scholarly journals Local network-level integration mediates effects of transcranial Alternating Current Stimulation

2017 ◽  
Author(s):  
Marco Fuscà ◽  
Philipp Ruhnau ◽  
Toralf Neuling ◽  
Nathan Weisz
Keyword(s):  
Occipital Cortex ◽  
Alternating Current ◽  
Functional Network ◽  
Local Network ◽  
Oscillatory Activity ◽  
Alpha Power ◽  
State Dependency ◽  
External Stimulation ◽  
Posterior Cingulate ◽  
Transcranial Alternating Current Stimulation

AbstractTranscranial alternating current stimulation (tACS) has been proposed as a tool to draw causal inferences on the role of oscillatory activity in cognitive functioning and has the potential to induce long-term changes in cerebral networks. However, the mechanisms of action of tACS are not yet clear, though previous modeling works have suggested that variability may be mediated by local and network-level brain states. We used magnetoencephalography (MEG) to record brain activity from 17 healthy participants as they kept their eyes open (EO) or closed (EC) while being stimulated either with sham, weak, or strong alpha-tACS using a montage commonly assumed to target occipital areas. We reconstructed the activity of sources in all stimulation conditions by means of beamforming. The analysis of resting-state data revealed an interaction of the external stimulation with the endogenous alpha power difference between EO and EC in the posterior cingulate. This region is remote from occipital cortex, which showed strongest EC vs. EO alpha modulation, thus suggesting state-dependency long-range effects of tACS. In a follow-up analysis of this online-tACS effect, we find evidence that this dependency effect could be mediated by functional network changes: connection strength from the precuneus, a region adjusting for a measure of network integration in the two states (EC vs. EO during no-tACS), was significantly correlated with the state-dependency effect in the posterior cingulate (during tACS). No analogous correlation could be found for alpha power modulations in occipital cortex. Altogether, this is the first strong evidence to illustrate how functional network architectures can shape tACS effects.

scholarly journals The hidden state-dependency of transcranial alternating current stimulation (tACS)

2020 ◽  
Author(s):  
Florian H. Kasten ◽  
Christoph S. Herrmann
Keyword(s):  
Markov Models ◽  
Alternating Current ◽  
Oscillatory Activity ◽  
Brain Disorders ◽  
State Dependency ◽  
Non Invasive ◽  
Transcranial Alternating Current Stimulation ◽  
Brain States ◽  
Invasive Techniques ◽  
The Brain

AbstractNon-invasive techniques to electrically stimulate the brain such as transcranial direct and alternating current stimulation (tDCS/tACS) are increasingly used in human neuroscience and offer potential new avenues to treat brain disorders. However, their often weak and variable effects have also raised concerns in the scientific community. A possible factor influencing the efficacy of these methods is the dependence on brain-states. Here, we utilized Hidden Markov Models (HMM) to decompose concurrent tACS-magnetoencephalography data into transient brain-states with distinct spatial, spectral and connectivity profiles. We found that out of four spontaneous brain-states only one was susceptible to tACS. No or only marginal effects were found in the remaining states. TACS did not influence the time spent in each state. Our results suggest, that tACS effects may be mediated by a hidden, spontaneous state-dependency and provide novel insights to the changes in oscillatory activity underlying aftereffects of tACS.

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

2019 ◽  
Author(s):  
Marina Fiene ◽  
Bettina C. Schwab ◽  
Jonas Misselhorn ◽  
Christoph S. Herrmann ◽  
Till R. Schneider ◽  
...  
Keyword(s):  
Phase Shift ◽  
Occipital Cortex ◽  
Alternating Current ◽  
Oscillatory Activity ◽  
Optimal Timing ◽  
Healthy Human ◽  
Oscillatory Phase ◽  
Transcranial Alternating Current Stimulation ◽  
Onset Delays ◽  
Human Participants

AbstractBackgroundOscillatory phase has been proposed as a key parameter defining the spatiotemporal structure of neural activity. To enhance our understanding of brain rhythms and improve clinical outcomes in pathological conditions, phase-specific modulation of oscillations by transcranial alternating current stimulation (tACS) emerged as a promising approach. However, the effectiveness of tACS in humans is still critically debated.ObjectiveHere, we investigated the phase-specificity of tACS effects on visually evoked steady state responses (SSRs) in 24 healthy human participants of either sex.MethodsTo this end, we used an intermittent electrical stimulation protocol and assessed the influence of tACS on SSR amplitude in the interval immediately following tACS.ResultsWe observed that the phase shift between flicker and tACS modulates evoked SSR amplitudes. The tACS effect size was dependent on the strength of flicker-evoked oscillatory activity, with larger effects in participants showing weaker locking of neural responses to flicker phase. Neural sources of phase-specific effects were localized in the parieto-occipital cortex within flicker-entrained regions. Importantly, the optimal phase shift between flicker and tACS associated with strongest SSRs was correlated with cortical SSR onset delays over the visual cortex.ConclusionsOverall, our data provide electrophysiological evidence for phase-specific modulations of oscillatory activity by tACS in humans. As the optimal timing of tACS application was dependent on neural conduction times as measured by SSR onset delays, data suggest that the interaction between tACS effect and SSR was cortical in nature. These findings corroborate the physiological efficacy of tACS and highlight its potential for controlled modulations of brain signals.

scholarly journals Perception of Rhythmic Speech Is Modulated by Focal Bilateral Transcranial Alternating Current Stimulation

2020 ◽  
Vol 32 (2) ◽  
pp. 226-240 ◽  
Author(s):  
Benedikt Zoefel ◽  
Isobella Allard ◽  
Megha Anil ◽  
Matthew H. Davis
Keyword(s):  
Speech Perception ◽  
Speech Processing ◽  
Alternating Current ◽  
Stream Segregation ◽  
Causal Role ◽  
Oscillatory Activity ◽  
Transcranial Alternating Current Stimulation ◽  
Neural Entrainment ◽  
Actual Speech

Several recent studies have used transcranial alternating current stimulation (tACS) to demonstrate a causal role of neural oscillatory activity in speech processing. In particular, it has been shown that the ability to understand speech in a multi-speaker scenario or background noise depends on the timing of speech presentation relative to simultaneously applied tACS. However, it is possible that tACS did not change actual speech perception but rather auditory stream segregation. In this study, we tested whether the phase relation between tACS and the rhythm of degraded words, presented in silence, modulates word report accuracy. We found strong evidence for a tACS-induced modulation of speech perception, but only if the stimulation was applied bilaterally using ring electrodes (not for unilateral left hemisphere stimulation with square electrodes). These results were only obtained when data were analyzed using a statistical approach that was identified as optimal in a previous simulation study. The effect was driven by a phasic disruption of word report scores. Our results suggest a causal role of neural entrainment for speech perception and emphasize the importance of optimizing stimulation protocols and statistical approaches for brain stimulation research.

scholarly journals Facilitated event-related power-modulations during transcranial alternating current stimulation (tACS) revealed by concurrent tACS-MEG

2018 ◽  
Author(s):  
Florian H. Kasten ◽  
Burkhard Maess ◽  
Christoph S. Herrmann
Keyword(s):  
Mental Rotation ◽  
Spatial Filtering ◽  
Alternating Current ◽  
Mental Rotation Task ◽  
Oscillatory Activity ◽  
Control Analysis ◽  
Alpha Band ◽  
Oscillatory Dynamics ◽  
Continuous Application ◽  
Transcranial Alternating Current Stimulation

AbstractNon-invasive approaches to modulate oscillatory activity in the brain receive growing popularity in the scientific community. Transcranial alternating current stimulation (tACS) has been shown to modulate neural oscillations in a frequency specific manner. Due to a massive stimulation artifact at the targeted frequency, only little is known about effects of tACS during stimulation. I.e. it remains unclear how the continuous application of tACS affects event-related oscillations during cognitive tasks. Depending on whether tACS merely affects pre‐ or post-stimulus oscillations or both, stimulation can alter patterns of event-related oscillatory dynamics in various directions or may not affect them at all. Thus, knowledge about these directions is crucial to plan, predict and understand outcomes of solely behavioral tACS experiments. Here, a recently proposed procedure to suppress tACS artifacts by projecting MEG data into source space using spatial filtering was utilized to recover event-related power modulations in the alpha band during a mental rotation task. MEG of twenty-five volunteers was continuously recorded. After 10 minutes of baseline measurement, they received either 20 minutes of tACS at individual alpha frequency or sham stimulation. Another 40 minutes of MEG were acquired thereafter. Data were projected into source space and carefully examined for residual artifacts. Results revealed strong facilitation of event-related power modulations in the alpha band during tACS application. Data provide first direct evidence, that tACS does not counteract top-down suppression of intrinsic oscillations, but rather enhances pre-existent power modulations within the range of the individual alpha (=stimulation) frequency.SignificanceTranscranial alternating current stimulation (tACS) is increasingly used in cognitive neuroscience to study the causal role of brain oscillations and cognition. However, online effects of tACS so far largely remain a ‘black box’ due to an intense electromagnetic artifact encountered during stimulation. The current study is the first to employ a spatial filtering approach to recover and systematically study event-related oscillatory dynamics during tACS, which can potentially be altered in various directions. TACS facilitated pre-existing patterns of oscillatory dynamics during the employed mental rotation task, but does not counteract or overwrite them. In addition, control analysis and a measure to quantify tACS artifact suppression are provided that can enrich future studies investigating tACS online effects.

scholarly journals Ameliorating Emotional Attention through Modulation of Neural Oscillations with Transcranial Alternating Current Stimulation

2021 ◽  
Author(s):  
Shuang Liu ◽  
Yuchen He ◽  
Dongyue Guo ◽  
Xiaoya Liu ◽  
Xinyu Hao ◽  
...  
Keyword(s):  
Emotional Processing ◽  
Emotion Processing ◽  
Alternating Current ◽  
Event Related Potential ◽  
Facial Emotion ◽  
Alpha Power ◽  
Emotion Identification ◽  
Transcranial Alternating Current Stimulation ◽  
Emotional Attention ◽  
P200 Amplitude

AbstractBackgroundNumerous clinical reports suggest that psychopathy like schizophrenia, anxiety and depressive disorder is accompanied by early attentional abnormalities in emotional information processing. In the past decade, the efficacy of transcranial alternating current stimulation (tACS) in changing emotional functioning has been repeatedly observed and has demonstrated a causal relationship between endogenous oscillations and emotional processing. However, tACS effects on emotional attention have not yet been tested.MethodsA total of 53 healthy participants were randomized to 2 groups, and they were subjected to active or sham tACS at individual alpha frequency (IAF) in the bilaterally dorsolateral prefrontal cortex (dlPFC). Participants and received this treatment for 20 min durations daily for 7 consecutive days. On days 1 and 7, electroencephalogram (EEG) recording of 8 minute resting with eyes open and closed. Responses to a facial emotion identification task were also recorded to measure alpha changes and event-related potential (ERP) alterations.ResultsOn day 7 after tACS, the active group showed a more clear elevation in alpha power at rest, especially in open state around stimulation area, compared to the sham group. ERPs revealed a significant larger P200 amplitude after active stimulation (p < 0.05), indicating attentional improvement in facial emotion processing. Additionally, a notable positive correlation (p < 0.05) between alpha power and P200 amplitude was found, providing an electrophysiological interpretation regarding the role of tACS in emotional attention modulation. In addition, the IAF-tACS showed an obvious advantage in alpha entrainment compared to an additional 10 Hz-tACS.ConclusionsThese results support a seminal outcome for the effect of IAF-tACS on emotional attention modulation, demonstrating a feasible and individual-specific therapy for neuropsychiatric disorders related to emotion processing, especially regarding oscillatory disturbances.

scholarly journals A Comparison of Closed Loop vs. Fixed Frequency tACS on Modulating Brain Oscillations and Visual Detection

2021 ◽  
Vol 15 ◽  
Author(s):  
Heiko I. Stecher ◽  
Annika Notbohm ◽  
Florian H. Kasten ◽  
Christoph S. Herrmann
Keyword(s):  
Closed Loop ◽  
Alternating Current ◽  
Alpha Activity ◽  
Stimulation Frequency ◽  
Alpha Power ◽  
Brain Oscillations ◽  
Stimulation Protocol ◽  
Fixed Frequency ◽  
Transcranial Alternating Current Stimulation ◽  
Synaptic Changes

Transcranial alternating current stimulation has emerged as an effective tool for the exploration of brain oscillations. By applying a weak alternating current between electrodes placed on the scalp matched to the endogenous frequency, tACS enables the specific modulation of targeted brain oscillations This results in alterations in cognitive functions or persistent physiological changes. Most studies that utilize tACS determine a fixed stimulation frequency prior to the stimulation that is kept constant throughout the experiment. Yet it is known that brain rhythms can encounter shifts in their endogenous frequency. This could potentially move the ongoing brain oscillations into a frequency region where it is no longer affected by the stimulation, thereby decreasing or negating the effect of tACS. Such an effect of a mismatch between stimulation frequency and endogenous frequency on the outcome of stimulation has been shown before for the parietal alpha-activity. In this study, we employed an intermittent closed loop stimulation protocol, where the stimulation is divided into short epochs, between which an EEG is recorded and rapidly analyzed to determine a new stimulation frequency for the next stimulation epoch. This stimulation protocol was tested in a three-group study against a classical fixed stimulation protocol and a sham-treatment. We targeted the parietal alpha rhythm and hypothesized that this setup will ensure a constant close match between the frequencies of tACS and alpha activity. This closer match should lead to an increased modulation of detection of visual luminance changes depending on the phase of the tACS and an increased rise in alpha peak power post stimulation when compared to a protocol with fixed pre-determined stimulation frequency. Contrary to our hypothesis, our results show that only a fixed stimulation protocol leads to a persistent increase in post-stimulation alpha power as compared to sham. Furthermore, in none of the stimulated groups significant modulation of detection performance occurred. While the lack of behavioral effects is inconclusive due to the short selection of different phase bins and trials, the physiological results suggest that a constant stimulation with a fixed frequency is actually beneficial, when the goal is to produce persistent synaptic changes.

scholarly journals Local Network-Level Integration Mediates Effects of Transcranial Alternating Current Stimulation

2018 ◽  
Vol 8 (4) ◽  
pp. 212-219 ◽  
Author(s):  
Marco Fuscà ◽  
Philipp Ruhnau ◽  
Toralf Neuling ◽  
Nathan Weisz
Keyword(s):  
Alternating Current ◽  
Local Network ◽  
Transcranial Alternating Current Stimulation

scholarly journals Effects of transcranial alternating-current stimulation to secondary motor areas on cortical oscillations in stroke patients

2019 ◽  
Author(s):  
Lutz A. Krawinkel ◽  
Marlene Bönstrup ◽  
Jan F. Feldheim ◽  
Robert Schulz ◽  
Winifried Backhaus ◽  
...  
Keyword(s):  
Phase Locking ◽  
Premotor Cortex ◽  
Alternating Current ◽  
Oscillatory Activity ◽  
Stroke Patients ◽  
Cortical Oscillations ◽  
Motor Network ◽  
Transcranial Alternating Current Stimulation ◽  
Phase Locking Value ◽  
Motor Areas

AbstractBackgroundThere is growing evidence that secondary motor areas are relevant for recovery after motor stroke. Communication among brain areas occurs via synchronization of oscillatory activity which can potentially be modulated via transcranial alternating-current stimulation (tACS).HypothesisWe hypothesized that tACS to secondary motor areas of the stroke-lesioned hemisphere leads to modulation of task-related connectivity among primary and secondary motor areas, reflected in metrics of EEG coupling in the frequency domain.MethodsWe applied focal tACS at 1mA peak-to-peak intensity to ipsilesional ventral premotor cortex (PMv) and supplementary motor area (SMA) in chronic stroke patients while they moved their impaired hand. To probe effects of stimulation on cortical oscillations, several task-related EEG-based connectivity metrics (coherence, imaginary coherence, phase-locking value, mutual information) were assessed before and after each stimulation.ResultsOverall, we found significant but weak modulations of the motor network by tACS. Stimulation of PMv reduced task-related coupling between (i) both primary motor cortices (M1) (coherence, −0.0514±0.0665 (mean±SD, active stimulation) vs. 0.0085±0.0888 (sham), p=0.0029) and (ii) between ipsilesional M1 and contralesional PMv (coherence, - 0.0386±0.0703 vs. 0.0226±0.0694, p=0.0283; phase-locking value, −0.0363±0.0581 vs. 0.0036±0.0497, p=0.0097) compared with sham stimulation.ConclusionsIn this exploratory analysis, tACS to the ipsilesional PMv induced a weak decrease of task-related connectivity between ipsilesional M1 and contralesional M1 and PMv. As an excess of interhemispheric coupling is under discussion as maladaptive phenomenon of motor reorganization after stroke (e.g., bimodal balance-recovery model), tACS-induced reduction of coupling might be an interesting approach to assist re-normalization of the post-stroke motor network.

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