Abstract
The interest in transcranial alternating current stimulation (tACS) has significantly increased in the past decade. It has potential to modulate brain oscillations in a frequency specific manner, offering the possibility to demonstrate a causal nature of oscillation behavior relationships. TACS is a strong candidate as a tool for clinical applications, however, to fulfill this potential, certain parameters have yet to be evaluated. First, little is known about long-lasting after-effects of tACS with respect to the modulations of rhythmic brain activity. Second, the power of endogenous brain oscillations might play a crucial role in the efficacy of tACS. We hypothesize that the after-effects of tACS depend on the endogenous power of oscillations. To this end, we modulated the power of endogenous occipital alpha oscillations via tACS. In two experiments, participants either had their eyes open or closed to keep endogenous alpha power either low or high while they were stimulated for 20 min with their individual alpha frequency (IAF) and simultaneously performing a vigilance task. After-effects on IAF power were evaluated over a course of 30 min with a pre stimulation period serving as baseline. After-effects were strongly dependent on IAF power. Enhanced IAF power was observed for at least 30 min after tACS under conditions of low endogenous IAF power, whereas, IAF power could not be further enhanced by tACS under conditions of high IAF power. The current study demonstrates, for the first time, a long lasting effect after tACS on endogenous EEG power in the range of the stimulation frequency. Additionally, we present conclusive evidence that the power of the endogenous oscillations has a critical impact on tACS efficacy. Long lasting after-effects foster the role of tACS as a tool for non-invasive brain stimulation and demonstrate the potential for therapeutic application to reestablish the balance of altered brain oscillations.
Highlights
Brain oscillations play a crucial role in motor, perceptual, and cognitive processes (Basar et al, 2001; Herrmann et al, 2004; Buzsáki, 2006; Schroeder and Lakatos, 2009) and alterations of these oscillations can be linked to psychiatric disorders (Herrmann and Demiralp, 2005; Uhlhaas et al, 2008)
Long lasting after-effects foster the role of transcranial alternating current stimulation (tACS) as a tool for non-invasive brain stimulation and demonstrate the potential for therapeutic application to reestablish the balance of altered brain oscillations
The current study focuses on the state-dependency of physiological tACS after-effects, the results are expected to generalize to task-related oscillations, as we assess general properties of oscillations
Summary
Brain oscillations play a crucial role in motor, perceptual, and cognitive processes (Basar et al, 2001; Herrmann et al, 2004; Buzsáki, 2006; Schroeder and Lakatos, 2009) and alterations of these oscillations can be linked to psychiatric disorders (Herrmann and Demiralp, 2005; Uhlhaas et al, 2008). Oscillatory brain activity is evoked by neuronal network activity of different spatial scales (Buzsáki and Draguhn, 2004). These oscillations, which manifest as changes of extracellular electric fields, in turn, can serve as a feedback signal to structure the activity of the neurons that generated it (Fröhlich and McCormick, 2010). Transcranial electrical stimulation (TES) with weak sinusoidally varying currents is a non-invasive brain stimulation technique that can mimick endogenous electric fields and is thought to directly modulate ongoing oscillatory brain activity as suggested by numerous studies (e.g., Marshall et al, 2006; Antal et al, 2008; Kanai et al, 2008; Pogosyan et al, 2009; Zaehle et al, 2010; Feurra et al, 2011; Neuling et al, 2012a; Polania et al, 2012). The power of the spontaneous brain activity in the range of the stimulation frequency can be enhanced (Marshall et al, 2006; Zaehle et al, 2010; Neuling et al, 2012a)
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