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The Mystery of the Human Cerebellum: What does it do?

April 6, 2004

Date: Tuesday, April 6, 2004
Woodward 149

Dr. Nancy Andreasen, <> Director, MIND Institute

Abstract: For many years the cerebellum has been viewed as a cerebral organ primarily dedicated to coordinating motor activity. During recent years, however, new evidence has emerged that indicates that the cerebellum may also be a key cognitive organ in the brain as well. Several models of cerebellar nonmotor or cognitive learning have been proposed. One model (Keele and Ivry, 1990, 1997) argues that the cerebellum functions as a clock or timekeeper, based primarily on lesion studies that indicate cerebellar injury leads to an impairment in the ability to estimate time intervals or imitate timed rhythm sequences. PET studies recently conducted in Iowa confirm that a variety of timing tasks produce robust activation in the cerebellum, as well as the thalamus and insula. Another model has been proposed by Ito (1997). He argues from cerebellar anatomy and suggests that the cerebellum is composed of large numbers of units that he refers to as microcomplexes. These provide an error-driven adaptive control mechanism. Microcomplexes are subunits within the cerebellum that facilitate its function. They receive dual inputs from mossy and climbing fibers; the climbing fibers detect errors and act to reorganize internal connections, while the mossy fibers "drive" the complex. The microcomplexes function like computer chips or microprocessors-they can be used to perform a vast array of different functions. They are connected to diverse brain regions (e.g., multiple different cortical areas) and therefore can play many diverse roles in brain function, including all types of cognitive processing. In addition, evidence has also emerged that suggests that cerebellar function is impaired in schizophrenia. Studies of schizophrenia using the tools of functional imaging have found a relatively consistent pattern of abnormalities in distributed brain regions that include the cerebellum. Abnormalities are seen in these studies in both the vermis and in the cerebral hemispheres in patterns that are task-related. Patients with schizophrenia have decreased blood flow in the cerebellum in a broad range of tasks that tap into diverse functional systems of the brain, including memory, attention, social cognition, and emotion. Vermal abnormalities are more frequently noted in tasks that use limbic regions (e.g., studies of emotion), while more lateral neocerebellar regions are abnormal in tasks that use neocortical regions (e.g., memory encoding and retrieval). It is therefore highly plausible that the symptoms and cognitive abnormalities of schizophrenia may arise because of malfunctions in a group of distributed brain regions and that the cerebellum is a key node in this malfunctioning group of regions.