Level of action of cathodal DC polarisation induced inhibition of the human motor cortex

Objective: To induce prolonged motor cortical excitability reductions by transcranial direct current stimulation in the human. Methods: Cathodal direct current stimulation was applied transcranially to the hand area of the human primary motor cortex from 5 to 9 min in separate sessions in twelve healthy subjects. Cortico-spinal excitability was tested by single pulse transcranial magnetic stimulation. Transcranial electrical stimulation and H-reflexes were used to learn about the origin of the excitability changes. Neurone specific enolase was measured before and after the stimulation to prove the safety of the stimulation protocol. Results: Five and 7 min direct current stimulation resulted in motor cortical excitability reductions, which lasted for minutes after the end of stimulation, 9 min stimulation induced after-effects for up to an hour after the end of stimulation, as revealed by transcranial magnetic stimulation. Muscle evoked potentials elicited by transcranial electric stimulation and H-reflexes did not change. Neurone specific enolase concentrations remained stable throughout the experiments. Conclusions: Cathodal transcranial direct current stimulation is capable of inducing prolonged excitability reductions in the human motor cortex non-invasively. These changes are most probabely localised intracortically.