Transcranial alternating current stimulation

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Transcranial alternating current stimulation (tACS) is a noninvasive means by which alternating electrical current applied through the skin and skull entrains in a frequency-specific fashion the neural oscillations of the underlying brain.[1][2]


Typically two electrodes are used: a stimulating one over the target cortex, and a reference one elsewhere, such as on the top of the head or on the neck. The size of the stimulation electrode is around 3 x 4 cm and the reference electrode has three times the surface area so as to reduce current density and limit stimulating the skin. They are held in place by elastic bands, and the hair and skin are saturated with saline solution for about 5–10 minutes. There is an initial sensation on the scalp but, after the initial few minutes, this fades.[3]

The alternating current applied is sinusoidal at a voltage of 5 to 15V.[citation needed] The current density under the stimulation electrode is about 83μA per square cm.[2][3]

Visual cortex[edit]

Transcranial alternating current stimulation applied over the visual cortex produces its effects depending upon frequency and illumination levels. When applied in an illuminated room, it induces most effectively the perception of continuously flickering light (phosphenes) with frequencies in the beta wave. But when in the dark such perception of flickering lights is most effective when the stimulation frequency when done in alpha wave range.[2] The phosphenes are reported to be seen in the far peripheral areas of vision. This suggests that tACS stimulates the anterior part of the visual cortex on its medial wall where the peripheral areas of the visual field are processed. The reason for this could be that the current flows mostly in the cerebrospinal fluid between the two cerebral hemispheres and enters the cerebral cortex from the forward part of the visual cortex.[2]

No effects are produced by current alternations at the theta wave or gamma wave frequencies.[2] These experiments make tACS a potentially interesting tool to investigate the causal role of oscillatory synchrony in cortical processing. But the claim that tACS phosphenes originate in the visual cortex has been disputed; the alternative claim is that current spread from the occipital electrode evokes phosphenes in the retina.[4][5][6][7] The frequency dependence of the phosphene strength can also be explained by retinal ganglion cell dynamics.[4] Hence experiments involving tACS applied at the visual cortex needs appropriate control studies to rule out the retinal interference.

Motor cortex[edit]

Transcranial alternating current stimulation applied over the motor cortex on one side brain at beta wave frequencies entrains activity in this range and increases the coherence between scalp-recorded electroencephalography (EEG) and electromyographic (EMG) activity in the first dorsal interosseous muscle of the opposite hand when it is held steady on a joystick.[8] The brain waves led muscle EMG by 41.6 ms suggesting that the synchronization with muscle activity is caused by the brain. There is also a small but significant slowing of the reaction time when a person moves the joystick to a target.[8]

Transcranial alternating current stimulation produces no effects upon motor evoked potentials.[9]

Frontal lobe[edit]

Transcranial alternating current stimulation applied over the frontal lobe targeting frontal midline theta (FMT) oscillations (4–8 Hz) can disrupt normal working memory performance and block demand-related FMT power increase.[10] Focused stimulation over the left prefrontal cortex with combination of theta (6 Hz) and gamma (80-100 Hz) oscillations can improve visual working memory performance and increase the neocortical integration.[11]

Electrophysiological evidence[edit]

While it was suggested that tACS in the individual alpha frequency range elevates the endogenous EEG alpha band power,[12] assessment of the direct effects of tACS on ongoing brain oscillations is challenging due to the stimulation artifacts.[13] Recently, however, Witkowski et al.[14] have introduced a novel stimulation protocol that capitalizes on the demodulation properties of the brain allowing for precise mapping of entrained brain oscillations, and thus investigating the mechanisms underlying tACS effects on physiology, brain function and behavior.

See also[edit]


  1. ^ Battleday, Ruairidh M.; Muller, Timothy; Clayton, Michael S.; Cohen Kadosh, Roi (2014-01-01). "Mapping the mechanisms of transcranial alternating current stimulation: a pathway from network effects to cognition". Neuroimaging and Stimulation. 5: 162. doi:10.3389/fpsyt.2014.00162. PMC 4237786Freely accessible. PMID 25477826. 
  2. ^ a b c d e Kanai R, Chaieb L, Antal A, Walsh V, Paulus W. (2008). Frequency-dependent electrical stimulation of the visual cortex. Curr Biol. 18(23):1839-43. PMID 19026538
  3. ^ a b Kanai R, Chaieb L, Antal A, Walsh V, Paulus W. (2008). Frequency-dependent electrical stimulation of the visual cortex Supplemental Data. Curr Biol. 18
  4. ^ a b Kar K, Krekelberg B. Transcranial electrical stimulation over visual cortex evokes phosphenes with a retinal origin. J Neurophysiol. 2012 Aug 1 PMID 22855777
  5. ^ Schwiedrzik, C.M. (2009). Retina or visual cortex? The site of phosphene induction by transcranial alternating current stimulation. Front Integr Neurosci 3,6
  6. ^ Schutter, D.J., and Hortensius, R. (2010). Retinal origin of phosphenes to transcranial alternating current stimulation. Clin Neurophysiol 121, 1080-1084
  7. ^ Ilkka Laakso and Akimasa Hirata: Computational analysis shows why transcranial alternating current stimulation induces retinal phosphenes. J Neural Engineering, Aug 10,2013, PMID 23813466
  8. ^ a b Pogosyan A, Gaynor LD, Eusebio A, Brown P. (2009). Boosting cortical activity at Beta-band frequencies slows movement in humans. Curr Biol. 19(19):1637-41. doi:10.1016/j.cub.2009.07.074 PMID 19800236
  9. ^ Antal, A., Boros, K., Poreisz, C., Chaieb, L., Terney, D., and Paulus, W. (2008). Comparatively weak after-effects of transcranial alternating current stimulation (tACS) on cortical excitability in humans. Brain Stimulation 1, 97–105. doi:10.1016/j.brs.2007.10.001
  10. ^ Chander BS, Witkowski M, Braun C, Robinson SE, Born J, Cohen LG, Birbaumer N, Soekadar SR. tACS Phase Locking of Frontal Midline Theta Oscillations Disrupts Working Memory Performance. Front Cell Neurosci 2016. doi:10.3389/fncel.2016.00120
  11. ^ Alekseichuk, Ivan; Turi, Zsolt; Lara, Gabriel Amador de; Antal, Andrea; Paulus, Walter. "Spatial Working Memory in Humans Depends on Theta and High Gamma Synchronization in the Prefrontal Cortex". Current Biology. 26 (12): 1513–1521. doi:10.1016/j.cub.2016.04.035. 
  12. ^ Zaehle, T., Rach, S., Herrmann, C.S. (2010). Transcranial Alternating Current Stimulation Enhances Individual Alpha Activity in Human EEG. PLoS ONE 5(11), e13766. doi:10.1371/journal.pone.0013766
  13. ^ Noury N, Hipp JF, Siegel M. Physiological processes non-linearly affect electrophysiological recordings during transcranial electric stimulation. Neuroimage 2016. doi:10.1016/j.neuroimage.2016.03.065
  14. ^ Witkowski M, Garcia-Cossio E, Chander BS, Braun C, Birbaumer N, Robinson SE, Soekadar SR. Mapping entrained brain oscillations during transcranial alternating current stimulation (tACS). Neuroimage 2016. doi:10.1016/j.neuroimage.2015.10.024