Alpha Power Increase After Transcranial Alternating Current Stimulation at Alpha Frequency (α-tACS) Reflects Plastic Changes Rather Than Entrainment

Brain Stimul. May-Jun 2015;8(3):499-508. doi: 10.1016/j.brs.2014.12.004. Epub 2014 Dec 20.

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.

Conclusion: 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.

Keywords: Alpha oscillations; Electroencephalogram; Entrainment; Spike-timing dependent plasticity; Synchronization; Transcranial alternating current stimulation.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adult
  • Alpha Rhythm*
  • Cerebral Cortex / physiology
  • Electroencephalography
  • Electroencephalography Phase Synchronization
  • Female
  • Humans
  • Male
  • Neuronal Plasticity / physiology*
  • Transcranial Direct Current Stimulation / methods*
  • Young Adult