Cochlear Implant

According to the National Institutes of Health–National Institute on Deafness and Other Communication Disorders, approximately 41,500 adults and 25,500 children have received cochlear implants in the United States. A cochlear implant is an electronic auditory prosthesis designed to provide individuals with severe to profound hearing loss access to speech and spoken language. An external microphone picks up and converts sound into an electrical signal. A speech or sound processor then manipulates the electrical signal based on one's understanding of speech perception. An external transmitter passes the electrical signal to an internal receiver, which then sends the electrical signal to an array of electrodes located in the cochlea that stimulate auditory nerve fibers. Although there is substantial variability in outcomes across individual cochlear implant recipients, cochlear implants generally transmit speech signals well enough for speech and spoken language understanding. However, cochlear implants do not transmit suprasegmental information, such as pitch and timbre, very well, leading to potential deficits in music perception and production.

There are several reasons why cochlear implants are poorer at transmitting pitch and timbre than traditional speech cues. One important factor is that the electrode array cannot be fully inserted into the cochlea. A typical electrode array will span one and one-half of the possible two and one-half turns of the cochlea. If there is ossification, surgeons can drill only a straight line into the cochlea, resulting in even fewer inserted electrodes. This means that the cochlear implant will transmit all of the environmental frequencies onto a limited portion of the basilar membrane of the cochlea, and the likelihood of these frequency bands matching the tonotopic portion of the basilar membrane is very low. In other words, the electrical stimulation is both frequency-compressed and frequency-shifted. Moreover, there are at most 24 electrodes providing the stimulation that approximately 13,000 inner ear hair cells would provide in a normal-hearing ear, and there are overlapping regions of stimulation from adjacent electrodes. Finally, there is a lack of fine-temporal structure in the stimulating pattern of electrical activity. These factors, among others, converge to provide limited pitch and timbre information to cochlear implant recipients. Nevertheless, cochlear implant users continue to report listening to and participating in music. What do they hear when they listen to music?