Ask Dr K: Science behind cochlear implants

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DEAR DOCTOR K >> I'm hard of hearing and want to learn more about cochlear implants. Are they a type of hearing aid?

DEAR READER >> A cochlear implant involves several small electronic devices that are surgically implanted in the ear. It can provide sound to people who are severely hard of hearing or deaf. It is not a type of hearing aid. In fact, to be eligible for a cochlear implant, a person must have hearing loss in both ears that is so extreme that even the best hearing aid has little or no effect.

To explain a cochlear implant, I first need to explain how the ear works. Sound waves travel into the ear canal. (When you get wax in your ear, it's in the ear canal.) The sound waves then hit a thin membrane called the eardrum, causing it to vibrate.

Behind the eardrum are several tiny bones. They begin to vibrate, and send the vibrations to a snail-shaped organ deep inside the ear: the cochlea (pronounced COKE-lee-ah). The vibrations then create waves in fluid that is inside the cochlea. Those fluid waves move tiny little hairs. When moved, the hairs send signals to the brain, along the main hearing nerve, the auditory nerve. The brain interprets those signals — and we hear.

Cochlear implant surgery bypasses most of this whole complicated process. It involves a small device worn above the ear, another device surgically implanted under the skin above the ear, and tiny wires that are surgically placed into the cochlea.

The devices above the ear and under the skin include a tiny microphone to capture sound waves and convert them into electrical signals. Those electrical signals get transmitted to the wires that lead into, and stimulate, the cochlea. The cochlea then sends signals to the brain along the main hearing nerve.

In other words, a cochlear implant bypasses the sick parts of the normal hearing apparatus: the eardrum, the little bones, and the fluid that moves the tiny hairs inside the cochlea. And it uses the healthy hearing nerve to transmit signals from the cochlea to the brain.

A person must learn how to interpret sounds through the implant. Learning to use a cochlear implant is hard work and can take months or years. Restoring hearing with a cochlear implant may help prevent several complications of living without hearing, which include depression and slowed thinking.

What led to the development of this miraculous new technology? Research, by many people. In particular, someone had to discover how the cochlea works; that discovery was honored with the Nobel Prize. Someone had to discover how to electrically stimulate the cochlea, and how the cochlea sends signals to the brain that it can understand. Finally, scientists had to invent miniaturized electronics that the cochlear implant uses, and the surgical techniques to place the electronics inside the body.

In short, it was research that has given the gift of hearing to thousands of people who had lost it, or never had it.

Dr. Komaroff is a physician and professor at Harvard Medical School. To send questions, go to AskDoctorK.com, or write: Ask Doctor K, 10 Shattuck St., Second Floor, Boston, MA 02115.


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