MILWAUKEE, Wis. — In a new finding, University of Wisconsin-Madison researchers have documented changes in the brain's emotional center that may explain the social impairment seen in children with autism.
And in a separate French study, scientists identified a gene that may explain why some people develop autism.
The UW authors found that males with autism who spent the least time looking at a person's eyes in photographs had a much smaller amygdala — the almond-shaped danger-detecting region of the brain — than males without autism.
In addition, males with a small amygdala also had difficulty distinguishing any face showing an emotion from those with neutral expressions, researchers say.
"This is a great first step," said Stephen Rao, a professor of neurology and director of the functional imaging research center at the Medical College of Wisconsin, who was not involved in the study.
"We pick up a lot of emotions from our ability to distinguish facial expressions, so that's why eye-tracking is good," he said.
Still, more studies that demonstrate the exclusivity of the amygdala need to be done before more conclusions are made, Rao said.
Autism, the most common condition in a group of developmental disorders known as autism spectrum disorders, is a serious brain disorder that begins in infancy and prevents children from developing normally.
Common signs, which can range from mild to severe, include impaired social interaction, problems with verbal and non-verbal communication, and unusual, repetitive, or severely limited activities and interests.
For the UW study, researchers analyzed data from 54 males, including 28 with autism spectrum disorders, who were ages 8 to 25. The participants were tested using magnetic resonance imaging and an eye-tracking device to record where they looked. This method allowed the authors to directly relate amygdala size with how much time the subjects spent looking at the eyes of pictured faces.
Participants in the autism group who had a small amygdala were significantly slower to identify happy, angry or sad facial expressions and spent the least amount of time looking at eyes on the photos. It took these males 40 percent longer than those with the large amygdala to recognize the emotional facial expressions, and those with the largest amygdala spent about four times longer looking at eyes than those with the smallest.
Furthermore, the size of the amygdala appeared stunted in those with little eye contact compared with participants in the control group. And those in the autism group with small amygdala had the most non-verbal social impairment as children.
The authors believe that the amygdala may first become hyperactive but over time begin to slowly degenerate. The same concept has been proposed for some forms of depression, which involves the brain region called the hippocampus.
Thus, children with the most limited interaction with people would begin to show amygdala changes early in life, while those with more social interaction would have fewer changes in the amygdala.
However, the researchers caution that these changes do not explain all autistic behavior but account for slightly more than half of the variability in non-verbal social impairment.
The study was published this month in the Archives of General Psychiatry.
Richard Davidson, co-author of the study and a professor of psychology and psychiatry at UW-Madison, said in a written statement that the findings have opened the door to future studies to clarify the role of the amygdala in autism.
In a separate finding, a French study published online this month in Nature Genetics found that mutations in a gene called SHANK3 may explain why a small group of individuals developed autism spectrum disorders.
The protein encoded by SHANK3 interacts with other proteins called neuroligins, which have a role in brain signaling. Mutations in two of the genes encoding neuroligins have previously been found in a small number of individuals with autism.
The researchers looked for the SHANK3 gene in more than 200 people with the disorders and identified it in three families. In the first family, a large part of the gene was missing in a person with autism, resulting in that person's inability to speak and mental retardation. Two brothers with autism in the second family had a smaller portion of their gene missing. Both brothers had severely impaired speech and severe mental retardation.
In the third family, a girl with autism had a deletion in SHANK3, while her brother, affected with a mild form of autism called Asperger syndrome, had an additional copy of SHANK3. The girl had severe language delay, while her brother was gifted in language development and had fluent speech.