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.
Highlights
Non-invasive brain stimulation (NIBS), including transcranial electric stimulation, has shown impressive effects ranging from short changes in neural activity to long lasting recovery maximization following neural injury (Hallett, 2005; Talelli and Rothwell, 2006; Hummel et al, 2008; Sandrini and Cohen, 2013)
The effects of 10 Hz retinofugal ACS (rACS) on neural oscillations showed the following characteristics: (1) 10 Hz stimulation resulted in an enhancement of α-power; (2) the post-stimulation α-peak did not significantly differ from baseline individual α frequency (IAF); and (3) there was no significant correlation between α-power and proximity to stimulation frequency
The specific enhancement of α-power over the occipital electrodes after 10 Hz rACS is consistent with studies conducted for transcranial ACS (tACS) (Antal et al, 2008; Zaehle et al, 2010; Helfrich et al, 2014; Vossen et al, 2015)
Summary
Non-invasive brain stimulation (NIBS), including transcranial electric stimulation (tES; Paulus, 2011), has shown impressive effects ranging from short changes in neural activity to long lasting recovery maximization following neural injury (Hallett, 2005; Talelli and Rothwell, 2006; Hummel et al, 2008; Sandrini and Cohen, 2013). Learning, and long-term memory formation (Marshall et al, 2006; Kuo and Nitsche, 2012; Santarnecchi et al, 2013), as well as clinical improvements including tumor growth slowing (Kirson et al, 2007) and tremor suppression in patients with Parkinson’s disease (Brittain et al, 2013). Despite these encouraging results, little is known about the mechanism of action (Zaghi et al, 2010). To efficiently and efficaciously apply tACS, especially in the visual system, it critically important for its users to further investigate the retinal as well as cortical mechanisms of action behind the observed effects
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