Cerebellar transcranial direct current stimulation modulates the effect of cerebellar transcranial magnetic stimulation on the excitability of spinal reflex.

Cerebellar transcranial magnetic stimulation (C-TMS) facilitates the ipsilateral soleus H-reflex, which reflects the excitability of the spinal motoneuron pool. This study aimed to investigate whether this facilitation of the spinal motoneuron pool excitability by C-TMS is affected by cerebellar transcranial direct current stimulation (ctDCS) in a polarity-specific manner. Eleven healthy adults participated in this study. The H-reflex was measured from the right soleus muscle by electrical stimulation of the right tibial nerve. The conditioning TMS over the right cerebellum was delivered 110 ms before tibial nerve stimulation. The H-reflex ratio was calculated as conditioned/unconditioned H-reflex amplitude, and measured for pre- or post-anodal, cathodal, or sham right ctDCS, for 15 minutes with 2 mA. The results showed that the H-reflex ratio was significantly increased by anodal ctDCS, reduced by cathodal ctDCS, and not affected by sham ctDCS, indicating that the effect of C-TMS on the H-reflex is modulated by ctDCS in a polarity-specific manner. This implies that the effect of C-TMS on the spinal motoneuron pool is affected by the change in excitability of the cerebellum.