Clinical applications of noninvasive electrical stimulation: problems and potential

Both transcranial direct current stimulation (tDCS) and repetitive transcranial magnetic stimulation (rTMS) can produce lasting aftereffects on cortical function that are thought to be due to the initial stages of synaptic potentiation/depression. They can also interact with processes of normal learning, to increase or decrease the rate of learning and retention. These features have spurred a number of investigators to test whether there is any clinical therapeutic potential for the methods to improve recovery of function after damage to the brain by injury or disease. The only condition where there is sufficient evidence is in certain forms of depression where excitatory rTMS is a recommended treatment protocol; there is insufficient evidence for any other condition. The problem facing investigators is the variety of possible paradigms that can be applied. Particularly for tDCS, only a small range of possible parameters has been tested, even in healthy volunteers; in addition, it is unclear whether stimulation should be applied at the same time as a behavioral therapy or whether stimulation should be applied at rest. Present trials give some evidence that can be used to address these questions, but until they are answered more securely it will be difficult to reach a consensus about "standard" protocols that can then be tested widely in multicenter trials.