Sustained Aftereffect of α-tACS Lasts Up to 70 min after Stimulation

Abstract

Transcranial alternating current stimulation (tACS) has been repeatedly demonstrated to increase power of endogenous brain oscillations in the range of the stimulated frequency after stimulation. In the alpha band this aftereffect has been shown to persist for at least 30 min. However, in most experiments the aftereffect exceeded the duration of the measurement. Thus, it remains unclear how the effect develops beyond these 30 min and when it decays. The current study aimed to extend existing findings by monitoring the physiological aftereffect of tACS in the alpha range for an extended period of 90 min post-stimulation. To this end participants received either 20 min of tACS or sham stimulation with intensities below their individual sensation threshold at the individual alpha frequency (IAF). Electroencephalogram (EEG) was acquired during 3 min before and 90 min after stimulation. Subjects performed a visual vigilance task during the whole measurement. While the enhanced power in the individual alpha band did not return back to pre-stimulation baseline in the stimulation group, the difference between stimulation and sham diminishes after 70 min due to a natural alpha increase of the sham group.

Integrating electric field modelling and neuroimaging to explain inter-individual variability of tACS effects

“…However, the impact of stimulation seems to depend on the strength and precision of the individual electric field and the precision of the stimulation frequency. Individual differences in electric fields explain tES variability 13 When the strength of tES-induced electric fields necessary to modulate neuronal activity is discussed, electric fields reaching the human brain are usually compared against thresholds derived from animal studies 11,[45][46][47] . Those thresholds are in the range of 0.2 V/m to 0.5 V/m 45 .…”

“…While tDCS is thought to exhibit its effect by changing neuronal excitability via alterations of neuron's resting membrane polarization 1, [8][9][10] , tACS is believed to work via the principles of neuronal entrainment 2,11 . In addition, both methods have been reported to cause changes outlasting the duration of stimulation by several minutes to more than an hour [12][13][14] , likely via NMDA-receptor mediated plasticity [14][15][16][17] .…”

Does alpha phase modulate visual target detection? Three experiments with tACS phase-based stimulus presentation

“…Firstly, perhaps tACS did not successfully phase-align neuronal alpha oscillations, and while naturally occurring alpha phase is functionally relevant, alpha tACS phase is not. While alpha tACS effects on EEG alpha power have been reported repeatedly (Kasten, Dowsett, & Herrmann, 2016;Neuling, Rach, & Herrmann, 2013), it is possible that mechanisms other than phase-alignment (i.e. stochastic resonance, Vossen, Gross, & Thut, 2015) underlie some of those effects.…”

Phase-specific manipulation of neural oscillatory activity by transcranial alternating current stimulation

“…However, phase-specific tACS effects already decayed after the first 1 sec time epoch. While lasting power enhancement after prolonged alpha tACS for up to 70 min has been repeatedly shown and ascribed to synaptic plasticity mechanisms (32)(33)(34)(35), evidence for outlasting effects on phase synchronization is still sparse. Assuming online phase alignment of neural activity by tACS, Stonkus et al (36) hypothesized that oscillatory entrainment echoes can outlast the stimulation period for a few cycles (36,37).…”