How Does Brain Symmetry Influence the Workings of Your Mind?

Research identifies link between brain symmetry and autism spectrum disorders.

Posted Nov 18, 2016

These stock images illustrate brain asymmetry between the left and right cerebral hemispheres of the cerebrum.
Source: Semnic/Shutterstock

Neuroscientists have identified intriguing symmetry and asymmetry differences between various brain regions in mice that carry autism-gene mutations and their autism-free counterparts. The unpublished results of this research were presented by Jacob Ellegood earlier this week at the 2016 Society for Neuroscience annual meeting in San Diego.

The left and right hemispheres of the cerebrum in red. 
Source: Life Science Databases/Wikimedia Common

Ellegood et al. found that ‘autistic’ mice showed more symmetry between the left and right hemispheres of the cerebrum (Latin for “brain”) when compared to controls without mutations in the autism genes. Interestingly, the 'autistic' mice showed more asymmetry between the left and right hemispheres of the cerebellum (Latin for “little brain”).

Ellegood and his colleagues at the Mouse Imaging Centre in Toronto identified three brain regions in the autistic mouse brain that showed significant imbalances of larger volumes on the right side and smaller volumes on the left side. Again, the cerebral hemispheres were found to be unusually symmetrical in mice with autism genes. These findings on brain symmetry are both intriguing and perplexing to neuroscientists.

That being said, the asymmetry of the cerebellum is of particular interest to the researchers. A wide range of studies have found that cerebellum size and functional connectivity is correlated with autism-related functions, such as emotion regulation and attention.

Along these lines, previous research has shown that humans with autism spectrum disorders (ASD) tend to process language more in the right hemisphere of their cerebral cortex, which is atypical. Hypothetically, this shift in language processing to the right side of the cerebrum might be a way of compensating for deficits of structure or connectivity within (and between) the left and right hemispheres of the cerebellum.

"Whatever the Cerebellum Is Doing, It's Doing a Lot of It"

Life Science Databases/Wikimedia Common
The left and right hemispheres of the cerebellum in red. 
Source: Life Science Databases/Wikimedia Common

Below is a brief summary of the dynamic relationship between both hemispheres of the cerebrum and both hemispheres of the cerebellum, along with some terminology:

In 1504, Leonardo da Vinci made wax castings of the human brain and coined the term cerebellum after noticing two small "little" brain hemispheres tucked neatly under the larger cerebral hemispheres.

Cerebellar is the sister word to cerebral and means “relating to or located in the cerebellum.” There are two cerebral hemispheres located in the cerebrum (left brain-right brain) and there are two cerebellar hemispheres (left and right) located in the cerebellum.

The left hemisphere of the cerebellum works in conjunction with the right hemisphere of the cerebrum to fine-tune and control muscle movements on the left side of your body. Conversely, the right hemisphere of the cerebellum works in conjunction with the left hemisphere of the cerebrum to fine-tune and control muscle movement on the right side of your body.

Courtesy of Larry Vandervert
These neuron counts of the cerebellum and cerebral cortex are based on studies by Lent, R., et al., 2012.
Source: Courtesy of Larry Vandervert

Although the cerebellum is only 10% of brain volume, the cerebellar hemispheres house well over 50% of your brain’s total neurons. Based on this disproportionate distribution of neurons, my father, Richard Bergland—who was a twentieth-century neuroscientist, neurosurgeon, and author of The Fabric of Mind (Viking)—would often say, "We don't know exactly what the cerebellum is doing. But whatever it's doing, it's doing a lot of it." 

Historically, experts believed the cerebellum was primarily responsible for ‘non-thinking’ functions such as balance, smooth motor control, and finely-tuned muscular coordination. And, they believed that the control of our cognitive functions—including social intelligence, logic, linguistics, creativity, etc.—were seated solely in the cerebral cortex. This viewpoint is rapidly evolving. 

Many thought leaders now believe that the cerebellum plays a pivotal role in many cognitive functions and creative capacity, as well as the regulation of our emotional interactions with the world around us and within ourselves.

What Is "Dysmetria of Thought"?

In recent years, Jeremy Schmahmann, a neurologist at Harvard Medical School, has worked closely with patients who have ataxia and cerebellar damage to specific regions within both hemispheres of the cerebellum.

Through his research at Massachusetts General Hospital, Schmahmann has developed a theory he calls “Dysmetria of Thought.” This is basically the hypothesis that the cerebellum fine-tunes cognitive thinking much the same way it fine-tunes muscle movements. Schmahmann has found that when there is structural of functional damage to a particular "microzone" of the cerebellum, it impacts the workings of the cerebral mind in a specific manner. 

Here is a YouTube video of Schmahmann describing his "Dysmetria of Thought" hypothesis:Based on the lateralized function of the cerebellar and cerebral hemispheres, one could speculate that changes in cerebellar structure and functional connectivity with the cerebral cortex have profound impacts on the fabric of someone's mind.

As an example, in September 2016, Maria Assunta Rocca, from the Division of Neuroscience at Vita-Salute San Raffaele University in Milan, Italy presented unpublished research reporting that gray matter volume in the left cerebellar hemisphere played a role in predicting levels of cognitive function on certain verbal and attention tests. (I wrote about these findings in a Psychology Today blog post, "Your Left Cerebellar Hemisphere May Play a Role in Cognition").

A 2008 study by researchers from the department of Neuroscience at the University of California, San Diego found differences in the lateralized function of left and right brain hemispheres in children with autism who failed to develop normal language skills. Unlike controls without ASD, functional magnetic resonance imaging (fMRI) showed that lateralized patterns of brain activity to speech in 2–3-year-old children were different. 

For example, the UCSD neuroscientists found that when children with autism heard a bedtime story, they displayed more activity on the right side of the cerebrum, rather than the left side. Children without autism are believed to process grammar and vocabulary more heavily on the left side of the brain.

"The Cerebellum Doesn't Get Enough Credit" 

In a recent statement about his 2016 presentation, Ellegood said, “The cerebellum doesn’t get enough credit.” Other research on the cerebellum has been found that cerebellar brain volumes are either unusually small or unusually large in mice with genetic models of autism.

Two years ago, at the annual 2014 Society for Neuroscience meeting in Washington D.C., researchers presented unpublished findings linking Purkinje cells in the cerebellum with ASD. (I reported on this research in a Psychology Today blog post "How Are Purkinje Cells in the Cerebellum Linked to Autism?")

A variety of other research has found cerebellar abnormalities in humans with ASD through postmortem studies of brain samples that showed loss of Purkinje cell volume. Over the past few years, multiple studies confirmed this phenomenon in both human and mouse brains.

Having a reduced number of Purkinje cells (the largest neurons in the cerebellum) is one of the most consistent hallmarks of ASD in postmortem studies of human autism brains. Purkinje cells represent the primary output of nerve signals from the cerebellum and have elaborate neuronal projections that connect with various regions of the left and right hemispheres of the cerebral cortex.

In autism spectrum disorders, the brain consistently shows defects in Purkinje cells, which have a single axon that projects from the cerebellum and creates connectivity from the cerebellum to an infinite number of cerebral brain regions.

More Research Is Needed on Symmetry and Asymmetry Differences Between All Four Brain Hemispheres

This rudimentary sketch illustrates how the ever-changing interconnectivity of brain hemispheres in the cerebrum and cerebellum might create a feedback loop that influences the workings of the human mind. 
Source: Photo and illustration by Christopher Bergland circa 2009.

Ellegood says that he and his colleagues are not clear as to why unusual symmetry in the cerebrum and asymmetry in other brain regions would accompany autism. One possibility is that functional asymmetry results from regular practice of a skill or cognitive capacity that relies heavily on one side of the brain.

“Certainly in the human population, if you juggle, if you play music, then that area of your head gets bigger,” Ellegood said in a statement. “So whether this finding is environmental or developmental is an open question.” Ellegood and colleagues have already given many of their lab mice a range of behavioral tests and plan to conduct more research. "It will be interesting to match the structural asymmetry with behavior," Albert Basson said in the statement.

Basson's lab at King’s College London collaborates with Ellegood on other autism-related research linked to the cerebral-cerebellar connection. Basson's team has pinpointed critical roles that the CHD7 gene plays in the development of the cerebellum. He is currently researching various functions and mechanisms of CHD7 in cerebellar and frontal cortex development. Stay tuned for updates on this exciting topic!