Pitch Is Key in Finding Source of Sounds

Brain uses different methods to locate origin of noise

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By
HealthDay Reporter

WEDNESDAY, Aug. 4, 2004 (HealthDayNews) -- A new British study sheds light on how you know where sounds are coming from, which could help scientists understand the complex interaction between the brain and the environment.

The research could possibly lead to improvements in devices like hearing aids and cochlear implants, the study author said.

Simply put, low-pitched sounds surround your head and enter both ears almost simultaneously. However, the brain is able to distinguish the small difference in time between when the sound enters each ear, and that is what tells you where the sound came from.

High-pitched sounds, on the other hand, are heard by only one ear with just a shadow of the sound heard in the other.

"The brain uses different paths to optimize its sensitivity to spacial cues to locate sounds," said lead researcher David McAlpine, a reader in auditory neuroscience at University College London.

McAlpine and his colleague Nicol S. Harper created this new model of how you track sound. Their report is in the Aug. 5 issue of Nature.

According to McAlpine, their model might help scientists develop new technology for tracking sounds in such noisy environments as crowded bars and restaurants, which could lead to refinements in devices that improve hearing.

This new model flies in the face of accepted theory, McAlpine said. For more than 50 years, the accepted theory of how humans locate the source of sounds was based on research in barn owls, he added.

Barn owls have special brain cells that are finely tuned to recognize specific sound pitches. "But in fact, barn owls are very different, not only to humans, but also to all other species," McAlpine said.

To find out how humans track sound, McAlpine's team had people roam througn London with microphones in their ears. The microphones registered the exact time sounds arrived at each ear for a range of noises that people commonly encounter.

McAlpine and Harper found that humans behave like small mammals when locating the source of a low-pitched sound.

Small mammals like gerbils are sensitive to low-pitched sounds, and like them, humans use the differences in the arrival time of sound at each ear to locate its source, McAlpine added.

"Different brain cells appear to be activated as sound reaches each ear, even though these time differences are far shorter than we can directly experience," McAlpine said. "Not in terms of pitch, but where the sound source is located."

In McAlpine's model, gerbils, guinea pigs and humans follow low-pitched sounds in the same way. However, human brains use a strategy similar to a barn owl to detect the source of high-pitch sounds.

Humans appear to pick and chose from the different strategies, depending on whether the sound is high-pitched or low-pitched, McAlpine said.

"The view that your sensitivity to spacial cues is dependent on neurons that are finely tuned to spacial positions, like barn owls, is not correct," McAlpine said. "For normal hearing, you use these spacial cues to extract signals from noise."

Both ears need to be functioning to do that, McAlpine noted. "If you don't have two ears, you have a big problem in noisy environments," he said.

More information

The National Institute on Deafness and Other Communication Disorders can tell you more about hearing problems.

SOURCES: David McAlpine, Ph.D., reader in auditory neuroscience, University College London; Aug. 5, 2004, Nature

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