Researchers at the University of Wisconsin have examined the age-dependent changes in the size of brain region named the amygdala on face processing in a group of individuals both affected and not affected with autism.

As children get older, the size of their brain increases, and so does the size of the amygdala. This recent study shows that this age-related increase in amygdala size does not occur in individuals with autism. In addition, those individuals with the smallest amygdala measured in adolescence also showed the greatest deficits in recognition of facial features, as well as a reduction in time spent looking at the eyes (eye gaze). Both of these behavioral characteristics are consistent with autism-specific impairments. Social communication deficits in early childhood were also linked to smaller amygdala size in adolescence, suggesting that this atrophy may begin earlier than the first changes in amygdala size are experimentally observed.

The amygdala has been well studied as a part of the brain circuit which controls emotional response to faces as well as fear-related memories. Changes in the size of this brain region as well as the number of brain cells in different areas of the amygdala have been associated with autism, however, this study specifically investigated whether the size of the amygdala is related to specific deficits in the ability to recognize facial expressions and examine specific facial features, especially the eyes. Because the group that showed the greatest deficits on this task also exhibited the smallest amygdala, this research provides further evidence that changes in the structure and function of this brain region are related to autism-specific behaviors such as deficits in face processing.

While the cause of this deceleration in brain size is not yet known, this research group, led by Dr. Richard Davidson, reports a hypothesis on why individuals with the most severe behavioral impairments show the smallest amygdala size. One explanation for this finding states that the amygdala of severely affected individuals may be hypersensitive and hyperactive, leading to a phenomenon called "excitotoxicity". That is, the amygdala in individuals with autism shows a maladaptive cellular response during development, leading to cell death and shrinkage of the size of the structure. Further studies are needed to determine if this hypothesis is true. The funders of this study, the National Institutes of Health, are proud to provide a detailed summary of this study which can be found here: http://www.nih.gov/news/pr/dec2006/nimh-04b.htm