Noninvasive brain stimulation techniques are gaining attention due to their safety in modulating brain dynamics. Promising applications include treatment of various central nervous system diseases and improvement in cognitive functions. Transcranial electrical stimulation provides noninvasive brain modulation using direct current, alternating current and random noise stimulation (Paulus, 2011).
A weak direct electrical current is applied through the scalp using two or more electrodes in the transcranial direct electrical stimulation tDCS technique. This induces brain excitability through cathodal hyperpolarization and anodal depolarization (Paulus, 2011). The induced effects depend on polarity, duration and intensity of electrical stimulation.
Transcranial alternating current stimulation is same as direct current in case of low intensity and electrodes. But it uses sinusoidal current through the scalp for electrical stimulation (Woods 2011). Several studies have been carried out that showed that transcranial direct/alternating current has a positive impact on motor, memory, perception and cognitive functions.
Cognitive processes include perception, memory, learning, and long-term memory formation. Induction of transcranial electrical brain stimulation enhances the cognitive functions. Direct current stimulation occurs through spontaneous cortical activity and alternating current modulate cognition by interfering with the oscillations of cortical networks (Kuo, 2012)
Transcranial stimulation of the brain through weak direct current induction serves a non-invasive and painless technique (Nitsche, 2000). In their study, induction of direct current through the scalp for modulating motor cortex excitation, showed up to 40% of the excitation changes that last for several minutes after end of stimulation. Stimulation was achieved by membrane polarization inducing anodal stimulation and inhibiting cathodal stimulation.
Weak direct current induction leads to cerebral excitability. Fregni et al (2005) evaluated “the effect of anodal stimulation of dorsolateral prefrontal cortex (DLPFC) on working memory”. A letter-based working memory task was performed by fifteen individuals during anodal stimulation of DLPFC. Out of these seven performed the same task but with cathodal stimulation. Results showed the increased performance of individuals with anodal stimulation.
Another study was carried out to investigate the association of slow oscillations on memory during sleep. Induction of transcranial slow oscillations of 0.75 Hz in early sleep increased the retention of declarative memory in healthy subjects and also improved the slow wave sleep and slow spindle activity in the frontal cortex. Stimulation of the brain by 5Hz oscillations during rapid eye movement sleep had no effect on declarative memory (Marshall, 2006). Based on this, another study was carried out to rule out the effect of slow oscillations during waking on brain and memory encoding. It was concluded that the effect of oscillation and memory depend on brain state, as when the awake brain transmitted stimulation by responding to oscillations and facilitated encoding. Transcranial oscillations didn’t improve memory when applied after learning, while it showed enhanced encoding of hippocampus dependent memory when induced during the process of learning (Kirov, 2009)
Transcranial alternating current stimulations have an enhanced effect on human cognitive functions. Antonenko et al (2016) conducted research on young and older healthy individuals. Transcranial alternating current of 6Hz was applied to the brain for 20 minutes during a language learning process. The results were in support of the evidence that alternating current improves human cognition through direct stimulation of task-related brain oscillations.
Antonenko D, Miriam Faxel, Ulrike Grittner,Michal Lavidor and Agnes Flöel, 2016. “Effects of Transcranial Alternating Current Stimulation on Cognitive Functions in Healthy Young and Older Adults”. Neural Plast: 4274127. Doi: 10.1155/2016/4274127. PMCID: PMC4889859. PMID: 27298740
Fregni, F, Boggio, P.S, Nitsche, M. et al, 2005. “Anodal transcranial direct current stimulation of prefrontal cortex enhances working memory”. Exp Brain Res: 166: 23. https://doi.org/10.1007/s00221-005-2334-6
Kirov R, Carsten Weiss, Hartwig R. Siebner, Jan Born, and Lisa Marshall, 2009. “Slow oscillation electrical brain stimulation during waking promotes EEG theta activity and memory encoding”. PNAS; September 8, 2009. 106 (36) 15460-15465; https://doi.org/10.1073/pnas.0904438106
Kuo M F, Michael A. Nitsche, 2012. “Effects of Transcranial Electrical Stimulation on Cognition”. Clinical EEG and Neuroscience, Volume: 43, issue: 3, page(s): 192-199. https://doi.org/10.1177/1550059412444975
Marshall, L., Helgadóttir, H., Mölle, M., and Born, J. (2006). “Boosting slow oscillations during sleep potentiates memory”. Nature 444(7119):610-3. doi: 10.1038/nature05278
Nitsche MA, Paulus W, 2000. “Excitability changes induced in the human motor cortex by weak transcranial direct current stimulation”. J Physiol. 2000 Sep 15; 527 Pt 3:633-9. PMID: 10990547 PMCID: PMC2270099
Paulus W, “Transcranial electrical stimulation (tES – tDCS; tRNS, tACS) methods”.Neuropsychol Rehabil. 2011 Oct; 21(5):602-17. Doi: 10.1080/09602011.2011.557292. Epub 2011 Aug 5.