Young children with autism appear more likely to have enlarged amygdala - the part of the brain associated with registering faces and with expressing key emotions, according to a study released on Monday.
Described in the May issue of Archives of General Psychiatry, the study compared the magnetic resonance imaging (MRI) results of 50 autistic children and 33 control children.
The children's brain scans were taken at age two and again at age four.
Compared to children from a control group who did not have autism, the autistic children were more likely to have enlarged amygdala.
"Amygdala growth trajectories are accelerated before age two years in autism and remain enlarged during early childhood," wrote lead author Matthew Mosconi and his colleagues at the University of North Carolina at Chapel Hill.
"Amygdala enlargement in two-year-old children with autism is disproportionate to overall brain enlargement and remains disproportionate at age four years."
The researchers said after observing the children that the enlarged amygdala appeared to be consistent with their inability to follow another person's gaze, and with the inability to share attention with others - fundamental behaviours thought to predict later social and language function in children with autism.
"Converging evidence from magnetic resonance imaging, head circumference and post-mortem studies suggests that brain volume enlargement is a characteristic feature of autism, with its onset most likely occurring in the latter part of the first year of life," the authors of the study wrote.
"The amygdala plays a critical role in early-stage processing of facial expression and in alerting cortical areas to the emotional significance of an event," the authors write.
"Amygdala disturbances early in development, therefore, disrupt the appropriate assignment of emotional significance to faces and social interaction."