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

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.

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Transient amplitude modulation of alpha-band oscillations by short-time intermittent closed-loop tACS

10.1101/2020.01.23.916676 ◽

2020 ◽

Author(s):

G. Zarubin ◽

C. Gundlach ◽

V. Nikulin ◽

A. Villringer ◽

M. Bogdan

Keyword(s):

Individual Differences ◽

Visual System ◽

Amplitude Modulation ◽

Closed Loop ◽

Alternating Current ◽

Stimulation Protocol ◽

Spatial Filters ◽

Alpha Oscillations ◽

Transcranial Alternating Current Stimulation ◽

Short Time

AbstractNon-invasive brain stimulation 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 are not yet understood. In the present study we used a closed-loop EEG transcranial alternating current stimulation protocol (EEG-tACS) 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) but not high (eyes closed).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 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.

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Alpha Oscillatory Activity Causally Linked to Working Memory Retention: Insights From Online Phase-Locking Closed-Loop Transcranial Alternating Current Stimulation

International Journal of Psychophysiology ◽

10.1016/j.ijpsycho.2021.07.163 ◽

2021 ◽

Vol 168 ◽

pp. S54

Author(s):

Xiaochu Zhang

Keyword(s):

Working Memory ◽

Closed Loop ◽

Phase Locking ◽

Alternating Current ◽

Oscillatory Activity ◽

Memory Retention ◽

Transcranial Alternating Current Stimulation

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Probing the Link Between Perception and Oscillations: Lessons from Transcranial Alternating Current Stimulation

The Neuroscientist ◽

10.1177/1073858419828646 ◽

2019 ◽

Vol 26 (1) ◽

pp. 57-73 ◽

Cited By ~ 4

Author(s):

Yuranny Cabral-Calderin ◽

Melanie Wilke

Keyword(s):

Time Windows ◽

Gamma Oscillations ◽

Alternating Current ◽

Brain Regions ◽

Causal Link ◽

Brain Oscillations ◽

Promising Tool ◽

Transcranial Alternating Current Stimulation ◽

Close Time ◽

Auditory Cortices

Brain oscillations are regarded as important for perception as they open and close time windows for neural spiking to enable the effective communication within and across brain regions. In the past, studies on perception primarily relied on the use of electrophysiological techniques for probing a correlative link between brain oscillations and perception. The emergence of noninvasive brain stimulation techniques such as transcranial alternating current stimulation (tACS) provides the possibility to study the causal contribution of specific oscillatory frequencies to perception. Here, we review the studies on visual, auditory, and somatosensory perception that employed tACS to probe the causality of brain oscillations for perception. The current literature is consistent with a causal role of alpha and gamma oscillations in parieto-occipital regions for visual perception and theta and gamma oscillations in auditory cortices for auditory perception. In addition, the sensory gating by alpha oscillations applies not only to the visual but also to the somatosensory domain. We conclude that albeit more refined perceptual paradigms and individualized stimulation practices remain to be systematically adopted, tACS is a promising tool for establishing a causal link between neural oscillations and perception.

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Effects of Transcranial Alternating Current Stimulation on Cognitive Functions in Healthy Young and Older Adults

Neural Plasticity ◽

10.1155/2016/4274127 ◽

2016 ◽

Vol 2016 ◽

pp. 1-13 ◽

Cited By ~ 28

Author(s):

Daria Antonenko ◽

Miriam Faxel ◽

Ulrike Grittner ◽

Michal Lavidor ◽

Agnes Flöel

Keyword(s):

Older Adults ◽

Language Learning ◽

Cognitive Functions ◽

Alternating Current ◽

Future Research ◽

Brain Oscillations ◽

Specific Patient ◽

Transcranial Alternating Current Stimulation ◽

Young Subjects ◽

Interaction With Age

Recently, transcranial alternating current stimulation (tACS) has emerged as a tool to enhance human cognitive processes. Here, we provide a brief summary of the rationale behind tACS-induced effects on task-relevant brain oscillations and associated cognitive functions and review previous studies in young subjects that have applied tACS in cognitive paradigms. Additionally, we present pilot data where we administered theta-tACS (6 Hz) over the temporoparietal cortex and a supraorbital reference for 20 min during implicit language learning in healthy young (mean/SD age: 22/2) and older (mean/SD age: 66/4) adults, in a sham-controlled crossover design. Linear mixed models revealed significantly increased retrieval accuracy following tACS-accompanied associative learning, after controlling for session order and learning success. These data provide the first implementation of tACS during cognitive performance in older adults and support recent studies suggesting that tACS in the theta frequency range may serve as a tool to enhance cognition, possibly through direct modulation of task-relevant brain oscillations. So far, studies have been heterogeneous in their designs, leaving a number of issues to be addressed in future research, including the setup of electrodes and optimal stimulation frequencies to be employed, as well as the interaction with age and underlying brain pathologies in specific patient populations.

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Transcranial Alternating Current Stimulation Enhances Individual Alpha Activity in Human EEG

PLoS ONE ◽

10.1371/journal.pone.0013766 ◽

2010 ◽

Vol 5 (11) ◽

pp. e13766 ◽

Cited By ~ 383

Author(s):

Tino Zaehle ◽

Stefan Rach ◽

Christoph S. Herrmann

Keyword(s):

Alternating Current ◽

Alpha Activity ◽

Human Eeg ◽

Transcranial Alternating Current Stimulation

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

10.1101/216176 ◽

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.

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Transcranial alternating current stimulation (tACS): from basic mechanisms towards first applications in psychiatry

European Archives of Psychiatry and Clinical Neuroscience ◽

10.1007/s00406-020-01209-9 ◽

2020 ◽

Cited By ~ 1

Author(s):

Osama Elyamany ◽

Gregor Leicht ◽

Christoph S. Herrmann ◽

Christoph Mulert

Keyword(s):

Psychiatric Disorders ◽

Cortical Neurons ◽

Alternating Current ◽

Brain Network ◽

Brain Oscillations ◽

Serious Adverse Events ◽

Double Blind ◽

Response Predictors ◽

Transcranial Alternating Current Stimulation ◽

The Brain

AbstractTranscranial alternating current stimulation (tACS) is a unique form of non-invasive brain stimulation. Sinusoidal alternating electric currents are delivered to the scalp to affect mostly cortical neurons. tACS is supposed to modulate brain function and, in turn, cognitive processes by entraining brain oscillations and inducing long-term synaptic plasticity. Therefore, tACS has been investigated in cognitive neuroscience, but only recently, it has been also introduced in psychiatric clinical trials. This review describes current concepts and first findings of applying tACS as a potential therapeutic tool in the field of psychiatry. The current understanding of its mechanisms of action is explained, bridging cellular neuronal activity and the brain network mechanism. Revisiting the relevance of altered brain oscillations found in six major psychiatric disorders, putative targets for the management of mental disorders using tACS are discussed. A systematic literature search on PubMed was conducted to report findings of the clinical studies applying tACS in patients with psychiatric conditions. In conclusion, the initial results may support the feasibility of tACS in clinical psychiatric populations without serious adverse events. Moreover, these results showed the ability of tACS to reset disturbed brain oscillations, and thus to improve behavioural outcomes. In addition to its potential therapeutic role, the reactivity of the brain circuits to tACS could serve as a possible tool to determine the diagnosis, classification or prognosis of psychiatric disorders. Future double-blind randomised controlled trials are necessary to answer currently unresolved questions. They may aim to detect response predictors and control for various confounding factors.

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The Benefits of Closed-Loop Transcranial Alternating Current Stimulation on Subjective Sleep Quality

Brain Sciences ◽

10.3390/brainsci8120204 ◽

2018 ◽

Vol 8 (12) ◽

pp. 204 ◽

Cited By ~ 6

Author(s):

Charles Robinson ◽

Natalie Bryant ◽

Joshua Maxwell ◽

Aaron Jones ◽

Bradley Robert ◽

...

Keyword(s):

Sleep Quality ◽

Slow Wave ◽

Closed Loop ◽

Slow Wave Sleep ◽

The United States ◽

Alternating Current ◽

Poor Sleep ◽

Subjective Sleep Quality ◽

Subjective Sleep ◽

Transcranial Alternating Current Stimulation

Background: Poor sleep quality is a common complaint, affecting over one third of people in the United States. While sleep quality is thought to be related to slow-wave sleep (SWS), there has been little investigation to address whether modulating slow-wave oscillations (SWOs) that characterize SWS could impact sleep quality. Here we examined whether closed-loop transcranial alternating current stimulation (CL-tACS) applied during sleep impacts sleep quality and efficiency. Methods: CL-tACS was used in 21 participants delivered at the same frequency and in phase with endogenous SWOs during sleep. Sleep quality was assessed in the morning following either verum or sham control stimulation during sleep, with order counterbalanced within participants. Results: Higher sleep quality and efficiency were found after verum stimulation nights compared to control. The largest effects on sleep quality were found immediately following an adaptation night in the laboratory for which sleep quality was reduced. Conclusions: Applying CL-tACS at the same frequency and phase as endogenous SWOs may offer a novel method to improve subjective sleep quality after a night with poor quality sleep. CL-tACS might be helpful for increasing sleep quality and efficiency in otherwise healthy people, and in patients with clinical disorders that involve sleep deficits.

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