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Here's a question that's puzzled scientists since the 1950s: How can someone in a room filled with voices pay attention to just one voice? Well, now scientists are beginning to get answers, and that's thanks to new technologies that can measure electrical activity in many parts of the brain. NPR's Jon Hamilton explains this latest effort to understand the so-called cocktail party effect.

JON HAMILTON, BYLINE: At a cocktail party, the brain has to make sense out of auditory chaos. And Elana Zion Golumbic, a researcher at Columbia University, says that's something people do really well.

ELANA ZION GOLUMBIC: It's also something we do all the time, not only in cocktail parties. You're on the street, you're in a restaurant, you're in your office - there are a lot of background sounds all the time, and you constantly need to filter them out and focus on the one thing that's important to you.

HAMILTON: Golumbic says experiments show that the human brain is much better at doing this than even the most powerful computer.

GOLUMBIC: But in terms of how we can actually do it, what's the brain doing to enable us to do this online filtering all the time; that has been a mystery.

HAMILTON: So a few years ago, Golumbic and other researchers began using a new technology to monitor people's brains as they worked to separate one stream of sounds from another. The technology involves a grid of electrodes placed on the surface of the brain. Golumbic says the experiments relied on volunteers who already had these electrodes in place. They were people awaiting surgery for severe epilepsy.

GOLUMBIC: And we bring in a cart with a computer and a screen and speakers. And what we do is, we actually show them movies.

HAMILTON: One movie shows a woman telling a brief story.

(SOUNDBITE OF MOVIE IN EXPERIMENT)

UNIDENTIFIED WOMAN: My good friend Jonathan has a pet parrot called Gypsy, who can speak.

HAMILTON: Another shows a man.

(SOUNDBITE OF MOVIE IN EXPERIMENT)

UNIDENTIFIED MAN: As a child, I never liked to clean up my room.

HAMILTON: And a third movie shows a man and a woman talking at the same time.

(SOUNDBITE OF MOVIE IN EXPERIMENT)

UNIDENTIFIED WOMAN, MAN: (Speaking at the same time) My good friend Jonathan....as a child I never liked...has a pet parrot...to clean up my room...called Gypsy, who can speak...because I thought it was easier for me to find things when everything was out......he can say his own name and likes to call out...compared to when they were hiding inside a closet or a drawer...hello, come on in...

HAMILTON: Just hearing those voices, it's pretty hard to pay attention to a single speaker. But volunteers were able to focus their attention by looking at the speaker they wanted to understand. And the activity in their brains revealed something remarkable. When a person's brain is in cocktail party mode, some areas - like those involved in processing sound - continue to respond to both voices. But Golumbic says other areas of the brain, like those devoted to language, appear to respond only to the selected speaker - in this case, the man.

GOLUMBIC: The other speaker has no representation in those areas. Those areas have completely filtered her out.

HAMILTON: Afterward, the volunteers couldn't remember the woman's story about Gypsy the parrot, but they had no trouble remembering the man who didn't like to clean his room.

Charlie Schroeder is a co-author of the new study, which appears in the journal "Neuron." He's also a neuroscientist at Columbia University and New York state's Nathan Kline Institute. Schroeder says the study offers some hints about how the brain attends to one voice while ignoring others. He says these hints come from the so-called exclusive sites in the brain, those areas that seem to respond only to the selected voice.

DR. CHARLIE SCHROEDER: In these exclusive sites, as the sentence unfolds, the brain's tracking of the signal becomes better and better and better.

HAMILTON: Schroeder says these areas seem to be constantly fine-tuning themselves, to detect the unique characteristics of a particular voice. He says it's also likely that these areas are using information from the first words of a sentence to predict what's going to come next.

Edward Chang of U.C.-San Francisco says this sort of research may eventually help people who have trouble tracking a single voice at cocktail parties. That includes many people with ADHD. Chang says he saw this close up, when a person with the disorder took part in one of his studies.

DR. EDWARD CHANG: This person did have ADHD, and had significant problems with ability to select the correct speaker.

HAMILTON: Chang says understanding how the brain solves the cocktail party problem also could help machines - like cellphones - do a better job understanding what we're trying to tell them. Jon Hamilton, NPR News.

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