different loudness, to build up something as close to the original sound as possible.

The hearing process using a cochlear implant can be summarized as follows (please refer to Figure 3):

Figure 3. The hearing process using a cochlear implant

1. Sounds and speech are detected by the microphone.
2. The information from the microphone is sent to the speech processor.
3. The speech processor analyses the information and converts it into an electrical code.
4. The coded signal travels via a cable to the transmitting coil in the headset. Radio waves from the transmitter coil carry the coded signal through the skin to the implant inside.
5. The implant package decodes the signal. The signal contains information that determines how much electrical current will be sent to the different electrodes.
6. The appropriate amount of electrical current passes down the appropriate lead wires to the chosen electrodes.
7. The position of the stimulating electrodes within the cochlea will determine the frequency or pitch of the sounds. The amount of electrical current will determine the loudness of the sounds.
8. Once the nerve endings in the cochlea are stimulated, the message is sent up to the brain along the hearing nerve. The brain can then try to interpret the stimulation as a meaningful sound.