THE RELEVANCE OF SENSORY INPUT FOR THE CEREBELLAR CONTROL OF MOVEMENTS

The performance of a motor task not only requires subjects to plan, pr epare, and initiate but also to monitor how a movement is performed. W e used positron emission tomography to examine to what extent the huma n cerebellum is involved in controlling motor output or sensory input from movements in normal subjects. In the first study, we compared the active performance of a motor task (flexion and extension of the righ t elbow) to the passive execution of the same movements. Passive movem ents were driven by a motor with the arm fixed in a guide hinge. Activ e movements (compared to rest) elicited increases of rCBF mainly in th e ipsilateral neocerebellar hemisphere and vermis of the posterior lob e. During passive movements, almost identical parts of the cerebellar hemispheres and vermis were activated (compared to the rest condition) . The direct comparison of active and passive movement conditions reve aled a small activation of the neocerebellar hemisphere of the posteri or lobe and cerebellar nuclei ipsilateral to the movement. Approximate ly 90% of cerebellar neuronal activity was related to sensory input. I n the second study, we compared the execution of a free selection joys tick movement task to a condition in which subjects simply imagined th e movements. The execution of movements (compared to rest) was associa ted with increases of rCBF in the ipsilateral neocerebellar hemisphere and vermis of the posterior lobe. During movement imagination, a smal l part of the ipsilateral cerebellar hemisphere and vermis of the post erior lobe was activated (compared to rest). The increase of rCBF duri ng movement imagination accounted for only 20% of the signal seen duri ng movement execution. Our results indicate that the neocerebellum may be much more concerned with sensory information processing than has b een considered previously. (C) 1997 Academic Press.