Simultaneously applying cathodal tDCS with low frequency rTMS at the motor cortex boosts inhibitory aftereffects

Background Transcranial direct current stimulation (tDCS) and repetitive transcranial magnetic stimulation (rTMS) can modulate cortical excitability. However, the combining effect of tDCS and rTMS at the same motor cortex is unknown. New methods: We have recently developed a new stimulation protocol that simultaneously combines two well documented methods. Eleven right-handed healthy subjects undertook four sessions at the left cortical representation area of right first dorsal interosseous muscle in randomized order, the order was counter balanced. Session one was the sham control without tDCS or rTMS. Session two involved cathodal tDCS with sham rTMS. Session three involved rTMS with sham tDCS. Session four was the combination of cathodal tDCS and rTMS (tDCS-rTMS). Cathodal tDCS was applied at 1 mA, and 1200 rTMS pulses were applied at 1 Hz with 90% resting motor threshold (RMT). All stimulation durations lasted for 20 min. RMT was monitored pre- and post-stimulation immediately after finishing the stimulation. Single pulse TMS induced MEP amplitudes were monitored before and after stimulation for 30 min afterwards. Results: Neither stimulus modality changed the value of RMT. As compared to sham stimulation, MEP amplitudes decreased in other three conditions. MEP amplitudes in tDCS-rTMS were much more inhibited than either tDCS or rTMS alone. Comparison with existing methods: We could reproduce inhibitory effect of 1 mA cathodal tDCS and 1 Hz rTMS in concordance with previous literature. The novel simultaneous tDCS-rTMS stimulation protocol can induce enhanced excitability change than either tDCS or rTMS. Conclusions: Simultaneous application of cathodal tDCS with low frequency rTMS produces a stronger inhibitory effect.