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 from University of Washington researchers lands squarely in the place code’s corner. The findings could lead to new cochlear implant developments, says lead author Bonnie Lau, a speech-language pathologist and postdoctoral fellow at the University of Washington Institute for Learning & Brain Sciences.
Cochlear Implant Issues
For someone who wears a cochlear implant, a surgically implanted electronic device that restores a sense of hearing, pitch is only weakly conveyed.
Pitch is one of the basic aspects of sound. It roughly corresponds to the periodicity of sound waves; sounds with a higher pitch have a higher repetition rate.
Most often associated with music and voices, pitch contributes an aesthetic quality to what we hear. Think of an ocean, or a symphony; without pitch,
“the things we love to listen to, those aesthetic aspects will be changed,”
Pitch also functions as a cue to distinguish sounds, especially in noisy places. When we don’t have access to it, listening in real-world environments like restaurants and public transportation is more difficult.
Place Code Vs. Time 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.”
Here’s how the two codes work.
In a time code, which relies on a phenomenon called “phase locking,” auditory nerve fibers respond to a time-based pattern in a sound wave by firing at the same place every cycle, transmitting information to the brain — a process that works only up to a certain frequency. But beyond a certain repetition rate, the auditory nerve fibers can’t follow the periodicity in a sound.
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.
Researchers tested 19 people with an average age of 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.
Timing Not Required
The participants listened to and 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.
“Our findings show that even when timing information is not available, you can still hear pitch,”
A separate study, supported by the National Institute on Deafness and Other Communication Disorders, is exploring whether a shortened electrode array in a cochlear implant can target exclusively high-frequency sounds.
Bonnie K. Lau, Anahita H. Mehta, Andrew J. Oxenham Superoptimal Perceptual Integration Suggests a Place-Based Representation of Pitch at High Frequencies Journal of Neuroscience 13 September 2017, 37 (37) 9013-9021; DOI: 10.1523/JNEUROSCI.1507-17.2017