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FEATURE ARTICLE

The Design and Function of Cochlear Implants

Fusing medicine, neural science and engineering, these devices transform human speech into an electrical code that deafened ears can understand

Michael Dorman, Blake Wilson

Ludwig van Beethoven was 28 years old when he first noticed a ringing and buzzing in his ears. Soon he was unable to hear high notes from the orchestra; speech became indistinct. By 1802, four years after the first symptoms, he was profoundly deaf.

Beethoven fell into a deep depression. He describes this period in his Heiligenstadt Testament, meant to be read after his death:

For me there can be no relaxation in human society; no refined conversations, no mutual confidences. I must live quite alone and may creep into society only as often as sheer necessity demands.... Such experiences almost made me despair, and I was on the point of putting an end to my life—the only thing that held me back was my art ... thus I have dragged on this miserable existence.

In 2001, Scott N. was 34 and had lost all of his hearing. A surgeon inserted 16 tiny electrodes into his inner ear, or cochlea, and connected them to a small package of electronics implanted under the skin. A year later, Scott came to author Dorman's laboratory at Arizona State University to test his understanding of speech. The results were extraordinary: Scott recognized 100 percent of more than 1,400 words, either in sentences or alone, without any prior knowledge of the test items. 

As impressive as this performance was, the cochlear implant did not restore normal hearing to Scott. The electrode array produced a stimulus that was only a crude mimicry of the signals in a normal cochlea. But as this example shows, a very high level of functionality can be restored by a neural prosthesis that does not recreate the normal system. For the thousands of people who have received a cochlear implant, even an imperfect restoration of hearing reconnects them to the world of sound. And it allows many of them to use that most critical toy of modern life, the cell phone.

Figure 1. The cochlea converts sound waves...Click to Enlarge Image

Although cochlear implants have a 40–year history culminating in the current generation of high–performance devices, hearing restoration is not universally welcomed. Among members of the Deaf community, the absence of hearing is not necessarily viewed as a disability. Some deaf parents refuse implants for their deaf children, triggering an impassioned debate between those who agree and those who challenge the decision. This article avoids that controversy to focus on the science of cochlear implants. But recent findings have influenced the temperature, if not the substance, of the debate. As we point out, hearing must be restored at a very early age if speech and language skills are to develop at a normal rate. The decision to use or forgo the implant cannot wait until the child—who must bear the consequences—reaches the age of consent.




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