Instructional assistant Jessica Reeder touches her nose to get Jacob Day, who is autistic, to focus his attention on her during a therapy session in 2007. (Rich Pedroncelli/Associated Press)

Toddlers with autism seem more likely to have a larger area of the brain linked with facial recognition and emotion, a brain scanning study suggests.

Using MRI brain scans, researchers found the brain's amygdala region was on average 13 per cent larger in toddlers with autism compared with children without the disorder.

The study by Dr. Joseph Piven, a psychiatry professor at the University of North Carolina at Chapel Hill and his colleagues appears in the May issue of the Archives of General Psychiatry.

Autism, or autistic spectrum disorder, is a neurodevelopmental disorder that affects aptitude for communication and personal interaction.

The structural abnormality appeared to be linked with the ability to recognize faces and emotions and joint attention — taking cues from an adult's gaze to pay attention to an object of interest, the researchers said.

The findings build on previous research showing the amygdala is involved in social and emotional perception in autism.

The MRI scans are currently a research tool and not a diagnostic one. But by understanding the pattern of early brain changes in autism, researchers hope to detect the disorder earlier, with the aim of predicting who is likely to benefit from early intervention.

Pinpointing growth time

In the study, the researchers took MRI scans of 50 autistic children and 33 children without the condition and gave tests to look for features of autism at age two (the earliest age the condition is generally diagnosed) and at age four.

The researchers took age, sex and IQ into account.

Scientists are trying to find out when the amygdala starts to grow larger in people with autism, and the results of this study suggest it happens by age two.

"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 two years in autism and remain enlarged during early childhood," the study's authors wrote.

The team continues to follow study participants to determine whether amygdala growth rates continue at the same rate, speed up or slow down in children with autism after age four.

The study was funded by the U.S. National Institutes of Health.