The components required for TDCS include a Constant Current Stimulator and surface electrodes soaked in normal saline. A Constant Current Stimulator provides a steady flow of direct current (e.g., 0 – 4mA) while constantly monitoring the resistance in the system. Saline soaked electrodes applied and secured onto the scalp over desired areas such as the left or right precentral gyrus region (corresponding to C3 or C4 of the International 10–20 EEG system) form terminals relaying currents across the scalp and through the underlying brain tissue. The direction of the current flow determines the effects on the underlying tissue. With an active electrode over C3 or C4, a reference electrode over a control region (e.g., supra-orbital region) and current flowing from the active to the reference electrode, the excitability of the brain tissue under the anodal electrode is increased and when the current flow is reversed, the excitability of the brain tissue under this electrode is decreased (the electrode that was previously the anode now becomes the cathode; ). Once switched on, the constant current stimulator produces a transient tingling sensation under the electrode which fades off in 30sec to 1min thereby making it ideal for blinding subjects (in sham-controlled studies) by turning it off after the initial sensory experience. McCreery and colleagues11
found that current densities below 25 mA/cm2
did not cause brain tissue damage and the protocols that apply 1–2 mA as in present day studies fall well within these limits. Recent studies on brain modeling and current density distribution have suggested that in spite of a fraction of the direct current being shunted through the scalp, TDCS carries adequate currents to the underlying cortex, sufficient for neuronal excitability shifts.12
Our own studies have shown that measures of cerebral blood flow can change in brain regions that are targeted by transcranial anodal direct current providing further proof that transcranially applied direct currents can affect tissue excitability as well as regional blood flow as an indirect marker of change in regional tissue excitability ().
The advantages of TDCS over other non-invasive brain stimulation methods include its ease of use, its large electrode size allowing influence over a larger neural network, a sham mode allowing controlled experiments and randomized controlled clinical trials, and its portability making it possible to apply stimulation while the patient receives occupational/physical therapy. Nevertheless, TDCS is limited by its poor temporal resolution and anatomical localization. Furthermore, inter-individual variation in conductivity due to differences in hair, scalp, and bone composition can interfere with the current that is carried to the brain. Last but not least, although single sessions and multi-day sessions have been done and found to be safe, the safety of prolonged periods of stimulation requires further studies.
TDCS by itself provides a sub-threshold stimulus that modulates the likelihood that neurons will fire by hyperpolarizing or depolarizing the brain tissue, without direct neuronal depolarization. The prolonged sensory, motor, and cognitive effects of TDCS have been attributed to a persistent bidirectional modification of post-synaptic connections similar to long-term potentiation (LTP) and long-term depression (LTD) effects.5,7
Dextromethorphan, an N-methyl-D-aspartate (NMDA) antagonist suppressed both anodal and cathodal TDCS effects, strongly suggesting the involvement of NMDA receptors in both types of DC-induced neuroplasticity.13
In contrast, Carbamazepine selectively eliminated anodal effects.14
Since Carbamazepine stabilizes the membrane potential through voltage-gated sodium channels (stabilizing the inactivated state of sodium channels), the results reveal that after-effects of anodal TDCS require a depolarization of membrane potentials.14,15
More studies are needed, particularly in humans to verify TDCS’s actions on brain tissue, its underlying mechanism, and the associated behavioral and cognitive effects.