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

Abstract

BackgroundResponses 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. ObjectiveWe 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. MethodsThirty-one healthy volunteers participated. The after-effects of cTBS with the conventional intensity of 80% of individual active motor threshold (AMT) (cTBS80%) 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 (cTBS65%) and 100% AMT (cTBS100%) in subjects who showed depression of MEP amplitudes after cTBS80%, as well as cTBS65% in subjects in whom facilitation of MEPs was induced by cTBS80%. ResultsCurrent directions in MEP measurement had no influence on the cTBS responses. In subjects whose MEPs were depressed by cTBS80%, cTBS100% partly induced MEP facilitation, while cTBS65% abolished the after-effects. In subjects who showed MEP facilitation by cTBS80%, cTBS65% partly induced MEP depression. ConclusionsStimulus intensity of cTBS influenced the responses to cTBS, and lowering stimulus intensity induced the expected after-effects of cTBS in some subjects.