Transcranial direct current stimulation (tDCS) is a form of neurostimulation which uses constant, low current delivered directly to the brain area of interest. It produces physical changes in the brain's structure as well as physiological changes in its response. TDCS increases cortical excitability, which can be measured in recordings of brain waves, and it also causes changes in the structure of the brain's connections that can be observed on an MRI. By using electricity to energize neural circuits in the cerebral cortex, researchers are hopeful that they have found a harmless and drug-free way to double the speed of learning.