Changes in intracortical circuits of the human motor cortex following theta burst stimulation of the lateral cerebellum

Objective: The cerebellum takes part in several motor functions through its influence on the motor cortex (M1). Here we applied the theta burst stimulation (TBS) protocol, a novel form of repetitive Transcranial Magnetic Stimulation (rTMS) over the lateral cerebellum. The aim of the present study was to test whether TBS of the lateral cerebellum could be able to modulate the excitability of the contralateral M1 in healthy subjects. Methods: Motor evoked potentials (MEPs) amplitude, short intracortical inhibition (SICI), long intracortical inhibition (LICI) and short intracortical facilitation (SICF) were tested in the M1 before and after cerebellar continuous TBS (cTBS) or intermittent TBS (iTBS). Results: We found that cTBS induced a reduction of SICI and an increase of LICI. On the other hand, cerebellar iTBS reduced LICI. MEPs amplitude also differently vary following cerebellar stimulation with cTBS or iTBS, resulting decreased by the former and increased by the latter. Conclusions: Although the interpretation of these data remains highly speculative, these findings reveal that the cerebellar cortex undergoes to bidirectional plastic changes that modulate different intracortical circuits within the contralateral primary motor cortex. Significance: Long lasting modifications of these pathways could be useful to treat various pathological conditions characterized by altered cortical excitability