The cerebellum receives input from regions of the cerebral cortex that plan and initiate complex and highly skilled movements; it also receives input from sensory systems that monitor the course of movements. This arrangement enables a comparison of an intended movement with the actual movement and a reduction in the difference, or “motor error.” The corrections of motor error produced by the cerebellum occur in real time and are stored over longer periods as a form of motor learning. Error correction is mediated by climbing fibers that ascend from the inferior olive to contact the dendrites of the Purkinje cells in the cerebellar cortex. Information provided by the climbing fibers modulates the effectiveness of the second major input to the Purkinje cells, which arrives via the parallel fibers from the granule cells. The granule cells receive information about the intended movement—and the actual performance of the movement—from the vast number of mossy fibers that enter the cerebellum from multiple sources. As might be expected, the output of the cerebellum from the deep cerebellar nuclei projects to circuits that govern all the major sources of upper motor neurons described in Chapter 17. The effects of cerebellar disease provide strong support for the idea that the cerebellum regulates the performance of movements. Thus, individuals with cerebellar disorders show severe ataxias in which the site of the lesion determines the particular movements affected, with the incoordination of movement on the same side of the body as the site of the lesion.