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MARY LOUISE KELLY, host:

If we listen to the world the way a microphone does, it would sound something like this.

(Soundbite of crowd talking over each other)

KELLY: That's because a microphone treats every sound in a crowded room the saw way. Luckily our brains filter out voices we don't want to hear so we can focus on just one. Scientists call that the cocktail party effect.

Now NPR's Jon Hamilton reports that researchers are finally beginning to understand how the brain does this trick, thanks to new research on birds and bats.

JON HAMILTON: Birds don't spend much time at cocktail parties, but Frederic Theunissen of U.C. Berkeley says they face a similar auditory challenge.

Professor FREDERIC THEUNISSEN (Psychology, U.C. Berkeley): Take a walk in the forest, it's clear that these animals are very vocal.

(Soundbite of birds)

Prof. THEUNISSEN: And sometimes when there's large groups of them it becomes a cacophony of sounds - just like when there's large groups of humans.

HAMILTON: And just like humans, some birds need to pick out just one voice in the crowd.

(Soundbite of bird)

Prof. THEUNISSEN:�In the species that I study, the zebra finch, they make lifelong couples, and it's very important for them to recognize their mate. So they do a lot of call of and call back to be able to stay in contact and, also, to kind of reinforce this kind of social bond between the pair.

HAMILTON: Theunissen thought there must be brain circuits in a zebra finch that filter out unwanted sounds. So his lab began studying their brains while the birds listened to audio recordings. They heard a familiar song all by itself, and the same song buried in white noise.

Prof. THEUNISSEN: We kind of knew before what the responses were going to be to songs alone. And so we were looking for brain areas where we would get a similar response when the song was embedded in noise.

HAMILTON: And they found some. In these areas, there were individual brain cells that acted like a dog waiting for his master's voice - they wouldn't respond to any other sound. Some of these cells also scanned relatively long stretches of sound, which allowed them to pick out a particular vocal feature.

Prof. THEUNISSEN: So it's able to detect that this feature was there, even though there's noise that's corrupting it.

HAMILTON: We probably do something similar when we're focusing on a guy with a nasal twang on the other side of the guacamole dip.

Another explanation for the cocktail party effect comes from bats. They tend to live in large colonies that make a racket when they fly out of their caves at the end of the day.

(Soundbite of bats)

Professor JAGMEET KANWAL (Physiology and Biophysics, Georgetown University): Basically, we can think that the bats have a cocktail party every evening.

HAMILTON: Jagmeet Kanwal studies mustached bats at Georgetown University. He says bats have to contend with two streams of sound - the first is from the constant echolocation noises they make to navigate and hunt mosquitoes. Much of that sound is beyond the range of the human ear. But to bats, it's as loud as a jet airplane. And amid that roar, Kanwal says, bats need to detect a separate stream of sound that they use to communicate.

Prof. KANWAL: And the communication sound is usually only one or two or three utterances and then it's gone.

(Soundbite of bats)

Prof. KANWAL: But it's very important for the animal to detect, because it may be, you know, I love you, or get out of my way, or whatever. So you don't want to lose that opportunity.

(Soundbite of laughter)

HAMILTON: Kanwal's lab found brain cells in bats that actually did a better job of detecting communication sounds when there was a lot of background noise.

Prof. KANWAL: So it's like somehow the system is designed that in fact, in its natural setting - when there's so much noise - somehow the communication sound is even louder.

HAMILTON: A closer look showed that these special brain cells were doing two things to cut through the noise: They were telling other brain cells in the area to stay quiet, to stop responding to the background noise, and at the same time they were amplifying their own response, so their voice would be heard above the din. Kanwal says the next step is to figure out if cells in the human brain do the same thing. The research on birds and bats was presented at the Society for Neuroscience meeting in San Diego.

Jon Hamilton, NPR News.

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