People with autism have fundamentally the same brain anatomy as typically developed individuals. Previous claims that differences in brain anatomy could explain the condition in general are wrong, says Israeli scientists, based on the world's broadest brain-imaging study to date.
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Therefore, anatomical measures of the brain alone are unlikely to help identify autism, spells out Dr. Ilan Dinstein of the departments of Psychology and Brain and Cognitive Sciences at the Ben-Gurion University of the Negev, who led the study.
Doctors have long hoped - even expected - to find answers to the origin of autism in brain structure. Indeed, numerous scientific papers did find differences. But these papers were fatally flawed, Dinstein explains, not least because each studied a small sample of people, typically 20 or 30.
"If among 20 subjects one has an especially small brain, it will skew the data of that tiny study significantly," he explains. "Then a paper is published reporting on anatomical differences."
Huge differences in 'normal' brains
Dinstein's innovation was first of all, to tap a worldwide collection of MRI scans from over 1,000 individuals (half with autism and half controls) ages six to 35 years old: 500 "normals" as control and 500 autism sufferers. His team also checked multiple areas of the brain (not only volume or, say, amygdala); and - for the first time - to methodically examine the hetereogeneity within each group, compared with the heterogeneity between the two groups.
The database, called the Autism Brain Imaging Data Exchange (ABIDE), provided unprecedented opportunity to conduct large-scale comparisons of anatomical MRI scans across autism and control groups.
For their study, the researchers divided each brain (figuratively) into 180 regions, and looked for differences in each one. They assessed multiple anatomical measures such as the volume, surface area and thickness of each brain region.
Some people have very small brains. Others have big ones. The differential in brain size alone within a group (whether the control group or autism group) can be as big as 90 percent. Yes, the guy next to you could have a brain nearly twice the volume of yours. But there no statistically significant difference was found in brain volume between the control group and the autism group.
The secrets of autism evidently lie elsewhere than in differences in brain anatomy, conclude the researchers from Ben-Gurion University of the Negev and Carnegie Mellon University in their paper “Anatomical Abnormalities in Autism?” published in the Oxford journal Cerebral Cortex.
Why did the other papers get it wrong?
Previous hypotheses suggested that autism might be associated with larger intra-cranial gray matter (the stuff of the brain itself), white matter (nervous tissue) or the volume of the amygdala. Some scientists thought it might be associated with smaller cerebellar, corpus callosum and hippocampus volume.
These papers got it wrong for four reasons, says Dinstein. One: The small sample size of the groups they studied, which could skew results. Two: Pressure to publish positive results. Three: They tended to focus on one specific area of the brain (such as the amygdala) and four: They did not factor in that there are vast anatomical differences between normal people's brains. (They knew it, they just didn't factor it in, explains Dinstein.)
"Since their samples were so small, what one would expect is that different papers would report different results, and they did," says Dinstein. "The inconsistency got so bad that one paper would find an enlarged amygdala among the autism group and another paper would report the opposite. Our study simply explains why this has been happening and puts an end to several ensuing debates.”
Dinstein also points out that autism has become a catchall phrase that actually covers a wide range of conditions. It is entirely possible that one specific kind within the autism spectrum is associated with a specific brain structure anomaly, he clarifies. “Expecting to find a single answer for the entire ASD population is nave," he says. "We need to move on to thinking about how to split up this very heterogeneous group of disorders into more meaningful biologically-relevant subgroups."