Transcranial Direct Current Stimulation (tDCS) is a novel method applied both in neurological and psychiatric patients with the aim to change the activity of nerve cells in the brain tissue. Although it was first developed in 1960s, real experiments started appearing during the past decade (M. A. Nitsche et al., 2008). The idea was that electrodes placed on different portions of the scalp can influence physiological changes in the brain. Here is something more about how this mechanism works and in which conditions it may have application.
Mechanism of Action of tDCS
Two centuries ago, it was noted that electrical stimulations can alter neurological symptoms and human behavior (Priori, 2003). Studies on animals have shown that delivering electrical current through the electrodes placed in the brain cause changes in electrical activity of different parts of the brain that can last for longer periods after the stimulation (Nitsche & Paulus, 2000). Interactions with protein synthesis and activity of neurotransmitters (chemicals responsible for impulse transmission between neural cells) have also been found. Experiments in humans and primates have shown that about 50% of currents applied on the surface of the scalp enter the brain (Rush & Driscoll, 1968).
There are two types of electrical stimulation: anodal and cathodal. Anodal stimulation seems to help with depressive symptoms, while cathodal stimulation acts on manic behavior (Carney, 1969). TDCS differs from other more invasive brain stimulation techniques as it does not activate depolarization (excitation and firing) of neural cells, but only modulates the activity of neural cells, making it more convenient and less invasive technique with fewer side effects.
Placing Electrodes for tDCS
Electrodes are prepared for placement on the scalp with sponges soaked with saline solution. Conductive gel is also used if rubber electrodes are used. The most important thing for the effectiveness of tDCS is proper placement of electrodes on the scalp (Priori, Berardelli, Rona, Accornero, & Manfredi, 1998). They should be placed to allow the current to pass through specific parts of the brain. Therefore, the position of electrodes is different for different conditions. Extensive experiments were conducted to investigate which orientation of the current flow is appropriate for specific conditions. For example, depressive patients benefit the most from electrodes placed on both sides of frontal region of the scalp (Ho et al., 2014). This direction of current flow affects prefrontal cortex on the left side, which is aimed to process and modify our emotional reactions generated in lower parts of the brain (limbic system). Alternative positioning techniques are also a focus of current studies.
Safety of tDCS
More invasive techniques which deliver higher current to the brain tissue have shown to be related with some level of tissue damage. For tDCS, the most commonly reported side effects are itching of the skin in contact with electrodes (Michael A. Nitsche et al., 2008). This side effect appears due to electrochemically produced toxins during the contact between the skin and the activated electrodes. Hyperemia and skin irritation are also possible, and they can be eliminated by using appropriate amount of conductive gel. Other side effects, which are not frequent include nausea, headache, and fatigue, occurring sporadically which disappear completely after the treatment is done (Poreisz, Boros, Antal, & Paulus, 2007). Generally speaking, prolonged electrical stimulation could cause accumulation of toxins in the brain tissue by same mechanisms, but this does not happen due to high capability of the brain tissue to eliminate unnecessary products. There are currently no evidences that tDCS can produce any unwanted deposits in the brain tissue. Furthermore, tDCS did not cause changes in EEG recordings, and MRI brain images (Iyer et al., 2005).
In conclusion, tDCS provides safe and effective long-term benefits in patients with psychiatric and neurological disorders. As adverse effects are usually mild and easy to overcome, this could be a promising technique in the near future which could replace more invasive brain stimulation techniques.
Carney, M. W. (1969). Negative polarisation of the brain in the treatment of manic states. Ir J Med Sci, 8(3), 133-135.
Ho, K. A., Bai, S., Martin, D., Alonzo, A., Dokos, S., Puras, P., & Loo, C. K. (2014). A pilot study of alternative transcranial direct current stimulation electrode montages for the treatment of major depression. J Affect Disord, 167, 251-258. doi: 10.1016/j.jad.2014.06.022
Iyer, M. B., Mattu, U., Grafman, J., Lomarev, M., Sato, S., & Wassermann, E. M. (2005). Safety and cognitive effect of frontal DC brain polarization in healthy individuals. Neurology, 64(5), 872-875. doi: 10.1212/01.WNL.0000152986.07469.E9
Nitsche, M. A., Cohen, L. G., Wassermann, E. M., Priori, A., Lang, N., Antal, A., . . . Pascual-Leone, A. (2008). Transcranial direct current stimulation: State of the art 2008. Brain Stimul, 1(3), 206-223. doi: 10.1016/j.brs.2008.06.004
Nitsche, M. A., Cohen, L. G., Wassermann, E. M., Priori, A., Lang, N., Antal, A., . . . Pascual-Leone, A. (2008). Transcranial direct current stimulation: State of the art 2008. Brain Stimul, 1(3), 206-223. doi: http://dx.doi.org/10.1016/j.brs.2008.06.004
Nitsche, M. A., & Paulus, W. (2000). Excitability changes induced in the human motor cortex by weak transcranial direct current stimulation. J Physiol, 527 Pt 3, 633-639.
Poreisz, C., Boros, K., Antal, A., & Paulus, W. (2007). Safety aspects of transcranial direct current stimulation concerning healthy subjects and patients. Brain Res Bull, 72(4-6), 208-214. doi: 10.1016/j.brainresbull.2007.01.004
Priori, A. (2003). Brain polarization in humans: a reappraisal of an old tool for prolonged non-invasive modulation of brain excitability. Clin Neurophysiol, 114(4), 589-595.
Priori, A., Berardelli, A., Rona, S., Accornero, N., & Manfredi, M. (1998). Polarization of the human motor cortex through the scalp. Neuroreport, 9(10), 2257-2260.
Rush, S., & Driscoll, D. A. (1968). Current distribution in the brain from surface electrodes. Anesth Analg, 47(6), 717-723.
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