Effects of 30Hz θ burst transcranial magnetic stimulation on the primary motor cortex

Theta Burst Stimulation (TBS) is a relatively new form of repetitive Transcranial Magnetic Stimulation (TMS) used to probe neuroplasticity in the human cortex. Thirty-Hz TBS, a variation of the originally described 50Hz TBS, has been shown to induce cortical changes in several nonmotor regions. However, its effects over the primary motor cortex have not been examined. Due to TMS device mechanical properties, 30Hz TBS is advantageous over 50Hz TBS in that it can be delivered at higher stimulation intensities. The goal of this pilot study is to examine the neurophysiologic effects of 30Hz TBS on the primary motor cortex (M1) of healthy adults. Eighteen right-handed adults (33±9.0 years; M:F=8:10) completed intermittent TBS (iTBS) or continuous TBS (cTBS) over left M1. TBS was performed with Magstim® SuperRapid2 with stimulation bursts (3 pulses at 30Hz) repeating every 200ms. For iTBS, each 2-s stimulation train was separated by 8s but there was no pause between trains for cTBS. Each TBS consisted of a total of 600 pulses delivered at an intensity of 90%*Resting Motor Threshold. Motor-Evoked Potentials (MEP) in the right first dorsal interosseous muscle were measured before, and one and ten minutes after TBS. Pre/post-TBS MEP amplitudes were compared using repeated-measures ANOVA. MEP amplitudes increased after 30Hz iTBS and decreased after 30Hz cTBS (TBS-Type*Time effect p=0.009). In conclusion, 30Hz TBS induced similar neurophysiologic effects over M1 as conventional 50Hz TBS.