CLIPAREA l Custom media/Shutterstock
Source: CLIPAREA l Custom media/Shutterstock

Autism spectrum disorders (ASDs) are typically characterized by impairments in social interaction, communication, as well as restricted or repetitive behaviors and interests. Repetitive and restricted behavior are two of the most widely recognized features of autism. These behaviors manifest as an autistic child's preoccupation with a narrow interest, inflexibility about routines or repetitive motions such as hand flapping.

Repetitive and restricted behaviors (RRB) in children with ASD fall into two categories: “lower-order” motor actions represented by stereotyped repetitive movements or “higher-order” more complex behaviors such as rituals, compulsion, insistence on things staying the same, and circumscribed interests that reflect cognitive inflexibility.

A new study from Stanford University School of Medicine reports that girls with autism display less repetitive and restricted behavior than boys with autism. The researchers were able to identify specific brain differences between boys and girls with autism that help explain these differences.

The September 2015 study, “Sex Differences in Structural Organization of Motor Systems and Their Dissociable Links with Repetitive/Restricted Behaviors in Children with Autism,” was published online in Molecular Autism. This study provides new evidence as to why boys and girls with ASD typically exhibit different behaviors.

What Are the Brain Structure Differences in Boys and Girls with Autism?

The researchers used two large public data bases to examine nearly 800 children with high-functioning forms of autism in the United States. The researchers gauged the severity of autism symptoms in girls and boys. 

Then, they examined symptom severity compared to structural MRI brain imaging data using a novel multivariate pattern analysis. A well-matched group of ASD girls and boys was compared with typically developing peers obtained from the Autism Brain Imaging Data Exchange (ABIDE).

The scientists found that overall girls and boys with autism didn’t differ on social behavior and communication skills, but that girls had less-severe repetitive and restricted behaviors. Typically, boys outnumber girls by four to one in terms of high-functioning autism diagnoses. In a press release, the study’s senior author, Vinod Menon, PhD, professor of psychiatry and behavioral sciences at Stanford said,

This replication provides the strongest evidence to date for gender differences in a core phenotypic feature of autism. We wanted to know which specific clinical manifestations of autism show significant gender differences, and whether patterns in the brain's gray matter could explain behavioral differences.

Understanding this is really quite crucial clinically. Girls and boys with autism differ in their clinical and neurobiological characteristics, and their brains are patterned in ways that contribute differently to behavioral impairments.

Kaustubh Supekar, PhD, the study's lead author added, "We found strong evidence for gender differences in autism. The discovery of gender differences in both behavioral and brain measures suggests that clinicians may want to focus diagnosis and treatments for autistic girls differently than boys.”

Supekar and Menon were interested in comparing the expression of core features of the disorder between sexes because they have long suspected girls with autism may display symptoms differently, causing them to be underdiagnosed or making it harder for them to get the most appropriate treatment.

"Autism has primarily been studied from the viewpoint of boys with the disorder," said Menon. "Understanding gender differences can help in identifying the behavioral skills that are most important to remediate in girls vis-a-vis boys."

The brain-scan analysis revealed several gender differences in brain structure between typically developing boys and girls that are consistent with the findings of earlier studies. Children with autism, however, had a dissimilar set of gender differences in their brains—specifically, in the motor cortex, supplementary motor area, and the Crus I subdivision of the cerebellum (Latin for “little brain”). 

The primary function of the cerebellum is related to fine-tuning motor control and proprioception. The cerebellum doesn’t initiate movements, but contributes to coordination, precision, and accurate timing. Both hemispheres of the cerebellum receive a wide range of input from sensory systems and integrate these inputs to fine-tune coordinated motor activity.

Damage to the cerebellum results in disorders in motor learning, dysmetria, impaired fine muscle movements, disrupted equilibrium, balance, and posture. It also appears that abnormalities in the cerebellum are linked to various behaviors associated with autism spectrum disorders. Jeremy D. Schmahmann, M.D. of Harvard Medical School has a theory called "Dysmetria of Thought" which is a hypothesis that the cerebellum fine-tunes cognitive function like it fine-tunes muscle movement. 

The Stanford researchers noted that many repetitive behaviors, such as hand flapping, have a motor component. The study demonstrated that patterns and volume of gray matter in these motor regions could be used to distinguish girls from boys with autism. 

The study concludes that one reason the severity of repetitive/restricted behaviors is lower in girls with ASD could be associated with sex differences in gray matter structure in cortical and cerebellar regions involved in motor control. These findings indicate that the brains of girls with ASD are structured differently from those of boys and that some of these differences are linked to sex differences in autistic behaviors.

ASD is diagnosed much more frequently in males than in females. There is emerging evidence that the clinical presentation of autistic behaviors is different in females and males. However, until this study, there haven't been systematic attempts to characterize neuroanatomical differences underlying the distinct different behavioral profiles observed in girls and boys with ASD.

How Are Abnormalities of the Cerebellum Linked to Autism Spectrum Disorders?

Life Sciences Database/Wikimedia Commons
Cerebellum (Latin for "little brain") in red. 
Source: Life Sciences Database/Wikimedia Commons

The cerebellum is one of the brain regions most often associated with autism. I've written extensively about neuroscientific research that correlates cerebellar abnormalities with autism spectrum disorders. Cerebellar means “of or pertaining to the cerebellum." Last month, I wrote a Psychology Today blog post, “More Research Links Autism and the Cerebellum” which sums up the latest findings. 

A February 2015 study, “Cerebellar Gray Matter and Lobular Volumes Correlate with Core Autism Symptoms,” was published in the journal Neuroimage. The researchers found reduced gray matter volume in subregions of the cerebelllum called “lobules” in children with autism. 

More specifically, structural differences and behavioral correlations converged on specific subregions or lobules of the cerebellum known as “cerebellar Crus I and II.” Across the board, children of both genders with ASD had less gray matter in right Crus I and II. Interestingly, the degree of cerebellar gray matter reduction in these regions predicted the degree of core autism behaviors and symptom severity.

Historically, post-mortem analyses of ASD individuals found that people with autism had reduced Purkinje cell size and quantity regardless of age, sex, or cognitive ability.

In a press release, the researchers stated, “Recent evidence also suggests that there are functional subregions within the human cerebellum for sensorimotor and cognitive processing; the anterior lobe connects to primary sensorimotor areas of the cerebral cortex, whereas the posterior lobe connects to prefrontal and parietal regions.”

Conclusions: The Cerebellum May Hold Clues for Gender Differences in ASD

Gaining a better understanding of how subregions of the cerebellum are linked to ASD in boys and girls may become a new frontier for autism research. Sam Wang, Associate Professor of Molecular Biology at Princeton University, wrote in his review of the highlights from the 2014 Society of Neuroscience conference:

One should not think of the cerebellum as a single monolithic object. I think that sharpening one's understanding of which part of the cerebellum might contribute to loss of function would be pretty interesting. 

A presentation by Peter Tsai and Mustafa Sahin addressed the question of whether early-life contributions by cerebellar Purkinje cell function can affect later behavioral function. I like the way they are examining whether rescue at different points in life can affect later behavioral outcomes. Their work points toward the possibility outlined in my recent review, that brain regions can have effects that reach far beyond what they themselves are thought to do in the adult animal.

Diedrichsen et al/Creative Commons
Lobules I-X of the cerebellum. 
Source: Diedrichsen et al/Creative Commons

Wang also commented on research which found that certain lobules of the cerebellum (specifically, VI and VII, near the midline) had effects on social interaction, but other lobules (IV and V) did not. This suggests that different parts of the cerebellum have different roles, just as different layers of the neocortex have different functions.

Recent studies have found evidence for reduced levels of restrictive and repetitive behaviors in girls, compared to boys with ASD and also differences in cerebellar brain volume of the Crus I and II lobules in both sexes. The fact that RRB severity is associated with sex differences in gray matter morphometry in distinct motor regions could be a revolutionary discovery. 

The latest study from Stanford University School of Medicine provides valuable insights into brain structure differences between boys and girls with autism. These findings are a call to action for the development of better tools to diagnose autism in both sexes—but especially in girls because their autistic symptoms often go undetected.

If you'd like to read more on this topic, check out my Psychology Today blog posts:

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