The brains of girls with autism show distinct structural differences from the brains of girls without autism, while the brains of boys with autism are indistinguishable from the brains of non-autistic boys, according to a new study.
According to a Spectrum News report this month, the study is one of the largest ever to examine sex differences in brain structure among people with autism. Until now, researchers have had trouble finding consistent differences between the brain structure of boys with autism and the brain structure of girls with the condition.
“We certainly thought we would see something in girls, but we didn’t expect this dramatic result where we didn’t see a thing in boys and then [we saw] all these differences in girls,” lead researcher Roger Jou said.
The study is also noteworthy for focusing on girls with autism, who are well known for being under-diagnosed compared with boys. Roughly four boys to every girl has an autism diagnosis, according to Spectrum’s report.
In conducting the study, researchers recruited 25 girls and 56 boys with autism, and 15 girls and 23 boys without it. The children’s brains were scanned using diffusion tensor imaging, which traces the flow of water along nerve tracts in the brain to reveal their location and integrity (meaning a measure of how orderly the tracts are). The researchers found that, compared with the non-autistic females, girls and women with autism have less integrity in several nerve tracts, including in one connecting the occipital lobe at the back of the head to the temporal lobe at the side. According to Jou, some of the girls’ challenges with language may be explained by the fact that the differences are mostly on the left hemisphere, rather than the right, since the main language areas of the brain are in the left hemisphere.
The researchers also found that autistic girls have autism traits similar to those of the boys with autism, with both genders scoring similarly on a test of autism severity. This fits with a theory known as the “female protective effect,” which suggests that biology factors, such as a lack of integrity of brain tracts, need to be more extreme in girls than in boys in order to result in autism.
The new study is limited by the wide age range of the participants, as structural differences in autistic boys and men might only show up at certain ages. To address this flaw in future studies, Jou says his team will recruit more participants and analyze the results by age.