Mapping motor representations in the human cerebellum

The cerebellum is a major motor structure. However, in humans, its efferent topographical organization remains controversial and indirectly inferred from neuroimaging and animal studies. Even central questions such as 'Can we evoke limb movements by stimulating the cerebellar cortex?' have no clear answer. To address this issue, we electrically stimulated the posterior cerebellum of 20 human patients undergoing surgery for tumours located outside this structure (e.g. pineal gland, quadrigeminal plate). Stimulation, delivered at a 60-Hz frequency for 2 s, evoked focal (single-joint) ipsilateral movements. Different regions were associated with the production of head (vermal lobule VI), face/mouth (hemispheric lobule VI) and lower-limb (hemispheric lobules VIIb-IX) responses. Upper-limb representations were more widely distributed. They intermingled with face/mouth representations in the superior posterior cerebellum (hemispheric lobule VI) and lower-limb representations in the inferior posterior cerebellum (hemispheric lobules VIIb-IX). No intra- or inter-limb somatotopy was found in these areas. Functionally, upper-limb (face/mouth movements) and upper limb-lower limb postural coordinations are major elements of our motor repertoire. Representation of these pairs of segments in common regions might favour the production of integrated motor behaviours. The intermediate region of the posterior cerebellum (hemispheric lobule VII and vermal lobules VII-VIII) was mostly silent. Latency results in conjunction with previous electrophysiological evidence in animals suggest that electrically evoked motor responses were not mediated by a cortical route but rather by brainstem structures. The potential role of this descending efferent pathway for fine motor control is discussed.