A cochlear implant (CI) is an electronic auditory prosthesis, which is used to bypass damaged hair cells in the inner ear and directly stimulate the auditory nerve with an electrical signal, provided that this is intact. The system consists of two parts: an electrode, which is surgically inserted into the inner ear (also called the cochlea), and a speech processor worn behind the ear. Sound information is transmitted to the implant via a coil, which is attached to the head with a magnet. A CI is mainly suitable for children with congenital hearing loss, children or adults with prelingually acquired hearing loss and adults with severe to profound hearing loss. In the normal hearing process, sound is transmitted from the auricle down the ear canal to the eardrum. The eardrum vibrates and passes these vibrations to three small bones in the middle ear (called ossicles) and on to the fluid-filled snail-shaped cochlea, where the auditory sensory cells (called hair cells) are located. These cells have small hair-like extensions. The sound pressure waves travelling through the fluid-filled inner ear bend these extensions out of shape and thus trigger electrical currents within the hair cells. These currents are passed on to the connected auditory nerve fibres and transmitted to the brain for further processing. The majority of people who are severely hard of hearing or profoundly deaf have damaged hair cells, but in most cases their auditory nerve is intact.
- The sound processor microphone picks up the sound waves from the air.
- The sound processor converts the sound waves into digital signals.
- The magnetic transmitter (coil) uses inductive transmission to send the digital signals through the skin to the implant beneath.
- The implant converts the received digital information into electrical signals and sends these via the electrode carrier to the auditory nerve.
- The auditory nerve passes these impulses onto the brain, which recognises them as sound.