December 18, 2006

Brain’s Fear Center Changes in Autism

Picture of a young boy

People with autism have trouble engaging in give-and-take interactions and relating to others. A new brain imaging study gives insight into how such social deficits relate to patterns of development in the amygdala, the brain’s fear hub.

People with autism spectrum disorders (ASD) are notably shy. Dr. Richard Davidson and his colleagues at the University of Wisconsin reported last year that the tendency to shy away from looking into someone's eyes was linked with hyperactivation of the amygdala, the almond-shaped danger-detector deep in the brain. The team set out to learn more about the relationship between amygdala volume and social impairment with funding from NIH's National Institute of Mental Health and National Institute of Child Health and Human Development.

The researchers enrolled 28 males, 8 to 25 years old, with ASD and, for comparison, a similar group of 26 with no known psychiatric disorders. The participants' brains were viewed using magnetic resonance imaging (MRI), and they were tested with eye-tracking and other measures of facial emotion processing. The results were published in the December 2006 issue of Archives of General Psychiatry.

Those with ASD who had a small amygdala were significantly slower than those with the largest fear hubs at identifying happy, angry or sad facial expressions, taking about 40% longer. They also spent considerably less time looking at eyes relative to other facial regions — about a quarter of the time as those with the largest amygdalae. There weren't similar patterns among the comparison subjects.

When the researchers divided the participants into those younger than 12.5 years old and those older, they found that the comparison group showed an increase in amygdala volume with age, but the ASD group didn't. Amygdala volumes weren't abnormally low in younger people with autism, but they were in older in ones.

Researchers have reported that some children with autism have abnormally enlarged amygdalae. The researchers propose an explanation that could resolve all these findings. They suggest that social fear in children with autism may initially trigger a hyperactive, abnormally enlarged amygdala. The constant barrage of signals between cells in the amygdala could overload and begin to kill them, eventually shrinking the structure.

The researchers caution that the amygdala changes they describe can't explain all autistic behavior, but their proposal may explain one way the brain changes in people with ASD. In any case, this study shows that measures such as eye gazing time will continue to help researchers clarify the relationships between genes, the brain and behavior in ASD.

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