Continuous theta burst stimulation at 30 hz does not modulate cortical excitability in a sham-controlled study

Abstract

Theta burst stimulation (TBS) can modulate cortical excitability but suffers from high inter-subject variability. Modified TBS frequency patterns (30 Hz) showed consistent inhibitory aftereffects, but further research into the time course of these effects is needed. This study aimed to investigate the efficacy of a 30 Hz continuous TBS (cTBS) protocol. Participants (n = 20) underwent an experimental session (real cTBS) and a control session (sham cTBS). To assess cortical excitability, Transcranial Magnetic Stimulation was applied over the primary motor cortex before cTBS, and at five timepoints after cTBS. Percentage change (PC) to baseline was analysed using a Linear Mixed Model. No difference in PC was found between real and sham cTBS (p = 0.696). Our results demonstrate a significant increase in PC over time (p = 0.006) at 30, (p = 0.01), 45 (p = 0.027), and 55 min (p = 0.024) post cTBS, irrespective of condition. Secondary analysis dividing the sample into responders and paradox-responders showed no significant predictors for cTBS responsiveness. We could not replicate previously reported suppressive effects of 30 Hz cTBS. Increases in MEP amplitudes over a 60-minute time window were independent of stimulation condition and marked by high inter-subject variability. Validations of modified TBS protocols are further needed to replicate findings and understand mechanisms underlying individuals' responsiveness.