The intensity of continuous theta burst stimulation, but not the waveform used to elicit motor evoked potentials, influences its outcome in the human motor cortex

摘要

Abstract Background Responses to continuous theta burst stimulation (cTBS) applied to the human primary motor cortex are highly variable between individuals. However, little is known about how to improve the after-effects of cTBS by adjusting the protocol characteristics. Objective We examined whether current directions adopted in the measurement of cortical motor excitability indexed as motor evoked potentials (MEPs) affect the responses to cTBS. We also tested whether the stimulus intensity of cTBS influences the after-effects. Methods Thirty-one healthy volunteers participated. The after-effects of cTBS with the conventional intensity of 80% of individual active motor threshold (AMT) (cTBS 80% ) were tested by measuring MEP amplitudes induced by not only posterior-anterior (PA) but also anterior-posterior (AP) and biphasic (PA-AP) currents. We also investigated cTBS with 65% AMT (cTBS 65% ) and 100% AMT (cTBS 100% ) in subjects who showed depression of MEP amplitudes after cTBS 80% , as well as cTBS 65% in subjects in whom facilitation of MEPs was induced by cTBS 80%. Results Current directions in MEP measurement had no influence on the cTBS responses. In subjects whose MEPs were depressed by cTBS 80% , cTBS 100% partly induced MEP facilitation, while cTBS 65% abolished the after-effects. In subjects who showed MEP facilitation by cTBS 80% , cTBS 65% partly induced MEP depression. Conclusions Stimulus intensity of cTBS influenced the responses to cTBS, and lowering stimulus intensity induced the expected after-effects of cTBS in some subjects. Highlights ? The intensity of continuous theta burst stimulation (cTBS) influenced its responses. ? The optimal intensity of cTBS was not 80% of active motor threshold in every subject. ? cTBS with low intensity induced expected responses of depression in some subjects. ? With relatively high intensity, cTBS tented to increase motor cortex excitability. ? The configuration of TMS in measurement of MEPs did not affect the responses to cTBS.