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

Alexandra Vossen, Joachim Gross, Gregor Thut
2015 Brain Stimulation  
Background: Periodic stimulation of occipital areas using transcranial alternating current stimulation (tACS) at alpha (a) frequency (8e12 Hz) enhances electroencephalographic (EEG) a-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
more » ... count 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 a-frequency. Methods: 12 healthy participants were stimulated at around individual a-frequency for 11e15 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 a-phase and power changes were compared after and between episodes of a-tACS across conditions and against sham. Results: a-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 a-aftereffects in response to a-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. Many human electrophysiological studies have mapped specific aspects of perception, memory, and cognition onto specific features of oscillatory brain activity, including phase and frequency (see e.g. Refs. [1e10]). Conventionally, such functional maps are established via non-invasive recording techniques such as electro/ magnetoencephalography (EEG/MEG) by examining task-related modulation of oscillatory brain activity or its covariation with behavioral performance measures. However, these maps are correlational by nature and do not permit the distinction between epiphenomenal and causal functional accounts. Recent attempts to demonstrate causal roles of oscillatory brain activity in implementing function have used non-invasive brain stimulation (NIBS, [11e14]) to promote natural neural frequencies [15] . To this end, an external, periodic electromagnetic force is applied over the appropriate (potentially task-relevant) brain area at the area's preferred oscillatory frequency with the aim to synchronize the intrinsic oscillations to the external force, and to assess the associated behavioral consequences (reviewed in [9, 16, 17] ). A promising
doi:10.1016/j.brs.2014.12.004 pmid:25648377 pmcid:PMC4464304 fatcat:xwu7bqtkkbfgtkl2ml7uv7npk4