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A study examining how the brain decodes pitch could inform further development of cochlear implants. (Credit: University of Washington)

For someone who wears a cochlear implant, a surgically implanted electronic device that restores a sense of hearing, pitch is only weakly conveyed. A new study could change how cochlear implants are designed.

For decades, scientists have debated how, exactly, humans perceive pitch, and how the ear and the brain transmit pitch information in a sound. There are two prevalent theories: place and time. The “time code” theory argues that pitch is a matter of auditory nerve fiber firing rate, while the “place code” theory focuses on where in the inner ear a sound activates. A new study bolsters support for the place code. Pinning pitch perception on a “place code” provides opportunities for improvement of cochlear implants that would not be possible if pitch were perceived only through a “time code.”

In a place code, different frequencies activate different parts of the inner ear, with pitch organized from high to low, like a musical scale. Where the activation is indicates the pitch of a sound. For the experiment, researchers tested 19 people (average age: 22) with a range of musical training (from no formal training to 15 years’ worth). Musical experience turned out to have no clear correlation with pitch perception in this study.

The participants compared a series of high-frequency tones (greater than 8,000 Hz) with specialized headphones in a soundproof booth; after each tone, participants used a computer to indicate which sound was higher in pitch. Researchers chose only very high-frequency tones, embedded in background noise, in order to eliminate the possibility of a time code and focused instead on whether a place code was at work. And when these ultra-high frequency pure tones were combined in a harmonic complex (think musical notes), participants’ pitch perception improved significantly.

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