While it is known that the human brain is divided into two hemispheres, what is not known is that despite the two develop differently, symmetry is found in the brain of autistic people.
The left and right halves develop differently, as each 'specialises' in certain functions. But brain asymmetry is sometimes affected in people with developmental or psychiatric disorders like autism, which is characterised by impaired social cognition, repetitive behaviour, and restricted interests.
"Previous studies have suggested that people with autism spectrum disorder are less likely to have the typical asymmetries for language dominance or hand preference," explained PhD student Merel Postema.
"However, it has not been clear whether asymmetry of the brain's anatomy is affected in autism, because different studies have reported different findings," Postema added.
To find if people with autism have a more symmetrical brain, researchers decided to do a large-scale study, based on brain scan data from 1,774 people with autism and 1,809 healthy controls that were collected in different countries over more than 20 years.
They found that the left and right cerebral hemispheres of the brain are indeed more similar in people with autism.
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In other words, people with ASD had less brain asymmetry. The reduced asymmetry was mostly found for cortical thickness, at various locations across the brain's surface.
In the healthy brain, the thickness of the cerebral cortex (the thin layer of gray matter that covers the brain) differs between the left and right hemispheres.
Importantly, the anatomical differences did not depend on age, sex, IQ, the severity of symptoms, or medication use.
"The very small average differences in brain asymmetry between affected people and controls mean that changes of brain asymmetry will not be useful in terms of clinical prediction", said study lead Clyde Francks.
As the bulk of the data were from children, the findings suggest that altered development of the brain's left-right axis is involved in autism, affecting widespread brain regions with diverse functions.