A cochlear implant is a surgically implantable device that provides hearing sensation to individuals with severe to profound hearing loss who cannot benefit from conventional hearing aids. By electrically stimulating the auditory nerve directly, a cochlear implant bypasses damaged or undeveloped sensory structures in the cochlea, thereby providing usable information about sound to the central auditory nervous system.

Although it has been known since the late 1700s that electrical stimulation can produce hearing sensations (see Simmons, 1966), it was not until the 1950s that the potential for true speech understanding was demonstrated. Clinical applications of cochlear implants were pioneered by research centers in the United States, Europe, and Australia. By the 1980s, cochlear implants had become a clinical reality, providing safe and effective speech perception benefit to adults with profound hearing impairment. Since that time, the devices have become more sophisticated, and the population that can benefit from implants has expanded to include children as well as adults with some residual hearing sensitivity (Wilson, 1993; Shannon, 1996; Loizou, 1998; Osberger and Koch, 2000).

The function of a cochlear implant is to provide hearing sensation to individuals with severe to profound hearing impairment. Typically, people with that level of impairment have absent or malfunctioning sensory cells in the cochlea. In a normal ear, sound energy is converted to mechanical energy by the middle ear, and the mechanical energy is then converted to mechanical fluid motion in the cochlea. Within the cochlea, the sensory cells—the inner and outer hair cells—are sensitive transducers that convert that mechanical fluid motion into electrical impulses in the auditory nerve. Cochlear implants are designed to substitute for the function of the middle ear, cochlear mechanical motion, and sensory cells, transforming sound energy into the electrical energy that will initiate impulses in the auditory nerve.

All cochlear implant systems comprise both internal and external components (Fig. 1). Sound enters the microphone, and the signal is then sent to the speech processor, which manipulates and converts the acoustic signal into a special code (i.e., speech-processing strategy). The transmitter, which is located inside the headpiece, sends the coded electrical signal to the internal components. The internal device contains the receiver, which decodes the signal from the speech processor, and an electrode array, which stimulates the cochlea with electrical current. The implanted electronics are encased in one of two biocompatible materials, titanium or ceramic. The entire system is powered by batteries located in the speech processor, which is worn on the body or behind the ear.

Figure 1..  

External and internal components of a cochlear implant system. Sound is picked up by the microphone located in the headpiece and converted into an electrical signal, which is then sent to the speech processor via a cable. The signal is encoded into a speech-processing strategy and is sent from the speech processor (body-worn or behind-the-ear) to the transmitter in the headpiece. The signal is transmitted to the internal receiver through transcutaneous inductive coupling of radio-frequency (RF) signals. The receiver/stimulator sends the signal to the electrodes, which stimulate the cochlea with electrical current.

The transmission link enables information to be sent from the external parts of the implant system to the implanted components. For all current systems, the connection is made through transcutaneous inductive coupling of radio-frequency (RF) signals. In this scheme, an RF carrier signal—in which the important code is embedded—is sent across the skin to the receiver. The receiver extracts the embedded code and determines the stimulation pattern for the electrodes. Most cochlear implant systems also employ back telemetry, which allows the internal components to send information back to the external speech processor to assess the function of the implanted electronics and electrode array.

The first cochlear implants consisted of a single electrode, but since the mid-1980s, nearly all devices have multiple electrodes contained in an array. Typically, cochlear implant electrodes are inserted longitudinally into the scala tympani of the cochlea to take potential advantage of the place-to-frequency coding mechanism used by the normal cochlea. Information about low-frequency sound is sent to electrodes at the apical end of the array, whereas information about high-frequency sounds is sent to electrodes nearer the base of the cochlea. The ability to take advantage of the place-frequency code is limited by the number and pattern of surviving auditory neurons in an impaired ear. Unfortunately, attempts to quantify neuronal survival with electrophysiologic or radiographic procedures before implantation have been unsuccessful (Abbas, 1993).

The first multi-electrode arrays were straight and thin (22 electrode bands on the 25-mm-long array) to minimize the occupied space within the scala tympani (Clark et al., 1983). Advances in electrode technology have led to the development of precurved, spiral-shaped arrays to follow the shape of the scala tympani, allowing the contacts to sit close to the target neurons (Fayad, Luxford, and Linthicum, 2000; Tykocinski et al., 2002). The advantages of the precurved array are an increase in spatial selectivity, a reduction in channel interaction, and a reduction in the current required to reach threshold and comfortable listening levels. In addition, the electrode contacts are oriented toward the spiral ganglion cells, and a “positioner,” a thin piece of Silastic, can be inserted behind the array to achieve even greater spatial selectivity and improve speech recognition performance (Zwolan et al., 2001).

For all systems, electrical current is passed between an active electrode and an indifferent electrode. If the active and indifferent electrodes are remote, the stimulation is termed monopolar. When the active and indifferent electrodes are close to each other, the stimulation is referred to as bipolar. Bipolar stimulation focuses the current within a restricted area and presumably stimulates a small localized population of auditory nerve fibers (Merzenich and White, 1977; van den Honert and Stypulkowski, 1987). Monopolar stimulation, on the other hand, spreads current over a wider area and a larger population of neurons. Less current is required to achieve adequate loudness levels with monopolar stimulation; more current is required for bipolar stimulation. The use of monopolar or bipolar stimulation is determined by the speech-processing strategy and each individual's response to electrical stimulation.

Two types of stimulation are currently used in cochlear implants, analog and pulsatile. Analog stimulation consists of electrical current that varies continuously in time. Pulsatile stimulation consists of trains of square-wave biphasic pulses. The pattern of stimulation can be either simultaneous or nonsimultaneous (sequential). With simultaneous stimulation, more than one electrode is stimulated at the same time. With nonsimultaneous stimulation, electrodes are stimulated in a specified sequence, one at a time. Typically, analog stimulation is simultaneous and pulsatile stimulation is sequential.

To represent speech faithfully, the coding strategy must reflect three parameters in its electrical stimulation code: frequency, amplitude, and time. Frequency (pitch) information is conveyed by the site of stimulation, amplitude (loudness) is encoded by the amplitude of the stimulus current, and temporal cues are conveyed by the rate and pattern of stimulation. The first multichannel devices extracted limited information from the acoustic input signal (Millar, Tong, and Clark, 1984). Advances in signal-processing technology have led to the development of more sophisticated processing schemes. One type of strategy is referred to as “n of m,” in which a specified number of electrodes out of a maximum number available are stimulated (Seligman and McDermott, 1995). With this type of processing, the incoming sound is analyzed to identify the filters (frequency regions) with the greatest amount of energy, and then a subset of filters is selected and the corresponding electrodes are stimulated.

In another approach, referred to as continuous interleaved sampling (CIS), trains of biphasic pulses are delivered to the electrodes in an interleaved or nonoverlapping fashion to minimize electrical field interactions between stimulated electrodes (Wilson et al., 1991). The amplitudes of the pulses delivered to each electrode are derived by modulating them with the envelopes of the corresponding bandpassed waveforms.

With analog stimulation, the incoming sound is separated into different frequency bands, compressed, and delivered to all electrodes simultaneously (Eddington, 1980). In the most recent implementation of this type of processing, the digitized signal is transmitted to the receiver; then, following digital-to-analog conversion, the analog waveforms are sent simultaneously to all electrodes (Kessler, 1999). Bipolar electrode coupling is typically used to limit the area over which electrical current spreads to reduce channel interaction, which is further reduced with the use of spiral-shaped electrodes.

Cochlear implant candidacy is determined only after comprehensive evaluations by a team of highly skilled professionals (see cochlear implants in adults: candidacy). The surgery is performed under general anesthesia and requires about 1–2 hours, either as an inpatient or outpatient procedure. Approximately 4 weeks following surgery, the individual returns to the clinic to be fitted with the external components of the system. Electrical threshold and most comfortable listening levels are determined for each electrode, and other psychophysical parameters of the speech-processing scheme are programmed into the speech processor. Multiple visits to the implant center are necessary during the first months of implant use as the individual grows accustomed to sound and as tolerance for loudness increases.

Most adults who acquire a severe to profound hearing loss after language is acquired (postlingual hearing loss) demonstrate dramatic improvements in speech understanding after relatively limited implant experience (Fig. 2). Improvements in technology have led to incremental improvements in benefit, which in turn have led to expanded inclusion criteria (Skinner et al., 1994; Osberger and Fisher, 1999). There are large individual differences in outcome, and although there is no reliable method to predict postimplant performance, age at onset of significant hearing loss, duration of the loss, and degree of preoperative residual hearing significantly affect speech recognition abilities (Tyler and Summerfield, 1996; Rubinstein et al., 1999). Many adults are able to converse on the telephone, and cochlear implants can improve the quality of life (Knutson et al., 1998). Adults with congenital or early-acquired deafness and children (see cochlear implants in children) also derive substantial benefit from cochlear implants.

Figure 2..  

Mean pre-and postimplant scores on speech perception tests for 51 adults with postlingual deafness. Performance was assessed on CNC monosyllabic words, Central Institute for the Deaf (CID) sentences, Hearing in Noise Test (HINT) sentences, and HINT sentences in background noise (+10 signal-to-noise ratio). Stimuli were recorded and presented in the sound field at 70 dB SPL.

Technological advances will continue, with higher processing speeds offering the potential to stimulate the auditory nerve fibers in a manner that more closely approximates that of normal hearing. Studies are under way to evaluate the benefit of bilateral implants (Gantz et al., 2002). New miniaturization processes will result in smaller behind-the-ear processors and, eventually, a fully implantable system with rechargeable battery technology. In the early days of cochlear implants, few people realized that this technology would become the most successful of all prostheses of the central nervous system.