Elsevier

Brain Stimulation

Volume 1, Issue 2, April 2008, Pages 97-105
Brain Stimulation

Original research
Comparatively weak after-effects of transcranial alternating current stimulation (tACS) on cortical excitability in humans

https://doi.org/10.1016/j.brs.2007.10.001Get rights and content

Objective

Interference with brain rhythms by noninvasive transcranial stimulation that uses weak transcranial alternating current may reveal itself to be a new tool for investigating cortical mechanisms currently unresolved. Here, we aim to extend transcranial direct current stimulation (tDCS) techniques to transcranial alternating current stimulation (tACS).

Background

Parameters such as electrode size and position were taken from those used in previous tDCS studies.

Methods

Motor evoked potentials (MEPs) revealed by transcranial magnetic stimulation (TMS), electroencephalogram (EEG)-power, and reaction times measured in a motor implicit learning task, were analyzed to detect changes in cortical excitability after 2-10 minutes of AC stimulation and sinusoidal DC stimulation (tSDCS) by using 1, 10, 15, 30, and 45 Hz and sham stimulation over the primary motor cortex in 50 healthy subjects (eight-16 subjects in each study).

Results

A significantly improved implicit motor learning was observed after 10 Hz AC stimulation only. No significant changes were observed in any of the analyzed frequency bands of EEG and with regard to the MEP amplitudes after AC or tSDCS stimulation. Similarly, if the anodal or cathodal DC stimulation was superimposed on 5, 10, and 15 Hz AC stimulation, the MEP amplitudes did not change significantly.

Conclusions

Transcranial application of weak AC current may appear to be a tool for basic and clinical research in diseases with altered EEG activity. However, its effect seems to be weaker than tDCS stimulation, at least in the present context of stimulus intensity and duration. Further studies are required to extend cautiously the safety range and uncover its influence on neuronal circuitries.

Section snippets

Subjects

Fifty subjects (24 men and 26 women) participated in the studies. None of the subjects took regular or acute medication. Participants gave informed written consent. The experiments were approved by the Ethics Committee of the University of Göttingen, and conformed to the Declaration of Helsinki. All subjects were right handed, according to the Edinburgh handedness inventory.14

Transcranial alternating current stimulation (tACS)

Ten healthy subjects (22-43 years old, mean age = 26.4 ± 8.0, 3 men) participated in the TMS study. Eight healthy

Results

All the subjects tolerated the stimulation; none of the experimental sessions were interrupted because of side effects of the stimulation. However, about half of the subjects noticed light flickering during higher frequency stimulation (30, 45 Hz) by using an intensity of 0.4 mA. As a result, we did not further increase the stimulation amplitude for safety reasons. Only 2 of the subjects reported a light burning sensation under the electrodes during the stimulation. Six subjects experienced a

References (28)

  • P.A. Merton et al.

    Stimulation of the cerebral cortex in the intact human subject

    Nature

    (1980)
  • M.A. Nitsche et al.

    Excitability changes induced in the human motor cortex by weak transcranial direct current stimulation

    J Physiol

    (2000)
  • M.A. Nitsche et al.

    Sustained excitability elevations induced by transcranial DC motor cortex stimulation in humans

    Neurology

    (2001)
  • A. Priori et al.

    Polarization of the human motor cortex through the scalp

    Neuroreport

    (1998)
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    This study was funded by the Bernstein Center for Computational Neuroscience (01GQ0432) (A.A.) and the Rose Foundation (C.P.).

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