Objective: To investigate the modulatory effect of a subthreshold low frequency repetitive transcranial magnetic stimulation (rTMS) train on motor cortex excitability.
Methods: The study consisted of two separate experiments. Subjects received a 10 min long subthreshold 1 Hz rTMS train. In the first experiment, (single pulse paradigm), cortical excitability was assessed by measuring the amplitude of motor evoked potentials (MEPs) before and after the rTMS train. In the second experiment, a paired pulse paradigm was employed.
Results: Corticospinal excitability, as measured by the MEP amplitude, was reduced by the rTMS train (experiment 1), with a significant effect lasting for about 10 min after the train completion. There was notable inter-individual as well as intra-individual variability in the effect. rTMS produced a significant decrease in intra-cortical facilitation as measured by the paired pulse paradigm (experiment 2). This effect lasted for up to 15 min following the train. Intra-cortical inhibition was not significantly affected.
Conclusions: Subthreshold low frequency rTMS depresses cortical excitability beyond the duration of the train. This effect seems primarily due to cortical dysfacilitation. These results have implications on the therapeutic use of rTMS.