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Sometime during childhood, our brains begin to organize in a way that helps us recognize human voices and the emotions they convey. But scientists haven't been sure exactly when that happens.

NPR's Jon Hamilton reports on new evidence that it happens well before children utter their first words.

JON HAMILTON: To figure this out, a team of researchers studied 32 infants. Half the babies were just 4 months old; the rest were 7 months old. The researchers had all of them listen to some common sounds.

(Soundbite of chickens and bells)

HAMILTON: And also some human voices.

(Soundbite of nonsensical utterances and Foreign language spoken)

HAMILTON: The voices included speech - but not in any language the children might recognize. During the experiment, the team measured activity in an area of the brain thats close to the ear and is eventually used to process human voices.

Tobias Grossmann, of the University of London, says in infants who were 4 months old, this area did not distinguish between the human and non-human sounds.

Dr. TOBIAS GROSSMANN (Research Fellow, University of London): The striking finding that was that at the age of 7 months, already they show brain responses that indicate that they process human voice very distinctly from other kinds of sounds.

HAMILTON: Then the researchers did a second experiment with the 7-month-old kids. They played them an unfamiliar word spoken with a happy intonation.

Unidentified Woman: (Foreign language spoken)

HAMILTON: And the same words spoken in an angry way.

Unidentified Woman: (Foreign language spoken)

HAMILTON: Once again, certain areas of the brain seemed to know the difference.

The findings provide strong evidence that sometime between the fourth and seventh month of life, we begin using specialized areas of the brain to process voice sounds. Grossmann says this discovery might have been made years ago if kids didnt act like kids. After all, several generations of high-tech scanners have been able to identify brain areas in adults that respond only to human voices.

But most young kids are just too squirmy to lie still for a scan, so researchers had to wait for a new technology that measures brain activity using beams of light. Grossmann says now, babies just wear a special helmet.

Dr. GROSSMANN: The infant can, in fact, be seated on the parent's lap while listening to different kinds of sounds.

HAMILTON: It makes sense that kids' brains become attuned to human voices so early. Their behavior shows that something must be changing in the brain. Most of the time, kids start responding to a parent's tone of voice within the first few months. Next, they learn to understand a few words. Then, they begin speaking.

But sometimes, things dont happen that way. And Rhea Paul, of the Yale Child Studies Center, says the new technique for measuring brain responses might help identify very young children with developmental disorders, including autism.

Dr. RHEA Paul (Principal Investigator, Child Studies Center, Yale University): One thing we know about children with autism is that they are almost universally delayed in their development of language.

HAMILTON: So, problems with the brain systems that recognize and process human voices could offer an early warning.

Paul says the brain areas that respond to intonation may be especially important to watch because children with autism often lack this ability. She says her lab found that toddlers with typical brains paid attention to which syllable in a word was being stressed, and to the rhythm of sentences.

Dr. PAUL: Our children with autism, on the other hand, paid attention to neither of those things.

HAMILTON: But Paul says that experiment was with children who were at least a year old. She says it's not clear yet whether there are differences in younger children. The new research appears in the journal "Neuron."

Jon Hamilton, NPR News.

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