EFFECT OF TRANSCRANIAL MAGNETIC STIMULATION OVER THE CEREBELLUM ON THE EXCITABILITY OF HUMAN MOTOR CORTEX

There have been conflicting reports over whether it is possible to sti mulate the human cerebellum through the intact scalp using transcrania l magnetic stimulation. Here we attempt to clarify the situation in no rmal subjects by comparing the various methods which have been used, E MG responses evoked by magnetic stimulation over the motor cortex coul d be suppressed by a prior magnetic stimulus over the cerebellum but t he onset latency of the effect varied according to the type of magneti c coil used. Inhibition began at a latency which ranged from 5 to 9 ms ec in different subjects if conditioning stimuli were given through a flat figure-of-eight coil held horizontally over the basal occiput. Th e effect lasted a further 6-10 msec. With a larger double cone coil, h eld vertically over the basal occiput, inhibition began earlier and at a more constant latency of 5 msec, It lasted only 3 msec. Stimulation of the C6/7 nerve roots in the brachial plexus with either an electri cal or magnetic stimulus also could suppress EMG responses evoked by c ortical stimulation. This began at a conditioning-test interval of 7 o r 8 msec and lasted for some 5 msec. We suggest that two types of moto r cortical suppression may be elicited from stimulation over the poste rior neck/skull: a cerebellar effect starting at 5 msec, and a periphe ral nerve effect starting later at 7/8 msec. Stimulation with a horizo ntal large figure-of-eight coil may produce a mixture of effects becau se the lower wing of the coil overlaps the posterior neck and can acti vate peripheral nerve fibres in the brachial plexus.