Abstract
Non-invasive brain stimulation (NIBS) techniques such as transcranial alternating current stimulation (tACS) have recently become extensively utilized due to their potential to modulate ongoing neuronal oscillatory activity and consequently to induce cortical plasticity relevant for various cognitive functions. However, the neurophysiological basis for stimulation effects as well as their inter-individual differences is not yet understood. In the present study, we used a closed-loop electroencephalography-tACS(EEG-tACS) protocol to examine the modulation of alpha oscillations generated in occipito-parietal areas. In particular, we investigated the effects of a repeated short-time intermittent stimulation protocol (1 s in every trial) applied over the visual cortex (Cz and Oz) and adjusted according to the phase and frequency of visual alpha oscillations on the amplitude of these oscillations. Based on previous findings, we expected higher increases in alpha amplitudes for tACS applied in-phase with ongoing oscillations as compared to an application in anti-phase and this modulation to be present in low-alpha amplitude states of the visual system (eyes opened, EO) but not high (eyes closed, EC). Contrary to our expectations, we found a transient suppression of alpha power in inter-individually derived spatially specific parieto-occipital components obtained via the estimation of spatial filters by using the common spatial patterns approach. The amplitude modulation was independent of the phase relationship between the tACS signal and alpha oscillations, and the state of the visual system manipulated via closed- and open-eye conditions. It was also absent in conventionally analyzed single-channel and multi-channel data from an average parieto-occipital region. The fact that the tACS modulation of oscillations was phase-independent suggests that mechanisms driving the effects of tACS may not be explained by entrainment alone, but rather require neuroplastic changes or transient disruption of neural oscillations. Our study also supports the notion that the response to tACS is subject-specific, where the modulatory effects are shaped by the interplay between the stimulation and different alpha generators. This favors stimulation protocols as well as analysis regimes exploiting inter-individual differences, such as spatial filters to reveal otherwise hidden stimulation effects and, thereby, comprehensively induce and study the effects and underlying mechanisms of tACS.
Highlights
We present the results of a study using a previously used closed-loop EEG-Transcranial alternating current stimulation (tACS) protocol (Zarubin et al, 2018) by which the stimulation signal was phase coupled with ongoing alpha oscillations
Alpha power values derived from the single electrode POz as well as the parieto-occipital cluster (POC), in contrast, are modulated by the factor STATE, but are not significantly modulated by tACS or any interaction between the experimental factors
Our study aimed to investigate the effects of tACS applied bilaterally over the visual cortex, tuned to neural alpha oscillations with a closed-loop EEG-tACS setup on visual alpha oscillations
Summary
Non-invasive brain stimulation (NIBS) technology has gained increasing attention in the last few years from the scientific community (Bergmann et al, 2016; Antal et al, 2017; Thut et al, 2017; Vosskuhl et al, 2018), clinical (Palm et al, 2014; Yavari et al, 2017), sports (Edwards et al, 2017; Angius et al, 2018), military (Nelson et al, 2016), and other fields. One of the reasons for this growing interest is the successful modulation of cognitive, motor, and perceptual functions in numerous studies in different domains such as motor function (Feurra et al, 2011a; Brittain et al, 2013; Angius et al, 2018), visual (Zaehle et al, 2010; Helfrich et al, 2014), auditory (Riecke et al, 2015), somatosensory (Feurra et al, 2011b; Gundlach et al, 2016, 2017), or linguistic processing (Riecke et al, 2018; Wilsch et al, 2018) and for higher cognitive functions such as decision making, creativity, or self-aware dreaming (Sela et al, 2012; Voss et al, 2014; Lustenberger et al, 2015) Another reason is the widespread availability of various experimental, clinical protocols, and instructions (Bergmann et al, 2016; Antal et al, 2017; Tavakoli and Yun, 2017). This can be explained by the complexity of the task: implementation and utilization of closed-loop tACS have various technical challenges and fundamental questions, which narrow and delay the development of this field
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