A human voice has no special ring to the autistic brain because areas related to reward and emotional context are not well wired to its center of voice recognition, a Stanford University study has found.
The findings, published Monday in the Proceedings of the National Academy of Sciences, lend support to the theory that social motivation lies at the heart of language and speech deficits that are endemic among children with autism spectrum disorder.
The social motivation theory of autism holds that deficits in communication and speech skills result from the brain’s diminished ability to build a social context for human voices. Infants do not experience a mother’s voice as soothing and cannot attach associations to its changes of tone, pitch and intonation.
“If the human voice, the mother’s voice, is not sufficiently engaging to a child because it is not perceived as rewarding enough, then the attention to the human voice gets limited and there’s no context for social communication and language-based communication,” said Stanford neurobiologist Vinod Menon, lead author of the study.
The under-connectivity researchers found between the speech-sensitive area of the cortex and reward and emotion centers of the brain correlated strongly with tests that showed communication and speech deficits among the autistic children whose brains were scanned, according to the study, which found no evidence that more fundamental areas of sound processing were different in autistic brains.
“The deficits are not in basic auditory processing; the deficits are in the way that they might attend, or be motivated to attend the human voice,” Menon said. “There’s a strong link between how aberrant the circuit is and how much language communication deficits there are.”
Studies comparing early learning that incorporates human voice and contact, versus solitary audio-visual methods without human contact, have suggested that cognitive skills likewise pass through a “social gateway” that is blocked in the autistic brain.
“Humans are a uniquely social species and children learn a huge proportion of their knowledge base through social interactions,” said Coralie Chevallier, a scientist at the Center for Autism Research at Children’s Hospital of Philadelphia, who has examined communication issues among autistics, but was not involved in the study.
“All things being equal, if a child is born with a slightly diminished propensity to take pleasure in social interactions, this might lead to drastic consequences for the development of social skills down the line,” Chevallier said.
The Stanford team used functional magnetic resonance imaging to examine an area of the superior temporal cortex that is known to be sensitive to human voice. They compared a group of 20 children diagnosed with autism, whose average age was 10, with 19 typically developing children of the similar age.
In the brains of the autistic subjects, the left side of the voice-sensitive area was under-connected with several areas that release dopamine as part of a complex reward and reinforcement system that helps the brain develop appropriate responses. The right side, meanwhile, showed less connectivity with areas such as the amygdala, a region that helps create emotional associations.
The right lobe of the voice-sensitive region of the brain has been shown to be sensitive to so-called prosodic elements of speech, including such things as the upward inflection of a question. While that sensitivity has been somewhat mysterious, the findings of the study suggest the amygdala may play a key role.
“I think the next question is whether the white matter pathways that link these areas might have some deficits, and if not, what is the cause of this,” Menon said.
The imaging of primary acoustic processing areas also suggested that signal pathways were equally connected in both groups. The problem, researchers concluded, lay downstream of that processing, in the interplay among higher-level processing regions of the brain.
The imaging was done while subjects were lying still in a scanning machine. It therefore reveals a static architecture, not an operating system. But Menon and his team likely will seek ways to test their findings with subjects conducting various tasks that require responding to sound.
The autistic children were considered to be high-functioning -- the autistic group, in fact, had slightly higher scores in vocabulary and reading comprehension than did the typically developing children. That is not unusual. Despite having difficulties with social communication, a great many people on the autism spectrum have high verbal skills, in addition to showing unusual talent in mathematics, visual memory and visual arts.
“One of the particular deficits in autistic children is not the language, per se, but the pragmatic, communicative aspect of language,” Menon said.
The study should offer encouragement for tutoring methods aimed at improving speech, particularly those centered on reward and reinforcement, Menon said. Those interventions, he suggested, may be simultaneously stimulating areas that are not well connected at a time when the brain is capable of revising its circuitry.
“Once you start to pair systems, the mantra is: fire together, wire together,” Menon said.