Modulation of Intracortical Inhibition and Facilitation of Motor Cortex Through Transcranial Direct Current Stimulation

Transcranial direct current stimulation (tDCS) can modulate cortical function transiently. This effect is monitored by motor evoked potentials (MEPs). The excitability of intracortical circuits in the motor cortex is assessed by paired pulse stimulation. In previous MEP studies, the modulation of short latency intracortical inhibition (SICI) and intracortical facilitation (ICF) with tDCS was demonstrated. In order to elucidate the neural mechanisms of tDCS, we evaluated the long latency intracortical inhibition (LICI) and short latency afferent inhibition (SAI) as well as SICI and ICF as objective parameters. There was a significant modulation of MEP amplitudes of single pulse TMS, however, SICI, ICF, LICI, and SAI were not significantly altered. Our results suggest that tDCS does not modulate neurotransmitter related to each parameter. Therefore, the modulation of MEP amplitudes of single pulse TMS may result from the change of membrane potentials.