On the mechanism of cerebellar contributions to cognition.

The cerebellum is highly stereotyped in its cellular circuitry. Output neurons in the nuclei with one exception excite their downstream targets in other parts of the nervous system. Yet the much more voluminous cerebellar cortex inhibits these output neurons. This has suggested that the desired output activity pattern is achieved by removing all unwanted activity patterns ('sculpting'). Lesions of the lateral cerebellum impair cognitive functions including speech. These lateral portions are active during imagined as well as overt movements. Imagined movements could be used to time task performances in the absence of an external clock. The intrinsic circuitry suggests that the cerebellar cortex links together and combines nuclear output activities. A linkage mechanism is consistent with the motor deficits in coordination after midline vermal section in humans and Purkinje cell recording in trained animals. The lateral cerebellum, which projects to frontal and parietal 'association' cortex, may link together cerebral 'cognitive units' as a substrate for coordinated thought.