Reproducibility of the effects of theta burst stimulation on motor cortical plasticity in healthy participants

Abstract

Objective: Theta-burst stimulation (TBS) is a repetitive transcranial magnetic stimulation (TMS) protocol, capable of enhancing or suppressing the amplitude of contralateral motor-evoked potentials (MEP) for several minutes after stimulation over the primary motor cortex. Continuous TBS (cTBS) produces a long-term depression (LTD)-like reduction of cortical excitability. The purpose of this study was to assess the test-retest reproducibility of the effects of cTBS and to investigate which neurophysiologic markers of cTBS-induced plasticity are most reproducible.

Methods: In ten healthy participants we evaluated in two different sessions the effects of cTBS (using AP-PA current direction, opposite to most commercial rTMS stimulators) on MEPs induced by single-pulse suprathreshold TMS (using AP-PA or PA current direction) over left motor cortex in the first dorsal interosseus (FDI) muscle.

Results: Results demonstrate that the marker of cTBS induced-plasticity with highest within-subject reproducibility is the modulation of corticospinal excitability measured 5min after cTBS.

Conclusion: Overall the effects of cTBS modulation show limited test-retest reproducibility and some measures of the cTBS effects are more reproducible than others.

Significance: Studies comparing cTBS effects in healthy subjects and patients need to proceed with care. Further characterization of the effects of TBS and identification of the best metrics warrant future studies.

Keywords: Neurophysiological markers; Primary motor cortex; Transcranial magnetic stimulation; Variability.