Magnetic stimulation intensity modulates motor inhibition

Repetitive transcranial magnetic stimulation (rTMS) is a standard tool in neuroscience research and therapy. Here we study one rTMS property that has not received adequate attention, the interaction of subthreshold intensity stimulation and low frequencies. We applied 1Hz rTMS over the motor cortex at three intensities, 40%, 80% and 100% of the resting motor threshold (rMT), and measured cortical excitability before and after the stimulation sessions. When comparing motor evoked potential (MEP) measured from the abductor pollicis brevis (APB) muscle before and after rTMS stimulation, we found that low intensity (40% MT) stimulation significantly decreased MEP magnitude, some smaller (non-significant) inhibition was found for the 80% MT intensity and increased MEP was found for the high intensity (100% MT) stimulation. Our results indicate that when explaining the input-output relationship of motor cortex induced activation as an intensity-dependent function, there might be a need to split it into separate functions associated with separate processes mediated by different cell types such as interneurons, pyramidal neurons and others.