Low-intensity, short-interval theta burst stimulation modulates excitatory but not inhibitory motor networks

Objective: Continuous theta burst stimulation (cTBS) administered at a low stimulus intensity can reduce the excitability of short interval intracortical inhibitory (SICI) networks without affecting the facilitatory intracortical motor networks involved in motor evoked potential (MEP) generation. We sought to determine whether low-intensity, facilitatory, short duration cTBS (300 stimuli over 20s; cTBS ₃₀₀) could modulate SICI without influencing cortical circuits involved in MEP generation. Methods: MEPs and SICI were assessed at baseline and 5min and 20min following cTBS ₃₀₀ applied at intensities of 60%, 65% or 70% of resting motor threshold (RMT). In addition, the effect of cTBS ₃₀₀ applied at 60% RMT on low level SICI (20% test MEP suppression) was examined. Results: Low-intensity cTBS ₃₀₀ facilitated MEP amplitude when applied at 70% RMT, and inhibited MEP amplitude when applied at 65% RMT. In contrast, none of the cTBS ₃₀₀ protocols had significant effects on moderate or low levels of SICI. Conclusions: The effects of cTBS ₃₀₀ on MEP generating motor networks are highly sensitive to stimulation intensity. Low-intensity cTBS ₃₀₀ does not have isolated, facilitatory effects on SICI networks. Significance: These results further highlight the difficulties of selectively facilitating the inhibitory circuits within M1 that are responsible for SICI with currently available rTMS paradigms.