Theta burst stimulation suppresses movement-related activity in the stimulated motor cortex but the motor system can cope with it

Background: Transcranial magnetic stimulation (TMS) can be used to examine the functional relevance of brains regions by inducing functional lesions. Several conditioning protocols have been established which induce a lasting suppression of cortical excitability and interfere with neuronal processing in the stimulated cortex beyond the time of TMS. Here we used continuous theta burst stimulation (cTBS) to suppress cortical excitability in the left primary motor hand area (M1) and examined how the cTBS-induced lesion effects changes regional activity in the motor system in the context of motor skill learning.

Methods: Ten healthy, right-handed subjects participated in a randomized, sham-controlled, cross-over study. In two separate sessions, fourty seconds of real cTBS at 80% of active motor threshold (AMT) or sham TBS were given 15 minutes prior to fMRI. During fMRI, participants had to lift two fingers of the right hand simultaneously while the rest of the hand rested on a flat surface. There were two types of alternating fMRI runs. In the „fast“ session“, movements were self paced and subjects had to perform as many moves as possible in 60 seconds. Between the five „fast“ blocks subjects performed four blocks in which the same task was performed at a constant rate (paced session). Two equally difficult versions of the task were used to avoid carry over effects in performance. During both tasks, whole-brain fMRI data were acquired. Finger movements were recorded with a three-dimensional ultrasound movement tracking system.

Results: All subjects markedly improved in performance during the course of the fMRI session (mean increase of 67%) without any effects of the type of TBS, task version, or order of experimental sessions. During learning, fMRI activation of the motor network, including ipsilateral M1, S1, PMC and SMA, decreased with time. Only the stimulated left M1 showed a decrease in task-related activity during the „fast“ sessions after real cTBS compared to sham TBS. This effect was already present in the first fMRI run and persistent over time. Effective connectivity analysis (PPI) revealed a change in task-related coupling between the stimulated left M1 and (i) the adjacent portion of the left M1 and (ii) the left and right PMd.

Conclusion: In the context of motor skill learning, „inhibitory“ cTBS led to a well-defined suppression in regional neuronal activity in the stimulated M1 but only when movements were performed at a fast rate. This „lesion effect“ did not result in any change in learning, presumably because of a compensatory change in connectivity between the lesioned cortex and connected motor and premotor areas.