Cerebello-frontal cortical projections in humans studied with the magnetic coil.

Focal stimulation over human cerebellum with a figure 8 magnetic coil (MC) results in an evoked wave recorded from bipolar scalp electrodes on the interaural line and more anteriorly. In 3 subjects, the wave responses along the interaural line had latencies of 8.8-13.8 msec, lasted 17.4-29.0 msec and had a maximum amplitude of 14.4-26.8 microV. The responses were recorded more anteriorly from leads midway between the interaural line and frontal leads; responses recorded from frontal leads were up to 3.5 msec later. The evoked wave was preceded by a diphasic EMG response with a latency of 1.2-2.0 msec. Analysis of the averaged responses recorded by adjoining bipolar leads indicated that the response was predominantly surface positive and crossed. Control experiments eliminated eye movement and somatosensory input as explanations of the evoked response, thereby identifying it as a cortical response. The surface positive wave in humans was compared with the responses recorded in cat and monkey to cerebellar stimulation. The responses in humans could reflect dysfacilitation through MC activation of Purkinje cells, or feed-forward facilitation through transsynaptic or antidromic activation of dentate neurons. The latency of the surface positive wave exceeds that of cerebellar inhibition of MC elicited hand muscle responses, but the discrepancy is at least partly accounted for by the extra delay required to set up the indirect cortico-spinal component required for motoneuron discharge. Estimates made of the cerebello-frontal cortical and peripheral feedback loop times suggest that the central has less than one quarter the delay of the peripheral loop, which would be especially advantageous during fast skilled movements of the fingers.