P148 Dose–effect of transcranial static magnetic field stimulation on excitatory and inhibitory intracortical circuits

Abstract

Question Transcranial static magnetic field stimulation (tSMS) is a new low-cost, non-invasive brain stimulation (NIBS) technique. When tSMS is applied for 10–15 min over the motor cortex, it leads to a short-lasting decrease of cortical excitability in healthy subjects, as measured by a decreased amplitude of motor-evoked potentials (MEPs), associated with an increase of short-latency intracortical inhibition (SICI). Here we aimed to test the effects of a longer application of tSMS (30 min) on excitatory and inhibitory intracortical circuits. Methods We performed 3 randomized double-blind sham-controlled experiments in a total of 31 right-handed healthy subjects. In experiment 1 we assessed MEP amplitudes before and after tSMS (or sham) applied for 30 min to the non-dominant motor cortex. In experiment 2 we tested the effects of 30 min of tSMS on SICI and I wave interaction. In experiment 3 we evaluated the effects of 10 min of tSMS on SICI and on the first peak of I wave interaction (short-latency intracortical facilitation, SICF). Results Prolonged application of tSMS (30 min) significantly decreased MEP amplitudes compared to sham till 30 min after the end of tSMS and, surprisingly, reduced SICI while increasing SICF. By applying tSMS for only 10 min, we found a significant and short-lasting increase of SICI with reduction of SICF. Conclusions We found that a prolonged application of tSMS leads to a long-lasting reduction in motor cortex excitability, similar to those typically induced by continuous theta-burst stimulation (cTBS) and cathodal transcranial direct current stimulation (tDCS). Moreover, SICI and SICF could be modulated in a bidirectional way by tSMS, depending on the application time, revealing a dose–effect of tSMS on excitatory and inhibitory intracortical circuits.