We hear when the cochlea, in the inner ear, stimulates the auditory nerve. Deafness occurs most commonly when tiny hair cells inside the cochlea are damaged as a result of a genetic defect, infection, loud noise or aging. Cochlear implants bypass the damage by receiving and converting sound into signals sent along electrodes to cells adjacent to the auditory nerve.

Worldwide, more than 40,000 children and adults depend on cochlear implants. Certain hair cells lining the cochlear ducts alongside auditory neurons are tuned to respond to specific frequencies. Therefore, implants such as the Nucleus or the Clarion have from eight to 22 electrodes that surgeons place at different positions to maximize the range of frequency stimuli forwarded to the brain. Recent research indicates that more electrodes won't improve performance as much as optimizing their placement; most implant wearers perceive loudness properly but can still have trouble sensing pitch correctly, making speech comprehension difficult. "Something is preventing the brain from extracting or assimilating all the coding information," says Philip Loizou, a professor of electrical engineering at the University of Texas at Dallas. "We don't know what yet."

This article was originally published with the title To Hear Again.

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