Exploring the after-effects of theta burst magnetic stimulation on the human motor cortex: a functional imaging study

Theta burst stimulation (TBS) is a protocol of subthreshold repetitive transcranial magnetic stimulation (rTMS) inducing changes in cortical excitability. From functional imaging studies with conventional subthreshold rTMS protocols, it remains unclear what type of modulation occurs (direction and dependency to neural activity) and whether putative effects are bound to unspecific changes in cerebral perfusion or require a functional challenge. In a within-subjects (n = 17) repeated measurement design including real TBS and a control session without stimulation, we examined neural activation in a choice-reaction task after application of intermittent TBS, a protocol, which enhances cortical excitability over the left motor cortex (M1). Brain activity was monitored by blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging interleaved with measuring regional cerebral blood flow (rCBF) at rest using MR-based perfusion imaging. On a separate day, TMS-induced compound muscle action potentials (cMAPs) amplitude of the right hand was measured after excitatory TBS. Compared to control, a significant decrease in BOLD signal due to right hand motor activity during the choice-reaction task was observed mainly in the stimulated M1 and motor-related remote areas after stimulation. This decrease might represent a facilitating effect, because cMAPs amplitude increased upon TBS compared to control. No changes in rCBF at rest were observed. The data demonstrate that subthreshold intermittent TBS targets both the stimulated cortical area as well as remote areas. The facilitation changing the efficacy of neural signal transmission seems to be reflected by a BOLD signal decrease, whereas the network at rest does not appear to be affected.