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Introduction

THE CEREBELLUM CONSTITUTES ONLY 10% of the total volume of the brain but contains more than one-half of its neurons. The cerebellar cortex comprises a series of highly regular, repeating units, each of which contains the same basic microcircuit. Different regions of the cerebellum receive projections from distinct brain and spinal structures and then project back to the brain. The similarity of the architecture and physiology in all regions of the cerebellum implies that different regions of the cerebellum perform similar computational operations on different inputs.

The symptoms of cerebellar damage in humans and experimental animals provide compelling evidence that the cerebellum participates in the control of movement. The symptoms, in addition to being diagnostic for clinicians, thus help define the possible roles of the cerebellum in controlling behavior.

Several fundamental principles define our understanding of the physiological function of the cerebellum. First, the cerebellum acts in advance of sensory feedback arising from movement, thus providing feedforward control of muscular contractions. Second, to achieve such control, the cerebellum relies on internal models of the body to process and compare sensory inputs with copies of motor commands. Third, the cerebellum plays a special role in motor and perceptual timing. Fourth, the cerebellum is critical for adapting and learning motor skills. Finally, the primate cerebellum has extensive connectivity to nonmotor areas of the cerebral cortex, suggesting it performs similar functions in the performance and learning of motor and nonmotor behaviors.

Damage of the Cerebellum Causes Distinctive Symptoms and Signs

Damage Results in Characteristic Abnormalities of Movement and Posture

Disorders that involve the cerebellum typically disrupt normal movement patterns, demonstrating the cerebellum’s critical role in movement. Patients describe a loss of the automatic, unconscious nature of most movements. In the early 20th century, Gordon Holmes recorded the self-report of a man with a lesion of his right cerebellar hemisphere: “movements of my left arm are done subconsciously, but I have to think out each movement of the right arm. I come to a dead stop in turning and have to think before I start again.”

This has been interpreted as an interruption in the automatic level of processing by cerebellar inputs and outputs. With a malfunctioning cerebellum, it seems that the cerebral cortex needs to play a more active role in programming the details of motor actions. Importantly, individuals with cerebellar damage do not experience the paralysis that can be associated with cerebral cortical damage. Instead, they show characteristic abnormalities in voluntary movement, walking, and posture that have provided important clues about cerebellar function.

The most prominent symptom of cerebellar disorders is ataxia, or lack of coordination of movement. Ataxia is a generic term used to describe the collective motor features associated with cerebellar damage. People with cerebellar disorders make movements that qualitatively appear jerky, irregular, and ...

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