Researchers said that some children with autism have weak brain connections in regions that link speech with emotional rewards, possibly signaling a new pathway in treatment.
The study in the Proceedings of the National Academy of Sciences is the first to suggest that the reason why children with autism display an insensitivity to human speech may be linked to faulty circuitry in the brain's reward centers.
"Weak brain connectivity may impede children with autism from experiencing speech as pleasurable," said Vinod Menon, senior author of the study and professor of psychiatry and behavioral sciences at Stanford University.
Researchers took magnetic resonance imaging brain scans of 20 children with a high-functioning type of autism; they had normal range IQs and could speak and read, but had a hard time in conversation or understanding emotional cues.
By comparing the scans to those of 19 children without autism, they found that the brains of youngsters with autism showed poor connections to brain regions that release dopamine in response to rewards.
On the left side of the brain, the autistic children showed weak connections to the nucleus accumbens and the ventral tegmental area.
And on the right side, in the voice-selective cortex where vocal cues and pitch are detected, there was a weak connection to the amygdala, which processes emotional cues.
Researchers also found that weaker connections meant worse communication abilities.
"The human voice is a very important sound; it not only conveys meaning but also provides critical emotional information to a child," said lead author Daniel Abrams, a postdoctoral scholar in psychiatry at Stanford.
"We are the first to show that this insensitivity may originate from impaired reward circuitry in the brain."
The findings also give weight to the theory that people with autism have a deficit in social motivation that explains their inattention to voices and words, rather than a sensory deficit that prevents them from hearing words.
"It is likely that children with autism don't attend to voices because they are not rewarding or emotionally interesting, impacting the development of their language and social communication skills," Menon said.
"We have discovered an aberrant brain circuit underlying a core deficit in autism; our findings may aid the development of new treatments for this disorder."
Further research could also reveal whether a type of therapy called pivotal-response training, which tries to motivate kids to use language for social interaction, has any effect on strengthening these brain circuits.
Autism affects as many as one in 88 in the United States, and about one in 100 in Britain.