May 4 (HealthDay News) -- Autistic toddlers seem more likely to have an enlarged amygdala, a brain area linked with facial recognition and emotions, University of North Carolina researchers report.
This brain abnormality appears to be tied to the ability to share attention and experiences with others, the team said.
"This study adds clarification to a potential fundamental brain mechanism underlying social deficits in autism. It provides potential insights into how this behavior develops," said lead researcher Dr. Joseph Piven, a professor of psychiatry.
"We found enlargement of the amygdala in very young children with autism at 2 years of age, and followed up again at age 4. The enlargement was stable over the 2- to 4-year-old time interval," he said.
The amygdala is a structure that has previously been implicated in social and emotional perception and in autism, Piven said.
"We also found that this enlargement was related to something called joint attention, or the ability of a young child to take cues from an adult about where to look in the visual field, for example, at an object of interest," he said.
This ability develops in a narrow window, between 9 and 15 months, and is thought to be a fundamental deficit in autistic individuals that predicts poor outcomes in social behavior and language, Piven explained.
"In another study, we have shown evidence that the brain in autism is normal in size until the end of the first year of life, at which time it overgrows," he said.
"Understanding the pattern of very early brain changes in autism and their relationship to particular behaviors could lead to enhanced early detection and could direct us to early interventions for these brain and behavior changes," Piven said.
The report is published in the May issue of the Archives of General Psychiatry.
For the study, Piven's team conducted MRI scans of 50 autistic children and 33 children without the condition. The children had brain scans and testing of certain behavioral features of autism at 2 and 4.
The researchers found autistic children were more likely to have an enlarged amygdala at 2 and 4.
However, the researchers did not find a relationship between amygdala size and other social behaviors at this age, such as social gestures or ritualistic/repetitive behavior, Piven said.
"We have found a very specific behavior, social-orienting, known to be a core neuropsychological mechanism in autism, to have a specific relationship to a very early change in a selected brain structure in autistic individuals," Piven said. The finding identifies "an important potential, fundamental mechanism underlying the development of autism," he added.
"Studying this relationship as these children develop -- does the amygdala continue to enlarge, stay the same or get smaller -- will shed important light on the neurobiological basis of autism," Piven said.
Dr. Jon Shaw, chief of the division of child & adolescent psychiatry at the University of Miami Miller School of Medicine, believes the study adds a potential new understanding to some of the social problems associated with autism.
"The number of studies that have suggested various neurobiological pathways to autism are almost infinite, confirming the complex neurodevelopmental processes intrinsic to the disorder and suggest the need for humility in interpreting neurobiological findings," Shaw said.
This study, however, suggests that one of the important facets in the neurodevelopmental mosaic that makes up autism is an alteration in amygdala functioning, Shaw said.
"The authors, taking a developmental approach, suggest that increased bilateral amygdala volume with alterations in functioning occurs early in autistic children and may be contributory to the enlarged head circumference found in a subset of children with autism," he said. Enlargement of the amygdala may also "contribute to the difficulties in interpreting interpersonal and social nuance though disturbances in social-orienting behaviors and reduced eye contact," Shaw said.
Piven's study comes on the heels of a report published online last week in the journal Nature that found that the inheritance of abnormal mutations of two common genes can raise the risk for developing autism.
SOURCES: Joseph Piven, M.D., professor, psychiatry, University of North Carolina at Chapel Hill; Jon Shaw, M.D., professor and chief, division of child & adolescent psychiatry, department of psychiatry & behavioral sciences, Miller School of Medicine, University of Miami; May 2009, Archives of General Psychiatry