Research shows where brain interprets 'pitch'


Johns Hopkins researchers have discovered a discrete region of the monkey brain that processes pitch, the relative high and low points of sound, by recognizing a single musical note played by different instruments.

Given the similarities between monkeys and man, humans may have a similar pitch-processing region in the brain too, which might one day help those with hearing and speech problems. The paper appears in the Aug. 25 issue of Nature.

By recording the activity of individual brain cells as monkeys listened to musical notes, the scientists identified single neurons, located in what they've called the brain's "pitch center," that recognize a middle-C as a middle-C even when played by two different instruments.

"Pitch perception is such a basic function of human and animal auditory systems, yet its source has remained elusive to researchers for decades," says Xiaoqin Wang, Ph.D., associate professor of biomedical engineering and neuroscience at the Johns Hopkins Institute for Basic Biomedical Sciences. "The discovery of a pitch-processing area in the brain solves an age-old mystery of auditory research."

According to Wang, pitch's importance to humans is found in facilitating our ability to follow a sequence of sounds we would recognize as "melodic" and combinations of sounds we identify as harmony. As a result, pitch gives meaning to the patterns, tones and emotional content of speech, like how raising our voice at the end of a sentence indicates a question, and cues the listener to the speaker's gender and age.

Although a melody or conversation is not as essential to monkeys, pitch perception is crucial for nonhuman primates to interpret the source and meaning of prey and predator calls or other sounds from the environment. Such information is crucial for the animal's survival.

Wang's team studied marmoset monkeys using single-neuron recording, a technique that measures the electrical activity of individual neurons in the brain. The researchers viewed each neuron's reaction as different notes were played by a computer.

The majority of pitch-selective neurons are located in a specific region of the monkey's brain near the primary auditory cortex, a region already known to interpret sounds.

"The auditory cortex has traditionally been thought to detect the complex spectrum contained within a sound; for example, with one set of neurons responding only to a trumpet and another set to a violin, even if playing the same note," says Wang. "But the neurons we found respond to a single musical note, regardless if played by a trumpet or violin."

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