The cerebellum

Abstract

For neuroscientists, the cerebellum has been a favorite target for experimental analysis, in part because of the geometric simplicity of its neuronal architecture and patterns of inputs and outputs, its distinct developmental patterning, and its remarkable capacity for physiologic and synaptic neuroplasticity to alter the function of specific neuronal circuits. Despite the uniformity of themodular structural organization across the cerebellum, and recent advances in understanding the connectional networks that define the function of the modular zones, remarkably little is known about the precise operations or even the essential computational functions carried out by the cerebellum. The organization of the neuronal machinery of the cerebellum, considering inputs, outputs, and intracerebellar connections, likely defines its general function of comparing cortical signals of intended movements or cognitive plans with sensory signals reporting on events associated with the actual movement or outcomes to generate feed-forward signals that can modify subsequent actions or strategies. The cerebellum receives an error signal through the climbing fibers about outcomes of the intended movementthat can activate mechanisms of neuroplasticity and learning to modify the next occurrence of the movement and tailor it to the appropriate environmental and sensory signals. A wide variety of experimental studies have generated convincing data demonstrating that cerebellar circuitry can be altered functionally by experience and that certain types of motor learning can be prevented or altered by cerebellar lesions. Neuroplasticity in defined cerebellar circuitry in the vestibulocerebellum is essential for adaptation of the vestibulo-ocular reflex, and learning-related plasticity in circuits of the spinocerebellum are necessary for classic conditioning of discrete somato-motor responses such as conditioned eyeblinks. Likewise, the lateral cerebellar hemispheres are important for learning motor and cognitive skills acquired with extensive practice. Nevertheless, the function of the cerebellum extends beyond motor programming and motor learning in the strict sense. Advances in the past decade provide structural and functional evidence that the cerebellum is involved in higher-order functions, including encoding and maintenance of working memory, regulation of planning of goal-directed behavior and correction of that behavior when conditions change, and even regulation of emotion and mood states. More precise knowledge of the functional mapping of the cerebellum, including specific modular topography and computational processes within and between modules, combined with advances in understanding molecular mechanisms of cerebellar plasticity in relation to behavioral change, should lead to more comprehensive accounts of cerebellar functions in motor and cognitive control and learning.