Although it has been known for half a century that unique structures evolved in the cerebellum of anthropoid apes and became greatly enlarged in the human brain, the function of these structures still remains unknown. In an attempt to explain their function, a new concept of cerebellar capabilities is proposed, which is based both on neural evidence and on information-processing theory. The phylogenetically newest structures of the cerebellum may contribute to mental skills in much the same way that the phylogenetically older structures contribute to motor skills. In both cases, the cerebellum can send signals from the dentate nucleus to the cerebral frontal cortex via the thalamus. Signals from the older part of the dentate nucleus certainly help the frontal motor cortex to effect the skilled manipulation of muscles, and signals from the newest part of the dentate nucleus may help the frontal association cortex to effect the skilled manipulation of information or ideas. How such mental skills could have evolved in higher primates in the course of phylogenetic and ontogenetic development is shown. The validity of this new concept of cerebellar function can be tested on humans by means of tomographic brain scans.

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Although the cerebellum has traditionally been regarded as a motor mechanism, recent behavioral evidence indicates that the human cerebellum is involved in a wider range of functions: in learning, in planning, in judging time, in some emotional and cognitive disorders such as autism, and in some normal mental activities such as the cognitive processing of words. This evidence suggests that the traditional view of cerebellar function now needs to be reassessed and enlarged to include nonmotor as well as motor functions in the human brain. Whereas the cerebellar connections to frontal motor areas enable the cerebellum to improve motor skills, cerebellar connections to adjacent association areas of the prefrontal cortex can enable the cerebellum to improve mental skills, and cerebellar connections to Broca's area can enable the cerebellum to improve language skills.

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 a5 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. D 1995 wi~cy-~iss, Inc.