The research, published in Proceedings of the National Academy of Sciences, might help explain why kids with autism have difficulty with the social and emotional aspects of human speech.
"Weak brain connectivity may impede children with autism from experiencing speech as pleasurable," said senior author Vinod Menon, PhD, a professor of psychiatry and behavioral sciences at Stanford and a member of the Child Health Research Institute at Lucile Packard Children's Hospital.
"The human voice is a very important sound; it not only conveys meaning but also provides critical emotional information to a child," explained leading author Daniel Abrams, PhD, a postdoctoral scholar in psychiatry and behavioral sciences.
A previous study in BMC Medicine showed that children with autism have a structural difference in brain connections compared to those without the disorder.
An indicator of autism is insensitivity to the human voice, Abrams said. "We are the first to show that this insensitivity may originate from impaired reward circuitry in the brain," he pointed out.
Kids with a high-functioning form of autism were the central point of the report. The children observed could speak and read and had 1Q scores in the normal range. However, they struggled having a conversation with another person or understanding emotional cues in someone's voice.
The investigators analyzed functional magnetic resonance imaging brain scans from 20 of these children and compared them to scans from 19 normally developing kids. A portion of the brain that responds selectively to the sound of human voices was given extra observation.
Past studies have demonstrated that in response to speech, adults with autism had low voice-selective cortex activity. However, until the current study, the connections between the voice-selective cortex and other brain regions in patients with the disorder had not been examined.
Menon and his team discovered that the voice-selective cortex on the left side of the brain in kids with a high-functioning form of autism was weakly connected to the nucleus accumbens and the ventral tegmental area (brain structures that release dopamine in response to rewards).
The voice-selective cortex on the right side of the brain, which identifies vocal cues like pitch and intonation, had a weak connection with the amygdala, which processes emotional cues.
The results indicated, according to the authors, that the weaker these connections in autistic kids, the worse their communication deficits.
After examining the degree of impairment in these brain connections, the scientists could predict the children's scores on the verbal part of a standard test of autism severity.
The report could help to improve some therapies currently being used for autism, said co-author Jennifer Phillips, PhD, a clinical associate professor of psychiatry and behavioral sciences at Stanford and also treats kids with autism at Packard Children's.
Pivotal-response training, for example, is intended to increase social use of language in kids who are able to speak some words but typically do not communicate with other people.
"Pivotal-response training goes after ways to naturally motivate kids to start using language and other forms of social interaction," Phillips explained.
Succeeding research could analyze whether brain connections leading from voice to reward centers become stronger by autism therapies.
The authors said:
"The findings also help resolve a long-standing debate about why individuals with autism show less-than-normal interest in human voices."
Two conflicting theories that could explain the phenomenon were examined:
- One theory: patients with autism have a deficit in their social motivation
- The other theory: autistic patients have sensory-processing deficits which impair their ability to fully hear human voices
Scientists now need to analyze the effects of the weak voice-to-reward circuit in autism.
"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. 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."
A recent study in Environmental Health Perspectives found that pregnant women exposed to high levels of air pollution are at increased risk of having a child with autism compared to women exposed to low levels.
Written by Sarah Glynn