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

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.