Summary: The left and right brain areas are more similar in those with ASD. Reduced asymmetry was identified in cortical thickness across various brain regions in those on the autism spectrum.
Source: Max Planck Institute
The left and right halves of our brains develop differently, as each hemisphere ‘specialises’ in certain functions. For example, for most people the left hemisphere–controlling the right hand–is dominant for language. 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. PhD student Merel Postema explains: “Previous studies have suggested that people with autism spectrum disorder are less likely to have the typical asymmetries for language dominance or hand preference. However, it has not been clear whether asymmetry of the brain’s anatomy is affected in autism, because different studies have reported different findings.”
Do people with autism have a more symmetrical brain? To settle this question, scientists from the international ENIGMA consortium of brain researchers decided to do a large-scale study, based on brain scans that were collected in different countries over more than 20 years. This is by far the largest ever study of this question, using brain scan data from 1,774 people with autism and 1,809 healthy controls.
The team found that the left and right cerebral hemispheres of the brain are indeed more similar in people with autism. 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”, says study leader Clyde Francks. “But the findings might inform our understanding of the neurobiology of autism spectrum disorder”.
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. For example, many of the affected brain regions overlap with the default mode network, which is a network of inter-connected brain regions that is particularly active during passive rest and mind-wandering, rather than when doing a specific task. Why this might relate to ASD may be a topic of future research.
Max Planck Institute
Marjolein Scherphuis – Max Planck Institute
The image is credited to Clyde Francks, MPI Nijmegen.
Original Research: Open access
“Altered structural brain asymmetry in autism spectrum disorder in a study of 54 datasets”. Postema, M.C., Van Rooij, D., Anagnostou, E., Arango, C., Auzias, G., Behrmann, M., … Francks, C..
Nature Communications doi:10.1038/s41467-019-13005-8.
Altered structural brain asymmetry in autism spectrum disorder in a study of 54 datasets
Altered structural brain asymmetry in autism spectrum disorder (ASD) has been reported. However, findings have been inconsistent, likely due to limited sample sizes. Here we investigated 1,774 individuals with ASD and 1,809 controls, from 54 independent data sets of the ENIGMA consortium. ASD was significantly associated with alterations of cortical thickness asymmetry in mostly medial frontal, orbitofrontal, cingulate and inferior temporal areas, and also with asymmetry of orbitofrontal surface area. These differences generally involved reduced asymmetry in individuals with ASD compared to controls. Furthermore, putamen volume asymmetry was significantly increased in ASD. The largest case-control effect size was Cohen’s d = −0.13, for asymmetry of superior frontal cortical thickness. Most effects did not depend on age, sex, IQ, severity or medication use. Altered lateralized neurodevelopment may therefore be a feature of ASD, affecting widespread brain regions with diverse functions. Large-scale analysis was necessary to quantify subtle alterations of brain structural asymmetry in ASD.