Cochlear implants (CIs) are one of the most successful neuroprosthetic devices to date. When the first CIs were implanted in people in the 1970s, most scientists thought they would only be able to serve as an aid to lip-reading; it was nearly inconceivable that implanted patients would be capable of understanding any speech without reading lips. The first CIs were huge – they used a giant rack of speech processing computer equipment, and were cumbersome and prone to infection.
Modern CIs are a true biomedical engineering success story. Due to engineering advances, contemporary CI speech processors fit comfortably behind the ear and communicate with internal hardware that is implanted in the patient via a wireless transmitted and receiver. Moreover, most patients implanted today, who may be completely deaf without the device, have significant speech recognition and can even talk on the telephone! Most children implanted today develop language very well, enjoy music, and attend mainstream schools!
In this post, I’ll describe a bit about how the cochlea, which translates sound waves into electrical signals that go to the brain, works. Then, I’ll describe how a CI works for people who are profoundly or severely hard of hearing.
Sound Reception in the Normal Cochlea
The initial site of sound reception in the inner ear is the cochlea. The cochlea is a bony, spiral-shaped cavity. The cochlea is filled with fluid, which moves in response to incoming sound vibrations. The cochlea includes a structure called the basilar membrane, which runs along the length of the cochlea, and the basilar membrane vibrates as the fluid in the cochlea moves. The basilar membrane is special in that it vibrates the most at different locations depending on the frequency of the incoming sound – so, the basilar membrane vibrates the most at one end of the cochlea for low-pitched sounds and it vibrates the most at the other end of the cochlea for high-pitched sounds.
The basilar membrane also serves as a base for approximate 3,000 inner hair cells. As the basilar membrane vibrates in a particular location depending on the pitch of a sound, corresponding inner hair cells deflect. This deflection triggers a cascade of reactions, which causes a nerve signal to originate and be transmitted to the brain. The location at which the nerve signal originates indicates the pitch of the sound.
The key takeaway is that the cochlea is tonotopically organized – the location along the length of the cochlea at which the nerve signal originates indicates the frequency (or pitch) of the incoming sound.
Sensorineural Hearing Loss
Sensorineural hearing loss occurs when the hair cells of the inner ear die or are otherwise not functional. If there are no or fewer inner hair cells at a particular location in the cochlea, it’s harder for nerve signals that encode the corresponding pitches to be generated and transmitted to the brain.
Hearing aids can be beneficial to people with mild to moderate hearing loss. Hearing aids amplify sounds to stimulate the remaining hair cells. However, when a person has lost most of their hair cells and there is little left to stimulate, hearing aids may be of no benefit. In that case, even though most of the hair cells have died, the auditory nerve might still be healthy. Such a patient might benefit from a CI, which bypasses the hair cells and directly stimulates the auditory nerve fibers. CIs have become standard clinical treatment for patients with profound hearing loss in both ears, and over 200,000 people worldwide have received CIs.
As described above, CIs bypass the hair cells of the cochlea and directly stimulate the auditory nerve. The device consists of an external microphone and signal (speech) processor and an internal electrode array. The microphone, which is located either behind the ear or on a coil magnetically attached to the scalp, picks up nearby sounds.
The speech processor then analyzes the incoming sounds from the microphone to determine how the electrodes within the cochlea should stimulate the auditory nerve. Since the cochlea is tonotopically organized, as described above, each electrode can be used to encode a different frequency, or pitch. The speech processor analyzes the sound detected from the microphone to determine what frequencies are present in the sound and the energy at each frequency. For example, the sound of a bird chirping might have a lot of energy at high frequencies (high pitches), which might cause higher numbered electrodes in the CI to be stimulated.
A transmitter then sends the sound processed information, which indicates how the electrodes should be stimulated, to a receiver, which is implanted in the patient. The transmitter (outside the body) and receiver (implanted inside the body) are magnetically coupled, which means that the information can be sent without a physical connection (reducing the risk of infection).
The electrodes are then stimulated according to the analysis performed by the speech processor. Modern CIs have between 12 and 22 electrodes, depending on the device manufacturer. When a single electrode is stimulated, the patient hears a sound, whose pitch depends on the location of the electrode – an electrode stimulated at one end of the cochlea will cause a low-pitched sound, and an electrode stimulated at the other end of the cochlea will cause a high-pitched sound. When a complex sound like speech is analyzed by the speech processor, lots of electrodes will be stimulated in rapid succession to convey the many frequencies that typical speech has!