Cerebellum and the sensory guidance of movement.

By the end of the 19th century the locations of the primary visual and motor areas of the cerebral cortex were well recognized. At that time it was generally assumed that for the visual control of movement visual areas must be linked to motor areas by way of a series of cortico-cortical fibres. Subsequent experimental evidence showed clearly, however, that skilled visuomotor performance is still possible after complete disconnection of interhemispheric and intracortical fibre systems. Preservation of skilled visuomotor performance after such lesions has often been thought to be mediated by ipsilaterally descending motor pathways. However, the evidence indicates that there must also be subcortical pathways that link sensory to motor areas of the brain. One such pathway involves the cerebellum. There is a massive input from cortical and subcortical visual areas to the pontine nuclei. Cells in the pontine nuclei respond vigorously to appropriate visual targets and they distribute their axonal terminals bilaterally in the cerebellar cortex. A cortico-ponto-cerebellar circuit would have remained intact in all cases in the literature in which there was complete disconnection of cortico-cortical fibres between visual and motor cortex. Lesions of the cortical sensory areas that project to the pons or interruption of the fibres within the internal capsule or basis pedunculi, that link cortical sensory areas with the pontine nuclei, can severely impair the sensory guidance of movement. This paper reviews the evidence for sensory input to the cerebellum and the possible role of a cortico-ponto-cerebellar circuit in the sensory guidance of movement.