Transcranial direct current stimulation: neurophysiology and clinical applications

Transcranial direct current stimulation (tDCS) is considered a noninvasive and well-tolerated brain stimulation technique with very few adverse side effects. Importantly, tDCS does not directly evoke neuronal firing (as induced by electroconvulsive or transcranial magnetic stimulation), but instead alters the resting membrane potential of pre- and post-synaptic neurons dependent on the current polarity in the stimulated brain region. Animal studies suggest changes in long-term potentiation occur via glutamate release in response to anodal tDCS, thereby affecting learning and memory. In clinical studies, a current not exceeding 2 mA/cm(2) is applied for 20-30 min via sponge electrodes placed above the target brain region. To date, a number of clinical studies have reported some promising effects when treating patients with depression, chronic pain, schizophrenia, dementia, Parkinson's disease and cerebral stroke. However, appropriately designed randomized controlled clinical trials are scarce and reported intervention effect sizes only vary from small to moderate, with little evidence for sustained long-term effects.