People with autism often struggle with social interaction. This difficulty is linked to a dysfunction in the cerebellum. Frank Van Overwalle has researched the underlying mechanisms and offers new insights. "Hopefully, these findings will help improve the support provided to people with autism."
The cerebellum is located below the cerebrum and behind the brainstem—essentially at the back of the skull. Despite its modest size, it plays a crucial role in motor control and coordination, which has long been established. Individuals with cerebellar damage often struggle to touch the tip of their nose with their index finger fluidly and effortlessly or walk a straight line without stumbling. These are not random tests occasionally performed by police during roadside checks; alcohol has a similar effect on the cerebellum as physical injury.
"The cerebellum is often called the motor brain," explains Prof. Emeritus Frank Van Overwalle (Brain, Body and Cognition Research Group, Department of Psychology). "It ensures you can perform movements automatically, without consciously thinking about how you do them. These are deeply ingrained actions you’ve practised countless times. For instance, if you step into a pothole while walking, your body immediately adjusts without conscious thought—that’s the cerebellum at work. The same happens in tennis: when your opponent hits a ball, you automatically adapt your swing—perhaps slightly harder, softer, more to the left, or to the right. The cerebellum automates and fine-tunes motor skills."
Interestingly, the cerebellum also influences cognitive, emotional, and social processes.
Frank Van Overwalle: "This is a more recent insight, one also observed in clinical practice. Patients who had tumours removed from the cerebellum sometimes displayed peculiar behaviours post-surgery, such as remaining silent for days or experiencing sudden emotional outbursts. Until about ten years ago, no research had been conducted on this social aspect. We decided to explore this gap."
How do you approach such research?
"The first step was a meta-analysis of existing studies. Research on brain activity often uses fMRI scans—functional Magnetic Resonance Imaging—a technique that measures changes in blood flow within the brain to identify which regions become active during specific tasks involving language, memory, emotions, and so on. We reviewed as many brain studies on social behaviour as possible, focusing on the cerebellum. It was often excluded from these studies because it was thought to play no role in social activities. Surprisingly, in 30% of these studies, we found cerebellar activation."
Did you conduct your own experiments with fMRI?
"Yes, that was the second step. We showed participants sequences of social actions—for example, a cartoon character performing tasks or navigating an obstacle course. When we asked participants to describe what the character was doing, there was little activity in the cerebellum. However, when we asked them to recount the sequence of actions or place them in the correct order, the cerebellum lit up."
"Autism is a developmental disorder and cannot be cured. However, our new findings could help improve the support available to individuals with autism."
What did you learn from this?
"That the cerebellum plays a crucial role in understanding the sequence of actions."
How does that relate to social processes?
"The order of actions is essential for making sense of social interactions. Imagine watching two people talk. Suddenly, one hits the other, and the other strikes back. Based on the sequence of actions, you might conclude that the first person is the aggressor and the second is defending themselves. Understanding this relies on recognising the order of events. This skill becomes even more critical when dealing with 'false beliefs' or 'outdated beliefs.'
"For example, in our experiments, participants watched a short cartoon where a boy is playing at a table with a sweet on it. The boy leaves the room, and a girl enters, eats the sweet, and leaves. When the boy returns and looks puzzled at the empty table, we understand his surprise because we know he didn’t see what happened. He has a ‘false belief.’ This ability to infer someone else’s thoughts, known as 'mentalising' or 'cognitive empathy,' allows us to understand others' perspectives."
That’s quite an achievement?
"Indeed. While great apes can do this to a limited extent, humans only develop this skill around the age of three or four. It’s a significant milestone in social intelligence."
Your research shows that people with autism struggle with this?
"Yes. Thinking about the sequence of actions or 'false beliefs' takes longer for individuals with autism; they find it more challenging. This is reflected in the time they take to respond to such scenarios—it takes just a bit longer."
Why is that?
"Something goes wrong during the development of the cerebellum. We see this, for example, in premature babies, whose cerebellums often show smaller or larger lesions at birth. When such lesions are present, the likelihood of developing autistic symptoms later is more than one in three."
How do you explain the link between autism and a dysfunction in the cerebellum?
"When something goes wrong there, social interaction becomes more challenging. This is evident in situations where others say or do something slightly different from what you expect. People without autism automatically adjust their responses, almost unconsciously—just like the walker or tennis player I mentioned earlier, who adjusted their movements instinctively. A dysfunction in the cerebellum may explain why people with autism lack that flexibility. It’s why unusual or unexpected events in social interactions can quickly lead to overstimulation and chaos for them. This also explains their preference for a familiar and predictable environment. You can observe the same tendency in their motor skills: people with autism often exhibit stereotypical movements."
Autism is often a genetic condition. Is this also visible in the cerebellum?
"Yes. Scans of adults with autism show that their cerebellums tend to have slightly less volume, in terms of fewer neural connections. This is particularly true in the posterior part of the cerebellum, which is associated with social and emotional functions. The motor region is located more towards the front. This difference can be explained evolutionarily. The social-emotional region is a more recent development compared to the motor region. Since the cerebellum is located behind the brainstem, it could only grow and develop backwards."
How significant is the genetic factor in autism?
"If you have a sibling with autism, the likelihood of also being autistic is 10%. For identical twins, that probability increases to 50%."
A cure seems impossible for a disorder so deeply intertwined with brain function.
"Autism is a developmental disorder—it cannot be cured. However, our new insights can help improve the support provided to people with autism. For example, caregivers already understand that individuals with autism struggle with mentalisation, or imagining what others are thinking, and they train for this. Now we’ve identified that understanding the sequence of actions is another challenge, and this can also be addressed. In fact, this was already happening in the 1960s, likely based on intuition. Children were told stories and explicitly taught how to respond to different plot twists. This helped them pick up behaviours they couldn’t learn spontaneously."
You are also exploring whether magnetic or electrical stimulation could help with these training methods.
"Yes, we’re looking into TMS and tES: transcranial magnetic stimulation and transcranial electrical stimulation. These techniques are already used as supplementary treatments for depression, alongside medication and psychotherapy. Electrodes are placed on the head, and magnetic or electrical pulses influence brain activity. TMS directly stimulates signal transmission in the brain, causing specific neurons to fire. tES, on the other hand, doesn’t work directly; it merely increases the likelihood of neurons firing.
We’ve conducted research on the impact of tES when the cerebellum is stimulated. When we ask healthy participants to sequence social actions after a session of tES, fMRI scans show that all brain regions responsible for social processes light up more strongly—they are more activated. Perhaps one day, we could use tES to support people with mild autism, helping them learn certain things more easily. But that’s still a long way off."
Your research was recently published in the prestigious Nature Reviews Neuroscience. Happy? Proud?
"Both, definitely—for the entire group of scientists involved. For me personally, it also felt like a career milestone. I recently retired, although I’m still continuing my work, albeit at a slower pace. I still find it absolutely fascinating."*
*This is a machine translation. We apologise for any inaccuracies.