Transcranial Electric Stimulation
Transcranial Electrical Stimulation (tES) involves passing a small electric current through the skull and the underlying cortex via two rubber electrodes. The stimulation can be targeted to certain brain areas by centring the active electrode over the brain area of interest, with the reference electrode placed in a task-neutral position. The electrical current travels via the shortest route between the two electrodes, thus stimulating the neural tissue in between. This current can either be direct, as in transcranial direct current stimulation (tDCS), or alternating in polarity, as in transcranial alternating current stimulation (tACS).
In tDCS, depending on whether the positive (anode) or the negative (cathode) electrode is placed on the scalp overlying the brain area of interest, the excitability of the underlying tissue is either increased (anodal) or decreased (cathodal). This change in cortical excitability, as measured by TMS induced MEP amplitude, can long outlast the stimulation period, making it an ideal method for increasing cortical excitability post training.
If it is applied during task, it has been demonstrated that tDCS is also capable of significantly modulating behaviour. For example, if anodal tDCS to M1 is applied during motor learning, this can enhancing the speed and retention of the task in healthy participants. It has also been shown that tDCS to M1 applied during motor training enhances gains in motor function in patients with hemiparesis due to stroke. There is also a growing literature surrounding the use of tDCS as a treatment for depression and other psychiatric disorders.