Transcranial alternating current stimulation and neurofeedback are two different types of non-invasive neuromodulation techniques, which can modulate brain activity and improve brain functioning. In this review, we compared the current state of knowledge related to the mechanisms of tACS and NFB and their effects on electroencephalogram activity and on aftereffects, including the duration of their persistence and potential behavioral benefits. Since alpha bandwidth has been broadly studied in NFB and in tACS research, the studies of NFB and tACS in modulating (...) alpha bandwidth were selected for comparing the online and offline effects of these two neuromodulation techniques. The factors responsible for variability in the responsiveness of the modulated EEG activity by tACS and NFB were analyzed and compared too. Based on the current literature related to tACS and NFB, it can be concluded that tACS and NFB differ a lot in the mechanisms responsible for their effects on an online EEG activity but they possibly share the common universal mechanisms responsible for the induction of aftereffects in the targeted stimulated EEG band, namely Hebbian and homeostatic plasticity. Many studies of both neuromodulation techniques report the aftereffects connected to the behavioral benefits. The duration of persistence of aftereffects for NFB and tACS is comparable. In relation to the factors influencing responsiveness to tACS and NFB, significantly more types of factors were analyzed in the NFB studies compared to the tACS studies. Several common factors for both tACS and NFB have been already investigated. Based on these outcomes, we propose several new research directions regarding tACS and NFB. (shrink)

Transcranial magnetic stimulation can cause measurable effects on neural activity and behavioral performance in healthy volunteers. In addition, TMS is increasingly used in clinical practice for treating various neuropsychiatric disorders. Unfortunately, TMS-induced effects show large intra- and inter-subject variability, hindering its reliability, and efficacy. One possible source of this variability may be the spontaneous fluctuations of neuronal oscillations. We present recent studies using multimodal TMS including TMS-EMG, TMS-tACS, and concurrent TMS-EEG-fMRI, to evaluate how individual oscillatory brain state affects TMS signal (...) propagation within targeted networks. We demonstrate how the spontaneous oscillatory state at the time of TMS influences both immediate and longer-lasting TMS effects. These findings indicate that at least part of the variability in TMS efficacy may be attributable to the current practice of ignoring oscillatory fluctuations during TMS. Ignoring this state-dependent spread of activity may cause great individual variability which so far is poorly understood and has proven impossible to control. We therefore also compare two technical solutions to directly account for oscillatory state during TMS, namely, to use tACS to externally control these oscillatory states and then apply TMS at the optimal brain state, or oscillatory state-triggered TMS. The described multimodal TMS approaches are paramount for establishing more robust TMS effects, and to allow enhanced control over the individual outcome of TMS interventions aimed at modulating information flow in the brain to achieve desirable changes in cognition, mood, and behavior. (shrink)

Non-invasive transcranial electrical brain stimulation (tES) techniques, including transcranial direct current stimulation (tDCS), transcranial alternating current stimulation (tACS) and transcranial random noise stimulation (tRNS), can alter neuronal activity and related brain functions. However, tES effects seem to be modulated by various influencing factors, leading to high inter-individual variability in tES effects and often only low effect sizes, or even no effects. The present thesis therefore aimed to investigate methodological and physiological influencing factors of tDCS, tACS and tRNS that have not (...) been sufficiently examined so far. A first study investigated the influence of montage and individual functional performance level on the effects of anodal tDCS over the left dorsolateral prefrontal cortex (DLPFC) in healthy adults. Compared with sham stimulation, a multichannel montage led to stronger effects than a bipolar montage. For both montages the effects of stimulation were dependent on the functional performance level of participants. A second study investigated the effects of multichannel tDCS over the left DLPFC in healthy children and adolescents, considering the influence of concurrent target task performance during stimulation and individual head anatomy. tDCS did not influence the target outcome but led to transfer effects on non-target task performance and neurophysiological activity, that were only partly influenced by task performance during stimulation. The individual head anatomy had no influence on stimulation effects. A third study investigated tACS and tRNS effects on motor cortex excitability in healthy children and adolescents in comparison to adults. The individual response to sham stimulation was investigated as marker for the individual physiological brain state. Motor cortex excitability was not modulated by age but by individual response to sham stimulation. All studies provide important insights into the modulatory factors of stimulation effects. Based on these results, future studies should aim at individualising tES application. (shrink)

GraphicalThirty healthy participants received 60 trials of intermittent SO tACS at an intensity of 2 mA. Motor cortical excitability was assessed using TMS-induced MEPs acquired across different oscillatory phases during and outlasting tACS, as well as at the start and end of the stimulation session. Mean MEP amplitude increased by ∼41% from pre- to post-tACS ; however, MEP amplitudes were not modulated with respect to the tACS phase.Converging evidence suggests that transcranial alternating current stimulation may entrain endogenous neural oscillations to (...) match the frequency and phase of the exogenously applied current and this entrainment may outlast the stimulation. However, observing entrainment in the electroencephalograph during stimulation is extremely difficult due to the presence of complex tACS artifacts. The present study assessed entrainment to slow oscillatory tACS by measuring motor cortical excitability across different oscillatory phases during and outlasting stimulation. 30 healthy participants received 60 trials of intermittent SO tACS at an intensity of 2 mA peak-to-peak. Motor cortical excitability was assessed using transcranial magnetic stimulation of the hand region of the primary motor cortex to induce motor evoked potentials in the contralateral thumb. MEPs were acquired at four time-points within each trial – early online, late online, early offline, and late offline – as well as at the start and end of the overall stimulation period. A significant increase in MEP amplitude was observed from pre- to post-tACS and from the first to the last tACS block. However, no phase-dependent modulation of excitability was observed. Therefore, although SO tACS had a facilitatory effect on motor cortical excitability that outlasted stimulation, there was no evidence supporting entrainment of endogenous oscillations as the underlying mechanism. (shrink)