title: Comparatively weak after-effects of transcranial alternating current stimulation (tACS) on cortical excitability in humans.
abstract: 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.
authors.affiliation: Department of Clinical Neurophysiology, Georg-August University, Göttingen, Germany. aantal@gwdg.de
authors.forename: Andrea
authors.initials: A
authors.lastname: Antal
authors.forename: Klára
authors.initials: K
authors.lastname: Boros
authors.forename: Csaba
authors.initials: C
authors.lastname: Poreisz
authors.forename: Leila
authors.initials: L
authors.lastname: Chaieb
authors.forename: Daniella
authors.initials: D
authors.lastname: Terney
authors.forename: Walter
authors.initials: W
authors.lastname: Paulus
mesh_terms: D000328:Adult; D004569:Electroencephalography; D019054:Evoked Potentials, Motor / Q000502:physiology*; D005260:Female; D006801:Humans; D007858:Learning / Q000502:physiology; D008297:Male; D009044:Motor Cortex / Q000502:physiology*; D011897:Random Allocation; D011795:Surveys and Questionnaires; D050781:Transcranial Magnetic Stimulation / Q000379:methods*; D004561:Transcutaneous Electric Nerve Stimulation / Q000379:methods*; D055815:Young Adult