title: Alpha Power Increase After Transcranial Alternating Current Stimulation at Alpha Frequency (α-tACS) Reflects Plastic Changes Rather Than Entrainment.
abstract: BACKGROUND
  Periodic stimulation of occipital areas using transcranial alternating current stimulation (tACS) at alpha (α) frequency (8-12 Hz) enhances electroencephalographic (EEG) α-oscillation long after tACS-offset. Two mechanisms have been suggested to underlie these changes in oscillatory EEG activity: tACS-induced entrainment of brain oscillations and/or tACS-induced changes in oscillatory circuits by spike-timing dependent plasticity.
  
  
  OBJECTIVE
  We tested to what extent plasticity can account for tACS-aftereffects when controlling for entrainment "echoes." To this end, we used a novel, intermittent tACS protocol and investigated the strength of the aftereffect as a function of phase continuity between successive tACS episodes, as well as the match between stimulation frequency and endogenous α-frequency.
  
  
  METHODS
  12 healthy participants were stimulated at around individual α-frequency for 11-15 min in four sessions using intermittent tACS or sham. Successive tACS events were either phase-continuous or phase-discontinuous, and either 3 or 8 s long. EEG α-phase and power changes were compared after and between episodes of α-tACS across conditions and against sham.
  
  
  RESULTS
  α-aftereffects were successfully replicated after intermittent stimulation using 8-s but not 3-s trains. These aftereffects did not reveal any of the characteristics of entrainment echoes in that they were independent of tACS phase-continuity and showed neither prolonged phase alignment nor frequency synchronization to the exact stimulation frequency.
  
  
  CONCLUSIONS
  Our results indicate that plasticity mechanisms are sufficient to explain α-aftereffects in response to α-tACS, and inform models of tACS-induced plasticity in oscillatory circuits. Modifying brain oscillations with tACS holds promise for clinical applications in disorders involving abnormal neural synchrony.
authors: School of Psychology, University of Glasgow, 58 Hillhead Street, Glasgow G12 8QB, United Kingdom. Electronic address: a.vossen.1@research.gla.ac.uk.
authors: Alexandra
authors: A
authors: Vossen
authors: Institute of Neuroscience and Psychology, University of Glasgow, 58 Hillhead Street, Glasgow G12 8QB, United Kingdom.
authors: Joachim
authors: J
authors: Gross
authors: Institute of Neuroscience and Psychology, University of Glasgow, 58 Hillhead Street, Glasgow G12 8QB, United Kingdom. Electronic address: Gregor.Thut@glasgow.ac.uk.
authors: Gregor
authors: G
authors: Thut
mesh_terms: D000328:Adult; D000513:Alpha Rhythm; D002540:Cerebral Cortex / Q000502:physiology; D004569:Electroencephalography; D058407:Electroencephalography Phase Synchronization; D005260:Female; D006801:Humans; D008297:Male; D009473:Neuronal Plasticity / Q000502:physiology*; D065908:Transcranial Direct Current Stimulation / Q000379:methods*; D055815:Young Adult
keywords: Alpha oscillations; Electroencephalogram; Entrainment; Spike-timing dependent plasticity; Synchronization; Transcranial alternating current stimulation
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