Review: The Cerebellum and Cognition
Key findings
- Damage to the cerebellum can result not just in cerebellar motor syndrome (ataxia) but also vestibular cerebellar syndrome and/or cerebellar cognitive affective syndrome (CCAS)
- CCAS manifests as impairments in executive function, visual–spatial disorganization, impaired visual–spatial memory, personality changes and difficulties with language
- The recently developed CCAS/Schmahmann syndrome scale is a quick way to test for CCAS in patients with cerebellar disease
- Recent findings about impairment in emotion attribution and metalinguistics in cerebellar diseases are further evidence that the cerebellum regulates the speed, consistency, capacity and appropriateness of cognitive processing
- Investigation of the role of the cerebellum in Alzheimer's disease, frontotemporal dementia, multiple sclerosis and other diseases is expected to yield novel neurological and neuropsychiatric treatments
For almost 200 years, it was believed that the cerebellum is engaged only in motor control—the term cerebellar syndrome was synonymous with ataxia. Today, neurologists refer to a triad of cerebellar clinical phenomena:
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- The cerebellar motor syndrome
- The cerebellar vestibular syndrome
- The cerebellar cognitive affective syndrome (CCAS), also known as Schmahmann's syndrome
In Neuroscience Letters, Jeremy D. Schmahmann, MD, director of the Ataxia Unit at Massachusetts General Hospital, describes the syndrome that bears his name and highlights other findings of the cognitive implications of cerebellar damage.
The Story of CCAS
Beginning in the 1830s, patient reports hinted that damage to the cerebellum might be relevant to cognition. In the 1990s, researchers documented that cerebellar stroke resulted in language problems including agrammatism, decreased verbal fluency and inability to detect one's own errors in language processing.
Dr. Schmahmann and a colleague conducted the study, published in Brain, that defined CCAS. They found that 20 patients with lesions confined to the cerebellum had clinically meaningful deficits in executive function, visual-spatial performance and language processing, as well as dysregulation of affect.
The deficits were most pronounced in 13 post-stroke patients, particularly the 10 whose infarcts affected the posterior inferior cerebral artery. The stroke patients improved over time, but their executive function remained abnormal.
Dr. Schmahmann's group later demonstrated, in a study published in Neuroscience, that the location of a cerebellar stroke determines the motor versus cognitive consequences. Of 39 stroke patients, 13 were not ataxic, and in all of them, the lesion was confined to the posterior lobe.
Understanding CCAS Today
Since that seminal study, Dr. Schmahmann's group and others have more fully characterized the CCAS as including:
- Impairments in executive function (e.g., planning, set-shifting, abstract reasoning, verbal fluency, working memory), often with perseveration, distractibility or inattention
- Visual–spatial disorganization and impaired visual-spatial memory
- Personality changes with blunting of affect or disinhibited and inappropriate behavior
- Difficulties with language production, including dysprosodia, agrammatism and mild anomia
CCAS arises from damage to the cerebellar posterior lobe and may occur separately or together with the cerebellar motor syndrome and the vestibular syndrome.
Testing for CCAS
Dr. Schmahmann and colleagues recently developed and validated the CCAS/Schmahmann syndrome scale, a brief test for detecting the CCAS in a patient with cerebellar disease. They have found it to be more sensitive for that purpose than the Mini-Mental State Test or the Montreal Cognitive Assessment. For now, the scale is suitable only for testing adults.
The Dysmetria of Thought Theory
More recently, in a study published by The Cerebellum, Dr. Schmahmann's group determined that cerebellar damage effects emotion attribution—the ability to detect the mental states of others based on immediately observable information. Impaired emotion attribution is associated with deficient social skills and autism spectrum behaviors.
The researchers have also found that cerebellar disease impairs metalinguistics, the use of language to express thoughts and emotions. In one of their studies, also published in The Cerebellum, all patients with cerebellar damage were impaired in the ability to interpret ambiguities in the utterances of others, understand metaphor, make logical inferences and construct meaningful sentences appropriate to the context.
Dr. Schmahmann developed the dysmetria of thought theory to encompass these and other findings of the role of the cerebellum. It holds that the cerebellum regulates not only the rate, rhythm, force and the accuracy of movements, but also the speed, consistency, capacity and appropriateness of cognitive processes.
Accordingly, when the cerebellum is damaged, the manifestation should be consistent across nervous system functions.
Future Directions
Investigations into the cognitive neuroscience of the cerebellum are extending into neuropsychiatry, Dr. Schmahmann explains. An example is his group's recent hypothesis that the cerebellum contributes to the cognitive and psychiatric symptoms of Alzheimer's disease.
The cerebellum has also been implicated in the behavioral, executive and language deficits in patients with frontotemporal dementia and the cognitive impairment that accompanies relapsing-remitting multiple sclerosis. As evidence continues to accumulate about how the cerebellum contributes to higher-order functions, it should be possible to move from theory to novel treatments for these and other diseases.
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