What is Transcranial Direct Current Stimulation (tDCS)?

Transcranial direct current stimulation (tDCS) is a form of non-invasive, low voltage electrical stimulation that stimulates neurons in targeted areas of the brain. In a common tDCS set-up, researchers use two electrodes to deliver the stimulating current; one electrode is the anode which delivers the inward current, while the other is the cathode which receives the outward current. The anode is normally placed over the target region of the brain over the scalp, and the cathode is placed either over an uninvolved brain region, or on the shoulders of the participant (Holgado et al., 2019). The electrodes are connected to a battery that delivers the current, usually between 1 and 2 mA, through the electrodes, increasing the excitability of neurons inside the target region of the brain. Specifically, anodal tDCS increases and cathodal tDCS decreases excitability. These changes to polarity are immediate and are linearly correlated to the magnitude of the electric field being applied (Bikson et al., 2004).

In addition to these immediate changes, tDCS induces changes in excitability beyond the period of stimulation. In an early study conducted by Nitsche and Paulus (2000), it was found that neuronal excitability increased for 60 minutes after 13 minutes of stimulation at 1 mA. This early work by Nitsche and Paulus (2000) and Bikson et al. (2004) focused on the modulation of neuronal polarisation and has become the physiological foundation and rationale for further studies that applied tDCS in clinical settings (see Antal et al., 2011; Fregni et al., 2006; Stagg et al., 2009; Vines et al., 2008).

As literature developed from these influential findings, it became apparent that these well-established neuronal mechanisms of action proposed by Nitsche and Paulus (2000) did not sufficiently account for all empirical findings relating to tDCS. For example, research reporting an enhancement in facets of performance after the application of cathodal tDCS rather than anodal tDCS (see Friehs & Frings, 2019; Heinen et al., 2016; Moliadze et al., 2018) was counter to the predicts of Nitsche and Paulus’ (2000) mechanistic explanation. This indicated a more complex relationship between the stimulatory effects of anodal and cathodal tDCS, which initiated further investigation of alternative mechanisms involved in the application of tDCS. A wide array of alternative mechanisms has been theorised, including electrotaxis (Keuters et al., 2014) and changes in the blood brain barrier (Shin et al., 2016).

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Peer-Reviewed Resources

Antal, A., Kriener, N., Lang, N., Boros, K., & Paulus, W. (2011). Cathodal transcranial direct current stimulation of the visual cortex in the prophylactic treatment of migraine. Cephalalgia, 31(7), 820-828. https://doi.org/10.1177%2F0333102411399349

Bikson, M., Inoue, M., Akiyama, H., Deans, J. K., Fox, J. E., Miyakawa, H., & Jefferys, J. G. (2004). Effects of uniform extracellular DC electric fields on excitability in rat 23 hippocampal slices in vitro. The Journal of physiology, 557(1), 175-190. https://doi.org/10.1113/jphysiol.2003.055772

Fregni, F., Boggio, P. S., Nitsche, M. A., Marcolin, M. A., Rigonatti, S. P., & Pascual-Leone, A. (2006). Treatment of major depression with transcranial direct current stimulation. Bipolar Disorders, 8(2), 203–204. https://doi.org/10.1111/j.1399- 5618.2006.00291.x

Friehs, M. A., & Frings, C. (2019). Cathodal tDCS increases stop-signal reaction time. Cognitive, Affective, & Behavioral Neuroscience, 19(5), 1129-1142. https://doi.org/10.3758/s13415-019-00740-0

Heinen, K., Sagliano, L., Candini, M., Husain, M., Cappelletti, M., & Zokaei, N. (2016). Cathodal transcranial direct current stimulation over posterior parietal cortex enhances distinct aspects of visual working memory. Neuropsychologia, 87, 35-42. https://doi.org/10.1016/j.neuropsychologia.2016.04.028

Holgado, D., Zandonai, T., Ciria, L. F., Zabala, M., Hopker, J., & Sanabria, D. (2019). Transcranial direct current stimulation (tDCS) over the left prefrontal cortex does not affect time-trial self-paced cycling performance: Evidence from oscillatory brain activity and power output. PloS one, 14(2), Article e0210873. https://doi.org/10.1371/journal.pone.0210873

Keuters, M. H., Aswendt, M., Tennstaedt, A., Wiedermann, D., Pikhovych, A., Rotthues, S., Fink, G. R., Schroeter, M., Hoehn, M., & Rueger, M. A. (2015). Transcranial direct current stimulation promotes the mobility of engrafted NSCs in the rat brain. NMR in Biomedicine, 28(2), 231–239. https://doi.org/10.1002/nbm.3244

Moliadze, V., Lyzhko, E., Schmanke, T., Andreas, S., Freitag, C. M., & Siniatchkin, M. (2018). 1 mA cathodal tDCS shows excitatory effects in children and adolescents: insights from TMS evoked N100 potential. Brain research bulletin, 140, 43-51. https://doi.org/10.1016/j.brainresbull.2018.03.018

Nitsche, M. A., & Paulus, W. (2000). Excitability changes induced in the human motor cortex by weak transcranial direct current stimulation. The Journal of physiology, 527(3), 633-639. https://doi.org/10.1111/j.1469-7793.2000.t01-1-00633.x

Shin, D. W., Khadka, N., Fan, J., Bikson, M., & Fu, B. M. (2016, March). Transcranial direct current stimulation transiently increases the blood-brain barrier solute permeability in vivo. In Medical Imaging 2016: Biomedical Applications in Molecular, Structural, and Functional Imaging (Vol. 9788, p. 97881X). International Society for Optics and Photonics. https://doi.org/10.1117/12.2218197

Stagg, C. J., Best, J. G., Stephenson, M. C., O’Shea, J., Wylezinska, M., Kincses, Z. T., Morris, P. G., Matthews, P. M., & Johansen-Berg, H. (2009). Polarity-sensitive modulation of cortical neurotransmitters by transcranial stimulation. Journal of 33 Neuroscience, 29(16), 5202-5206. https://doi.org/10.1523/JNEUROSCI.4432- 08.2009

Vines, B. W., Cerruti, C., & Schlaug, G. (2008). Dual-hemisphere tDCS facilitates greater improvements for healthy subjects’ non-dominant hand compared to uni-hemisphere stimulation. BMC Neuroscience, 9(1), 103. https://doi.org/10.1186/1471-2202-9-103

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