THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

10.7 Prosthetics for Hearing 235external transmitterimplantedreceiver/stimulatormicrophone, battery packand speech processorFigure 10.8. The components of a cochlear implant. There are five main components,only two of which are inside the body. A microphone above the ear receives sound wavesthat are directed to a tiny computer behind the ear. The computer transforms the inputinto specifications for stimuli to be conveyed to the implanted electrodes. The disk-shapetransmitter uses high-bandwidth radio waves to send these instructions to a receiver justunder the skin. The receiver converts the instructions into electrical stimuli and relays themto the appropriate electrode in the cochlea, which, in turn, excites neurons in the auditorynerve. (Courtesy of Michael F. Dorman and American Scientist.)Adults who lost their hearing and later receive a cochlear implant can associatethe new stimulation patterns with their recall of what speech should sound like.Children born deaf do not have this advantage, but it has been found that congenitallydeaf children who received cochlear implants during their first or second yearcan learn spoken language as well as, or almost as well as, children with normalhearing. Children receiving implants later in life have greater difficulty in copingwith signals from an implant.Ongoing cochlear implant research is now focused on the combination of electricand acoustic stimulation (EAS). A number of hearing-impaired people have someability to hear low frequencies but little or no sensitivity to higher frequencies.When an electrode array is inserted about two thirds of the way into the cochlea,hearing at 1 kHz and above may possibly be restored by electrical stimulation.