A comparison of two different continuous theta burst stimulation paradigms applied to the human primary motor cortex

ObjectiveThe application of repetitive transcranial magnetic stimulation (rTMS) in bursts at theta frequencies (TBS) may produce lasting neuroplastic changes in the human cortex. However, there exists high variability in subjects’ responses, possibly due to non-optimal stimulation characteristics. Here we compare the efficacy of two variations of continuous TBS (cTBS) for producing neuroplastic change in the human primary motor cortex (M1).MethodsThe two cTBS paradigms were: (1) standard cTBS (cTBSstd) (three stimuli at 50 Hz, repeated at 5 Hz), and (2) modified cTBS (cTBSmod) (three stimuli at 30 Hz, repeated at 6 Hz with intensity). Motor evoked potentials (MEPs) were recorded from the right first dorsal interosseous muscle before, as well as at 0, 5, 10, 20 and 30 min following each paradigm.ResultsBoth cTBSstd (P = 0.05) and cTBSmod (P < 0.0001) induced a suppression of MEP amplitudes. However, MEP suppression following cTBSmod was greater (ANOVARM; P = 0.02). Experiments using magnetic brainstem stimulation provided evidence that cTBSmod induced MEP suppression through cortical mechanisms.ConclusionsThe neuroplastic response of the human M1 to cTBS is highly dependent on the stimulation parameters employed.SignificanceThese findings may have significant implications for the clinical application of cTBS paradigms.

► We show that slight modifications in the stimulation parameters used for the application of continuous theta burst stimulation (cTBS) can have a significant impact on its efficacy for inducing human motor cortical neuroplasticity. ► Whereas the standard paradigm (cTBSstd) induced a neuroplastic response that was short-lived and highly variable, a modified variant (cTBSmod) induced a lasting change in motor evoked potential (MEP) amplitude that was consistent between subjects. ► This is likely due to differences in inter and intra-burst frequencies between the two paradigms.