Transcranial Direct Current Stimulation (tDCS)
How does TDCS work?
TDCS involves applying a weak direct electrical current (ranging from 1 - 2mA) to the cerebral cortex (via two electrodes that are placed on the scalp) from the positively charged anodal electrode to the negatively charged cathodal electrode (Paulus 2011; Nitsche and Paulus 2011). The current delivered is too weak to induce neuronal firing, instead the applied current induces a change in the resting membrane potential surrounding the neurons, and thus increasing or decreasing the probability of neuronal firing. Consequentially, this results in strengthening or weakening synapses (neuron-to-neuron communication/connections).
Anodal stimulation (with the positively charged electrode) leads to resting membrane potential depolarization in the cortical region underneath the stimulating electrode, thus increasing the probability of neuronal firing.
Cathodal stimulation (with the negatively charged electrode) leads to resting membrane potential hyperpolarization under the region of interest, thus decreasing the probability of neuronal firing ((Nitsche and Paulus 2000; Nitsche et al. 2003, Steenbergen et al. 2016).
TDCS is thought to be particularly effective during training(learning), as training promotes the formation of synaptic connections. Hence, applying TDCS during training can enhance synaptic formation and thus strengthening the targeted neuronal circuitry. Different regions of the brain (and thus different neuronal circuits) can be targeted by placing the electrodes in various desired locations - the specific placement of electrodes on the head is called a montage.
Various scientific studies use different montages to obtain different desired effects based on what we know currently about specific functions of different brain regions.
What is TDCS used for?
Currently, tDCS's role is being explored for use in depression, schizophrenia, aphasia, addiction, epilepsy, chronic pain (migraine, fibromyalgia), attention deficit, and motor rehabilitation in patients with stroke (Alonso-Alonso et al. 2007, neuromodec.com).
What Does TDCS look like?
TDCS devices are small, portable, battery charged (usually with one or two 9-volt batteries) devices. The battery connects to two electrodes (one positively charged and the other negatively charged) via two wires. The electrodes are usually sponges that are soaked in saline (salt) water prior to placement on the head. The saline water is important for the conduction of electricity to the brain. When both electrodes are placed on the head and the machine is turned on, current flows freely from the anode to the cathode and a complete electrical circuit is created.
TDCS is not FDA approved. Many different types are available for sale on the internet. Be wary of the one you buy if you decide to do so. There are certain DIY devices that have been shown to do the opposite of what they are marketed to do (Steenbergen et al. 2016). This is likely because the electrodes in these devices were in a fixed position connected to a head piece. This fixed position was not a traditional montage used in the scientific experiments that use TDCS. This is why I personally prefer devices in which the electrodes are connected to wires that flow freely - the electrodes can be placed in any desired montage.
However it is important that the tDCS user is appropriately educated/trained on how to correctly place the electrodes. Incorrect placement can lead to undesired results, and if used incorrectly, over time may potentially cause undesired structural changes in the brain.
It is also important to know how to use the machine correctly. For instance, the electrodes need to be appropriately soaked in saline prior to placement on the head. If they are not soaked properly or if the electrodes are too aged, the current will not deliver.
More reading on tDCS:
Alonso-Alonso, M., Fregni, F., & Pascual-Leone, A. (2007). Brain stimulation in poststroke rehabilitation. Cerebrovasc Dis, 24 Suppl 1, 157-166. doi:10.1159/000107392
Nitsche MA, Liebetanz D, Lang N, Antal A, Tergau F, Paulus W (2003b) Safety criteria for transcranial direct current stimulation (tDCS) in humans. Clin Neurophysiol 114:2220–2222
Nitsche MA, Paulus W (2000) Excitability changes induced in the human motor cortex by weak transcranial direct current stimula- tion. J Physiol 527:633–639
Nitsche MA, Paulus W (2011) Transcranial direct current stimula- tion–update 2011. Restor. Neurol Neurosci 29:463–492
Steenbergen, L., Sellaro, R., Hommel, B., Lindenberger, U., Kuhn, S., & Colzato, L. S. (2016). "Unfocus" on foc.us: commercial tDCS headset impairs working memory. Exp Brain Res, 234(3), 637-643. doi:10.1007/s00221-015-4391-9