The sense of hearing is so complex that it is not surprising to know that deafness can take multiple forms. For example, the middle ear may fail to transmit sounds to the inner ear, or the inner ear or auditory nerve may not function, or the brain may not respond to input from the nerve.
What causes hearing loss? Some babies are born deaf because of a genetic mutation, chromosomal abnormality, or prenatal exposure to disease. Other people lose their hearing later because of disease, exposure to loud noise, or injury. Earwax or an ear infection can cause short-term deafness. And nearly everyone suffers some hearing loss later in life as the ear becomes less sensitive to higher frequencies.
Hearing aids can sometimes help treat hearing loss. By amplifying sounds, a hearing aid moves the eardrum more than normal, helping the person hear more clearly. If the middle ear cannot transmit sound, however, a conventional hearing aid is useless. Bone-conduction aids solve this problem by transmitting sound waves to the bones of the skull. The vibrations stimulate the cochlea directly, bypassing the middle ear.
A cochlear implant may restore some hearing to a person who is profoundly deaf (figure above). A surgeon places the device under the skin behind the ear. A microphone in the implant picks up sound; a processor then decomposes it into separate frequency components. Electrodes placed directly in the cochlea stimulate the parts of the auditory nerve corresponding to each frequency. By sending signals directly to the nervous system, cochlear implants compensate for nonfunctioning parts of the middle and inner ear.