Scientists have found that autistic toddlers have an enlarged amygdala, a brain area known to be linked with functions like the processing of faces and emotion.
Researchers at the University of North Carolina at Chapel Hill also say that this brain abnormality appears to be associated with the ability to share attention with others, a fundamental ability thought to predict later social and language function in children with autism.
"Autism is a complex neurodevelopmental disorder likely involving multiple brain systems," the authors write as background information in the article.
"Converging evidence from magnetic resonance imaging, head circumference and postmortem 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," write the authors.
Dr. Matthew W. Mosconi conducted a magnetic resonance imaging study involving 50 autistic children and 33 control children.
The children participating in the study underwent brain scans along with testing of certain behavioural features of autism at ages 2 and 4, including a measure of joint attention that involves following another person's gaze to initiate a shared experience.
The researchers observed that compared to control children, those children with autism were more likely to have amygdala enlargement both at age 2 and age 4.
"These findings suggest that, consistent with a previous report of head circumference growth rates in autism and studies of amygdala volume in childhood, amygdala growth trajectories are accelerated before age 2 years in autism and remain enlarged during early childhood. Moreover, amygdala enlargement in 2-year-old children with autism is disproportionate to overall brain enlargement and remains disproportionate at age 4 years," the authors write.
According to them, amygdala volume in autistic children was found to be associated with an increase in joint attention ability at age 4, which suggests that alterations to this brain structure may be associated with a core deficit of autism, the authors note.
"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. Amygdala disturbances early in development, therefore, disrupt the appropriate assignment of emotional significance to faces and social interaction," the authors write.
The researchers are continuing with the follow-up of the research participants so as to determine whether amygdala growth rates continue at the same rate or undergo another period of accelerated growth or a period of decelerated growth in autistic children after age 4.
The study has been published in the Archives of General Psychiatry, one of the JAMA/Archives journals.