Transcranial magnetic stimulation induces current pulses in transcranial direct current stimulation electrodes

Transcranial direct current stimulation (tDCS) is a noninvasive neuromodulation technique where weak direct current is administered through electrodes placed on the subject´s head. Transcranial magnetic stimulation (TMS) is a noninvasive method for focal brain stimulation where small intracranial currents are induced by a pulsed magnetic field. TMS can be applied simultaneously with tDCS to probe brain excitability or to effect synergistic neuromodulation. Delivering TMS simultaneously with tDCS can induce electric current pulses in the tDCS electrodes even when the tDCS device is turned off or is set to 0 mA output, as long as the electrodes are connected to the tDCS current source. The output impedance of commercial tDCS devices is in the range of 2-5 kΩ which can allow substantial currents to be induced by TMS. In a rat TMS-tDCS setup, the induced currents are comparable to the tDCS current magnitude. To mitigate the induced currents, the area of the loop formed by the tDCS electrode leads should be minimized and the impedance of the tDCS circuit at TMS pulses frequencies (1-10 kHz) should be maximized.