Topography of EEG responses evoked by transcranial magnetic stimulation to the cerebellum

The evoked potential induced by stimulating the cerebellum by transcranial magnetic stimulation (TMS) was measured and the reaction map of the TMS over the cerebellum for observing the neuronal connectivity in the brain was obtained in this study. An electroencephalogram (EEG) measurement system to eliminate the electromagnetic interaction emitted from TMS was used. EEG activities 5 ms after TMS stimulation were successfully measured. There were several components of evoked potential after stimulation, at 10, 25, and 50 ms. A large response of evoked potential appeared at about 10 ms after TMS when the cerebellum was stimulated. Reaction maps of the stimulation at the occipital area were obtained using the 9-ms evoked potential component. From the reaction map, it is possible to consider that the fast component of evoked signal by TMS to the cerebellum was conducted posteriorly to anteriorly along the pathways of the neuronal fiber exiting the cerebellum to the cerebral cortex. The response at the right brain was large when the left cerebellum was stimulated. The response at the left brain was large when the right cerebellum was stimulated. The response map of the TMS gave information of the neuronal connectivity. TMS will become a useful tool for the study of cortical reactivity and neuronal connectivity of the brain.