Hearing impairment is a common disorder, affecting many children and adults. Its severity varies widely among individuals. Mild loss of hearing sensitivity can almost always be treated effectively by acoustic hearing aids. These devices electronically amplify weak sounds to enable people with hearing loss to hear most sounds comfortably. However, sometimes the degree of impairment in both ears is so severe that no amount of amplification can restore adequate hearing sensitivity. At present, the cochlear implant is the only effective form of treatment when profound hearing impairment results from extensive loss or dysfunction of hair cells. Located in the inner ear (or cochlea), hair cells convert the mechanical vibrations of sound waves into electrical impulses, which travel along specialized nerve fibers to the brain, where hearing sensations are perceived. They are susceptible to permanent damage from various causes, such as infections, exposure to excessive levels of noise, and physical trauma.
Many types of cochlear implant have been developed and evaluated. All are based on the same functional principle: to replace the role of the hair cells by delivering electric stimuli directly to the auditory nerve fibers. The stimuli are small electric currents conducted through electrodes that are placed inside the cochlea during a surgical operation. Most commonly, the stimuli comprise brief pulses of current that produce nerve impulses similar to those resulting from hair cell activity in acoustic hearing.
In a normal cochlea, there are many thousands of hair cells and nerve fibers. Researchers have found that the structures of the cochlea are organized in a regular pattern, such that some nerve fibers respond best to sounds having high frequencies (or high pitch), whereas others are more responsive to lower frequency sounds. Therefore, modern cochlear implants have an array of multiple electrodes that are positioned to elicit hearing sensations that vary in pitch.
In addition to the electrode array, a miniature electronic device is surgically placed under the skin near the implanted ear. This device receives instructions transmitted from an external sound processor that controls how much current should be delivered by each electrode over time to evoke the intended sensations of hearing. Generally, the perceived loudness depends on the current level and the pitch depends on the location of the active electrode, as well as the rate at which the current pulses are presented.
The sound processor is usually worn on the external ear and has a similar appearance to a conventional hearing aid. Its transmitter is magnetically coupled to the implanted receiver so that information is conveyed across the intact skin. The implant also receives electrical power via this coupling. Thus, the batteries required by the system are located in the external processor, where they may be replaced easily.
The processor picks up sound with a microphone that converts acoustic waves into electric signals. These signals are continuously analyzed by dividing them into separate frequency bands. Typically, the number of bands equals the number of implanted electrodes. The level in each frequency band controls the current level of the pulses delivered by the associated electrode. By this means, the frequency components comprising a complex sound are transformed into a corresponding pattern of electric nerve stimulation. Consequently, the cochlear implant user experiences hearing sensations that have variations of pitch and loudness related appropriately to the original sound.
The performance of cochlear implants has improved steadily since the earliest practical devices were developed in the 1970s. Much of the improvement is a product of progress in sound processor design. With the latest devices, most implant recipients can understand speech and recognize many other sounds, at least in favorable listening conditions. However, performance is poorer in situations with high levels of background noise, and perception of music is often unsatisfactory. Future advances will enable cochlear implants to provide better hearing of all types of sounds to people who obtain insufficient benefit from acoustic hearing aids.
References:
- Advanced Bionics Corporation, http://www.advanced.com
- Clark, G. , Tong, Y. C., & Patrick, J. F. (Eds.). (1990). Cochlear prostheses. Edinburgh, UK: Churchill Livingstone. Cochlear, Inc., http://www.cochlear.com
- MED-EL, http://www.medel.com
- Schindler, A. (1999). Description of the Clarion Multi Strategy Cochlear Implant. Annals of Otology, Rhinology, and Laryngology, 108(Suppl. 177, Part 2).