The basics of tDCS:
Transcranial Direct-Current Stimulation (tDCS) is a contemporary, portable, non-invasive neuromodulatory technique that delivers a low electric current to the scalp. A fixed current between 1 and 2 mA is typically applied. tDCS works by applying a positive (anodal) or negative (cathodal) current via electrodes to an area, facilitating the depolarization or hyperpolarization of neurons, respectively. The positioning of the anode and cathode electrodes is used to influence how current flows, and where in the brain it does. The current delivered by tDCS is not considered strong enough to trigger an action potential in a neuron; its sub-threshold effect works by bringing the neurons closer to, or farther from firing. Plainly, tDCS augments the resting voltage of the neuronal membrane to prod a neuron’s activity in a desired direction. In this way, tDCS may work by strengthening or weakening synaptic transmission between neurons by augmenting synaptic plasticity which is, in turn, the cellular basis of learning. tDCS is often combined with training. Training in itself produces learning (synaptic plasticity), and concurrent tDCS amplifies these effects (enhances synaptic plasticity). Some areas tDCS is currently being explored include: depression, schizophrenia, aphasia, addiction, epilepsy, chronic pain (migraine, fibromyalgia), attention, and motor rehabilitation.
What does the tDCS device look like?
tDCS devices are small battery powered devices. There is usually a control panel that allows you to program the device (e.g., to set the duration and intensity of stimulation). Electrodes are placed on the head and held in place by headgear — usually an elastic strap. A cable connects each electrode to the stimulator. When the stimulator is turned on, current flows from the device to the electrode, and subsequently through the brain. Research and clinical grade stimulators have many features that help ensure stimulation is tolerable and reliable. This includes an impedance meter and a current meter.
What does tDCS feel like? What are the side-effects?
Research on the side-effects of tDCS is ongoing, but so far the established side-effects are minor, and restricted to the electrode location. They include temporary skin redness, itching, and tingling. Other suggested side-effects of tDCS include headache, nausea, and dizziness. It should be noted that these latter three side-effects have been illustrated to occur at nearly the same rate as sham stimulation (fake stimulation).1 When tDCS is applied inadequately, other side-effects can occur such as a phosphene which is a temporary, non-dangerous flash of light. This can occur if electrodes are placed too close to the eye. Additionally, incorrect tDCS administration can elicit standard skin burns. There is no scientific evidence that demonstrates lasting injury or irreversible side-effects from tDCS. Nonetheless it should be noted that all of the tolerability and safety data on tDCS comes from controlled human trials using specialized equipment and strictly controlled protocols (e.g., limiting current duration, number of sessions).
What don’t we know about tDCS?
While questions remain about the best applications for tDCS, there are decades of research denoting its implicated mechanism. Recent work suggests glial activation and the alteration of intracellular cAMP and calcium concentrations to largely contribute to tDCS’s effects.2 It is also understood that the plasticity of the human brain can allow lasting excitability changes as a result of tDCS application, namely, long-term potentiation (LTP) and long-term depression (LTD).
Where can I get tDCS?
In the United States tDCS has the regulatory status of “investigational”. This gives no indication of efficacy; it means the FDA has not issued an opinion. Typically, The FDA does not issue an opinion until companies show interest it in marketing a device. In the United States, companies are not allowed to market tDCS for a clinical indication such as “treatment of depression” or “treatment of epilepsy.” Doctors in the United States are allowed to provide “off-label” treatment, that is, treatment that is not approved by the FDA for the given indication. Research centers worldwide are allowed to test tDCS in controlled clinical trials. In such trials every subject must sign an informed consent sheet. You can find a listing of tDCS trials here: clinicaltrials.gov. In the EU, tDCS is approved for the treatment of pain and depression.
1Brunoni, A. R., Amadera, J., Berbel, B., Volz, M. S., Rizzerio, B. G., & Fregni, F. (2011). A systematic review on reporting and assessment of adverse effects associated with transcranial direct current stimulation.International Journal of Neuropsychopharmacology, 14(8), 1133-1145.
2Monai, H., Ohkura, M., Tanaka, M., Oe, Y., Konno, A., Hirai, H., … & Hirase, H. (2016). Calcium imaging reveals glial involvement in transcranial direct current stimulation-induced plasticity in mouse brain. Nature communications, 7.