Enhanced cortical facilitation after intermittent theta burst stimulation with increased stimulation intensity

Objective: Intermittent theta burst stimulation (iTBS) can modulate the neuronal activity in the primary motor cortex. However, the responsiveness to the protocol shows high inter-individual variability which remains not fully understood. This study investigated whether responsiveness to iTBS depends on stimulation intensity. Methods: We conducted a sham-controlled, single-blinded experiment. Sixteen healthy volunteers participated in three sessions: iTBS at 80% of the active motor threshold (80%AMT), iTBS at 80% of the resting motor threshold (80%RMT), and a sham iTBS. Stimulation was applied to the hand representation of the primary motor cortex. The aftereffects of iTBS were assessed by changes in the amplitude of motor evoked potentials (MEPs) measured in posterior-anterior (PA) and anterior-posterior (AP) coil orientations to understand if there is differential effect in cortical networks. Results: Our results showed significant facilitation of both PA and AP MEPs following iTBS at 80%RMT but not after iTBS at 80%AMT or sham stimulation. Additionally, the number of responders was greater following iTBS at 80%RMT compared to iTBS at 80%AMT. Conclusions: Our results suggest that higher intensity iTBS may more effectively increase cortical excitability. Significance: This finding has the potential to improve the efficacy of various iTBS applications and facilitate their clinical application.