The Underestimated Cerebellum

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New evidence on the structure and function of the cerebellum, which is summarized in this review, is beginning to clarify the role of the cerebellum in the human brain. The new evidence challenges the traditional concept that the cerebellum serves essentially as a motor mechanism. Instead, a more powerful role is suggested in which the cerebellum contributes to other functions as well, by sending its output to other locations in the cerebral cortex besides the well-known motor areas. Structural evidence about the cerebellar output to such cerebral targets was obtained by using a new anatomical tracing technique on the monkey, which shows that the cerebellum sends a significant projection of nerve fibers to cognitive areas of the prefrontal cortex. Congruent with this anatomical evidence is the neuroimaging evidence obtained on normal human brains, which shows that the cerebellum is strongly activated when the brain performs some cognitive and language functions. Both structurally and functionally, therefore, the cerebellum is underestimated when it is regarded solely as a motor mechanism. Instead, it can be regarded as a more versatile information-processing mechanism whose circuitry carries out two basic processes that are commonly performed by computers: 1) the cerebellar circuitry performs transformations on the streams of information flowing into it, and 2) it distributes the transformed streams to the right places in the brain at the right time. When such processing is performed repeatedly on motor, or cognitive, or language tasks, the cerebellum and its cerebral targets can learn through practice to perform these tasks automatically, thereby improving the speed of performance. This speed is needed, for example, in learning to speak a language fluently because such fluency requires a very rapid selection of words, which can be achieved if the search process for finding the words is performed automatically in the brain. We suggest to brain-mappers that new discoveries about these language and cognitive functions can be found by imaging those parts of the cerebro-cerebellar system that evolved uniquely in the human brain, and we indicate where to look.

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