Brain's Fear Center Likely Shrinks in Autism's Most Severely Socially Impaired
The brain's fear hub likely becomes abnormally small in the most severely socially impaired males with autism spectrum disorders, researchers funded by the National Institutes of Health's (NIH) National Institute of Mental Health (NIMH) and National Institute on Child Health and Human Development (NICHD) have discovered. Teens and young men who were slowest at distinguishing emotional from neutral expressions and gazed at eyes least — indicators of social impairment — had a smaller than normal amygdala, an almond-shaped danger-detector deep in the brain. The researchers also linked such amygdala shrinkage to impaired nonverbal social behavior in early childhood.
The new findings suggest that social fear in autism may initially trigger a hyperactive, abnormally enlarged amygdala, which eventually gives way to a toxic adaptation that kills amygdala cells and shrinks the structure, propose Richard Davidson, Ph.D., and colleagues at the University of Wisconsin, who report on their magnetic resonance imaging (MRI) study in the December 2006 Archives of General Psychiatry.*
In a related study, another research team led by Davidson found that well siblings of people with autism share some of the same differences in amygdala volume, and in the way they look at faces and activate social/emotional brain circuitry, particularly an area critical for face processing.
"Together, these results provide the first evidence linking objective measures of social impairment and amygdala structure and related brain function in autism," explained Davidson. "Finding many of the same differences, albeit more moderate, in well siblings helps to confirm that autism is likely the most severe expression of a broad spectrum of genetically-influenced characteristics."
While some people with minimal expression of these traits might be perceived as aloof or loners, those at the more severe end of the spectrum are unable to engage in give-and-take interactions and fail to develop age-appropriate peer relationships. Notably, they shy away from looking at eyes. Davidson's research team had reported last year linked such eye-gazing with hyperactivation of their fear hub.** Yet different studies have found the amygdala in autism to be variously enlarged, shrunken or even normal in size.
Reprinted with the permission of the National Institute of Mental Health. © 2008 NIMH.
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