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Most kids pay close attention to human voices long before they know what people around them are saying. But it's different for many children with autism spectrum disorders. They often appear indifferent to human speech, even after they've learned to talk. NPR's Jon Hamilton reports now on new research that may explain why.

JON HAMIILTON, BYLINE: One early sign of autism is when a toddler doesn't respond to his or her own name. Nancy Minshew, a child neurologist at the University of Pittsburgh, says this is when many parents first suspect something is wrong.

NANCY MINSHEW: They'll say - you know - I can call his name, and he never answers; and he doesn't come, and I don't know where he is. And they don't realize, you said the words but their brain didn't hear it.

HAMIILTON: Researchers have known about this lack of response to human voices since the 1940s, but it hasn't been clear what's causing the problem. So Vinod Menon and a team at Stanford University used a special type of MRI to compare the brains of 20 children who had autism, with the brains of 19 typical kids. Specifically, they looked at how certain areas of the brain were connected. And they found that in typical kids, there is a very strong connection between areas that respond to the human voice, and areas that release the feel-good chemical dopamine.

So when typical kids hear a human voice, they experience a feeling of pleasure that encourages them to pay attention. But Menon says in children with autism, that connection was pretty weak.

VINOD MENON: These findings suggest that the core brain circuits that relayed the human voice to the reward pathways are aberrant, and this may impede a child's ability to experience speech as a pleasurable stimulus.

HAMIILTON: Which means these children don't have the same motivation to listen to voices, and to figure out what the words mean. Menon says his study found that connections between voice areas, and areas involved in emotion-related learning, were also weaker. And, he says, the weaker the connections, the more trouble a child had communicating.

Coralie Chevallier, from the Children's Hospital of Philadelphia, says the finding seems to explain why speech often comes late to children with autism, even though the brain circuit involved in processing spoken words seems to function normally.

CORALIE CHEVALLIER: So it really looks like it's not that the circuit is broken but rather, that it's not spontaneously made use of.

HAMIILTON: It doesn't happen automatically because there's no reward. Chevallier says the new study supports a theory that children with autism lack a wide range of social skills because they simply aren't as motivated as other kids to acquire them. And she says if motivation is the problem, then many autism therapies are right to offer food, or other tangible rewards, to children when they do things like listen carefully.

CHEVALLIER: When you're reinforcing the child's behavior with M&Ms or a piece of cookie, you're providing an extrinsic reason for the child to do something they didn't want to do in the first place. So you're working on motivation.

HAMIILTON: Nancy Minshew, at the University of Pittsburgh, says the new study also offers a reminder of just how different the world can be for people with autism.

MINSHEW: If you just stop for a moment and listen in your day, there's all kinds of sounds. Our minds enable us to selectively pay attention to words and human voices. But in autism, that's not happening.

HAMIILTON: Minshew says even when people with autism do pay attention to words, they are likely to miss the emotional content conveyed through things like tone of voice.

MINSHEW: Most people overestimate what people with autism understand. We may speak the same language but they don't experience it in their mind or their brain the same way that we do.

HAMIILTON: The new research appears in the Proceedings of the National Academy of Sciences. Jon Hamilton, NPR News.

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