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

The 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).

The two cTBS paradigms were: (1) standard cTBS (cTBS(std)) (three stimuli at 50Hz, repeated at 5Hz), and (2) modified cTBS (cTBS(mod)) (three stimuli at 30Hz, repeated at 6Hz 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 30min following each paradigm.

Both cTBS(std) (P=0.05) and cTBS(mod) (P<0.0001) induced a suppression of MEP amplitudes. However, MEP suppression following cTBS(mod) was greater (ANOVA(RM); P=0.02). Experiments using magnetic brainstem stimulation provided evidence that cTBS(mod) induced MEP suppression through cortical mechanisms.

The neuroplastic response of the human M1 to cTBS is highly dependent on the stimulation parameters employed.

These findings may have significant implications for the clinical application of cTBS paradigms.