Rebound or Entrainment? The Influence of Alternating Current Stimulation on Individual Alpha

Abstract

Alternating current stimulation (ACS) is an established means to manipulate intrinsic cortical oscillations. While working towards clinical impact, ACS mechanisms of action remain unclear. For ACS’s well-documented influence on occipital alpha, hypotheses include neuronal entrainment as well as rebound phenomena. As a retinal origin is also discussed, we employed a novel form of ACS with the advantage that it specifically targets occipital alpha-oscillations via retinofugal pathways retinofugal ACS (rACS). We aimed to confirm alpha-enhancement outlasting the duration of stimulation with 10 Hz rACS. To distinguish entrainment from rebound effects, we investigated the correlation between alpha peak frequency change and alpha-enhancement strength. We quantified the alpha band power before and after 10 Hz rACS in 15 healthy subjects. Alpha power enhancement and alpha peak frequency change were assessed over the occipital electrodes and compared to sham stimulation. RACS significantly enhanced occipital alpha power in comparison to sham stimulation ( p < 0.05). Alpha peak frequency changed by a mean 0.02 Hz (± 0.04). A greater change in alpha peak frequency did not correlate with greater effects on alpha power. Our findings show an alpha-enhancement consistent with studies conducted for transcranial ACS (tACS) and contribute evidence for a retinal involvement in tACS effects on occipital alpha. Furthermore, the lack of correlation between alpha peak frequency change and alpha-enhancement strength provides an argument against entrainment effects and in favor of a rebound phenomenon.

Atypical Temporal Dynamics of Resting State Shapes Stimulus-Evoked Activity in Depression—An EEG Study on Rest–Stimulus Interaction

“…This adaptive mechanism (16) responds to the demands presented by a task and may adjust the sampling rate—increasing the PF would allow for more cycles in the same period of time—of incoming information required by this task. This is supported by findings from several studies (95, 96).…”

α‐tACS over the somatosensory cortex enhances tactile spatial discrimination in healthy subjects with low alpha activity

“…This discrepancy may be explained by an obscured overall effect of tACS caused by subject heterogeneity. Several previous studies have shown that alpha‐oscillation power is typically increased during α‐tACS (Helfrich, Schneider, et al, 2014), as well as after tACS (Haberbosch et al., 2019; Kasten et al., 2016; Neuling et al., 2012, 2013; Vossen et al., 2015; Zaehle et al., 2010). Conversely, a recent study has demonstrated that α‐tACS decreases alpha‐oscillation power (Gundlach et al., 2017).…”

“…Transcranial alternating current stimulation (tACS) may be effective for increasing ongoing oscillatory brain activity (Zaehle et al., 2010; Neuling et al., 2012; 2013; Helfrich, Schneider, et al, 2014; Vossen et al., 2015; Kasten et al., 2016; Haberbosch et al., 2019). In addition, tACS at specific frequencies has been often demonstrated to alter various sensory functions (Kanai et al., 2008; Laczó et al., 2012; Helfrich, Knepper, et al, 2014; Kar & Krekelberg, 2014; Strüber et al., 2014; Müller et al., 2015; Riecke et al., 2015; Rufener et al., 2016; Rufener et al., 2016; Wöstmann et al, 2018; Baltus et al., ; Fusco et al., 2018).…”

Transcranial magnetic stimulation entrains alpha oscillatory activity in occipital cortex

“…It is made The copyright holder for this preprint this version posted January 10, 2021. ; https://doi.org/10.1101/2021.01.09.426064 doi: bioRxiv preprint Our 4-pulse alpha-band TMS protocol provides an interesting alternative for occipital entrainment to that of transcranial alternating current stimulation (tACS) protocols. Alpha-band tACS protocols typically involves 10-20 minutes of stimulation followed by 1-3 minutes of testing period, during which enhanced alpha-power or ITPC has been recorded with concurrent EEG and taken as evidence of entrainment [81,82] (but see alternative interpretations: [83,84]). In any case, TMS protocols [49,51], including ours, provide more focal stimulation effects than tACS and can be effectively delivered in shorter 'bursts' of pulses, hence pinpoint more accurately the temporal dynamics of entrainment effects, enable trial-by-trial stimulation designs and concurrent EEG recordings.…”