The Cerebellum Modulates Thought, Emotions, and Relationships
November 13, 2016
For some time now I’ve been puzzled about the cerebellum. This pair of mini-hemispheres at the underside of the back of the brain contains over half of the brain’s neurons, though it constitutes only ten percent of the its volume. I knew it had a role in controlling movements, but I was surprised to learn at a recent lecture by Jeremy Schmahmann of the Massachusetts General Hospital that the cerebellum has an important role in the regulation of thoughts, emotions, and social behavior.
The input to the cerebellum is mainly from pontine nuclei which receive their afferents from the cerebral cortex. Its output is via efferent fibers connecting to cerebral cortical regions. In addition, there are bidirectional connections with the vestibular system, the spinal cord, and the brainstem.
Past anatomic research had emphasized the cerebellum’s connections with the motor system, but more recent studies have identified input via pontine nuclei from association areas of the parietal, frontal, temporal, and limbic cortices. This connectivity implies a role for the cerebellum in modulating, in Schmahmann’s words, “higher-order, behaviorally relevant information in the domains of attention, executive function, visual-spatial cognition, language and emotion.”
Studies of patients with cerebellar injuries, strokes, and tumors have found that the anterior lobe appears to regulate motor functions, while the posterior lobe subserves cognitive and emotional functions. Further, changes in emotional tone are associated with lesions in the medial structure known as the vermis (Latin for “worm”), while cognitive problems result from lesions in the lateral posterior lobe.
In a review article, Schmahmann discusses the brain's networks, noting that functional problems can result from problems in any of a network. In the motor system, injury to the prefrontal gyrus (the motor cortex) produces paralysis or clumsy movements, damage to motor areas of the putamen results in slow movements, problems in the anterior lobe of the cerebellum produce motor incoordination, and a lesion in the ventral lateral thalamic nucleus produces ataxic hemiparesis.
Similarly, executive function problems can result from damage to the dorsolateral prefrontal cortex or its connections to the lorsolateral caudate nucleus, medial dorsal thalamic nucleus, and the cognitive areas of the posterior cerebellum. In particular, striatal deficits may impair initiation of behavior and the ability to organize information into manageable quanta. The thalamus contributes an alerting or engagement function to thought. And the cerebellum regulates behavior around a homeostatic baseline.
This last is Schmahmann’s dysmetria of thought hypothesis. The cytoarchitecture of the cerebellum is remarkable uniform, unlike the cerebral cortex, where various regions such as the hippocampus and motor cortex differ in the layered arrangement neurons. The simpler, three-layered cerebellar cerebellar cortex is thought to transform or operate on (these are engineering terms) the cognitive and emotional information it receives in the same way it handles sensorimotor information: automatizing and optimizing behavior around a homeostatic baseline appropriate to the environmental and social context by regulating the sped, capacity, consistency, and appropriateness of cognitive and emotional processes. The precise nature of the "transform" is not known. Dysfunctions in this area produce a “cerebellar cognitive affective syndrome.”
This strongly implies a role for cerebellar dysfunction in psychiatric disorders, and Joseph Phillips and colleagues have recently reviewed the evidence for the cerebellum’s involvement in attention deficit hyperactivity disorder, autism, schizophrenia, bipolar disorder, major depression, and anxiety disorders.
What does this say about treatment? The cerebellum is another node in the circuits mediating cognition, emotions, and social relationships. I expect neuroimaging of brain circuits will be available for clinical use before long, and it will be possible to localize dysfunction to particular parts of an individual patient’s brain circuitry and target treatment interventions to specific sites.
In his 2010 review, Schmahmann found no empirical evidence for efficacy of medication to treat the cerebellar cognitive affective syndrome. Cognitive-behavioral therapy, specifically making patients consciously aware of their particular cognitive and emotional modulation problems and using strategies to compensate, had shown promising results. He also discussed early findings using deep brain stimulation with implanted electrodes as well as transcranial magnetic stimulation (TMS) using neuronavigation software to target the cerebellum. (The TMS devices now commerically available for treatment of depression do not use such software and are not capable of precision targeting of brain areas.)
At this point it would be hard to diagnose cerebellar dysfunction in a patient presenting with a psychiatric disorder, much less prescribe a targeted treatment, although a history of cerebellar damage or signs and symptoms of motor regulation problems would be suggestive. We now have to include the cerebellum as one of the brain areas important to understand as we try to make sense of our patients’ psychiatric problems.