You're at a crowded party with chaos and noise all around but somehow you focus on what your date says. Scientists have long puzzled over how we do this — filter out unimportant noise to concentrate on one voice out of many in a room.
At first, researchers suggested the brain distinguishes different sources of sound by analyzing which direction they come from. But people can still focus on a single voice among many over the telephone when they can't tell where the speaker is in relation to other speakers.
Now scientists have discovered that the auditory system probably sorts different sources of sound based on their unique pitch and suppresses less important ones.
"We think this is the major way the brain can do this cocktail party phenomenon," said Holger Schulze, a neuroscientist at the Leibniz-Institute for Neurobiology in Magdeburg, Germany.
He and his colleagues studied gerbils that have a very similar hearing mechanism to humans, so they think the findings likely apply to humans as well.
"It has been known for 20 or 30 years that it is possible without directional information to do the job," Schulze said. "But we didn't know how it works, where in the brain this happens. Now we know it's in one small area within the auditory cortex."
The auditory cortex is a region of the temporal lobe in the brain that processes sound information. The researchers think different voices are sorted into distinct areas arranged in a circle in the auditory cortex.
"Within a circular map you can connect each region with all other regions equally well," Schulze told LiveScience. "This is important if you want one region to be able to inhibit all other regions equally well. If it was linear, you could only inhibit your neighbors."
The scientists studied Mongolian gerbils, inserting microelectrodes into their brains to record the electrical activity of different neurons.
When the researchers played one sound, they could see a small area light up in the rodents' brains. Then they added another sound of a different pitch and observed a new area light up initially, but then diminish in activity as it became suppressed by the first.
"One area sends neurotransmitters to other areas to say, 'Stop listening,'" Schulze said. "But there must be some residual activity left so that if you hear something that catches your attention you can selectively switch focus."
Though we can distinguish individual voices without knowing where they are coming from, if we have directional information this process is even easier. Both the pitch-sorting mechanism and directional sorting are usually going on simultaneously, Schulze said.
The researchers hope their discovery could help people who wear hearing aids, since the devices currently can't sort sound well.
"This is the big deficit they have," Schulze said. "We will try to implement this mechanism from the brain into hearing aids, so that the hearing aids can segregate the sounds from different speakers."
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