Query: tacs     EEG

title: Artifact Removal in tACS EEG Recordings: A Combined Methodology Based on the Empirical Wavelet Transform*
abstract: Transcranial alternating current stimulation (tACS) is a non invasive brain stimulation technique that modulates brain activity, which yields promise for achieving desired behavioral outcomes in different contexts. Combining tACS with electroencephalography (EEG) allows for the monitoring of the real time effects of stimulation. However, the EEG signal recorded with simultaneous tACS is largely contaminated by stimulation induced artifacts. In this work, we examine the combination of the empirical wavelet transform (EWT) with three blind source separation (BSS) methods: principal component analysis (PCA) multiset canonical correlation analysis (MCCA) and independent vector analysis (IVA) aiming to remove artifacts in tACS contaminated EEG recordings. Using simulated data, we show that EWT followed by IVA achieves the best performance. Using experimental data, we show that BSS combined with EWT performs better compared to standard BSS methodology in terms of preserving useful information while eliminating artifacts.

title: P172 Inter areal phase synchrony modulates motion discrimination: A tACS EEG study

title: Signal Space Projection Suppresses the tACS Artifact in EEG Recordings
abstract: To probe the functional role of brain oscillations, transcranial alternating current stimulation (tACS) has proven to be a useful neuroscientific tool. Because of the excessive tACS caused artifact at the stimulation frequency in electroencephalography (EEG) signals, tACS EEG studies have been mostly limited to compare brain activity between recordings before and after concurrent tACS. Critically, attempts to suppress the artifact in the data cannot assure that the entire artifact is removed while brain activity is preserved. The current study aims to evaluate the feasibility of specific artifact correction techniques to clean tACS contaminated EEG data.  In the first experiment, we used a phantom head to have full control over the signal to be analyzed. Driving pre recorded human brain oscillation signals through a dipolar current source within the phantom, we simultaneously applied tACS and compared the performance of different artifact correction techniques: sine subtraction, template subtraction, and signal space projection (SSP) In the second experiment, we combined tACS and EEG on one human subject to demonstrate the best performing data correction approach in a proof of principle.  The tACS artifact was highly attenuated by SSP in the phantom and the human EEG; thus, we were able to recover the amplitude and phase of the oscillatory activity. In the human experiment, event related desynchronization could be restored after correcting the artifact. Comparison With Existing  The best results were achieved with SSP, which outperformed sine subtraction and template subtraction.  Our results demonstrate the feasibility of SSP by applying it to a phantom measurement with pre recorded signal and one human tACS EEG dataset. For a full validation of SSP, more data are needed.

title: EEG+tACS signals for artefact removal

title: A need for time varying models to suppress artefacts of tACS in the M/EEG
abstract: Rhythmic modulation of brain activity by transcranial alternating current stimulation (tACS) can entrain neural oscillations in a frequency and phase specific manner. However, large stimulation artefacts contaminate concurrent 'online' neuroimaging measures, including magneto and electro encephalography (M/EEG) restricting most analyses to periods free from stimulation 'offline' aftereffects) While many published methods exist for removing artefacts of tACS from M/EEG recordings, they universally assume linear artefacts: either time invariance (i.e. an artefact is a scaled version of itself from cycle to cycle) or sensor invariance (i.e. artefacts are scaled versions of one another from sensor to sensor) However, heartbeat and respiration both nonlinearly modulate the amplitude and phase of these artefacts, predominantly via changes in scalp impedance. The spectral symmetry this introduces to the M/EEG spectra may lead to false positive evidence for entrainment around the frequency of tACS, if not adequately suppressed. Good electrophysiological evidence for entrainment therefore requires that tACS artefacts are fully accounted for before comparing online spectra to a control (e.g. as might be observed during sham stimulation) Here I outline an approach to linearly solve templates for tACS artefacts, and demonstrate how event locked perturbations to amplitude and phase can be introduced from simultaneous recordings of heartbeat and respiration effectively forming time varying models of tACS artefacts. These models are constructed for individual sensors, and can therefore be used in contexts with few EEG sensors and with no assumption of artefact collinearity. I also discuss the feasibility of this approach in the absence of simultaneous recordings of heartbeat and respiration traces.

title: Investigating Nuisance Effects Induced in EEG During tACS Application
abstract: Transcranial alternating current stimulation (tACS) in the frequency range of 1 100 Hz has come to be used routinely in electroencephalogram (EEG) studies of brain function through entrainment of neuronal oscillations. It turned out, however, to be highly non trivial to remove the strong stimulation signal, including its harmonic and non harmonic distortions, as well as various induced higher order artifacts from the EEG data recorded during the stimulation. In this paper, we discuss some of the problems encountered and present methodological approaches aimed at overcoming them. To illustrate the mechanisms of artifact induction and the proposed removal strategies, we use data obtained with the help of a schematic demonstrator setup as well as human subject data.

title: A set of electroencephalographic (EEG) data recorded during amplitude modulated transcranial alternating current stimulation (AM tACS) targeting 10 Hz steady state visually evoked potentials (SSVEP)
abstract: Transcranial alternating current stimulation (tACS) can affect perception, learning and cognition, but the underlying mechanisms are not well understood. A promising strategy to elucidate these mechanisms aims at applying tACS while electric or magnetic brain oscillations targeted by stimulation are recorded. However, reconstructing brain oscillations targeted by tACS remains a challenging problem due to stimulation artifacts. Besides lack of an established strategy to effectively supress such stimulation artifacts, there are also no resources available that allow for the development and testing of new and effective tACS artefact suppression algorithms, such as adaptive spatial filtering using beamforming or signal space projection. Here, we provide a full dataset comprising encephalographic (EEG) recordings across six healthy human volunteers who underwent 10 Hz amplitude modulated tACS (AM tACS) during a 10 Hz steady state visually evoked potential (SSVEP) paradigm. Moreover, data and scripts are provided related to the validation of a novel stimulation artefact suppression strategy, Stimulation Artifact Source Separation (SASS) removing EEG signal components that are maximally different in the presence versus absence of stimulation. Besides including EEG single trial data and comparisons of 10 Hz brain oscillatory phase and amplitude recorded across three conditions (condition 1: no stimulation, condition 2: stimulation with SASS, condition 3: stimulation without SASS) also power spectra and topographies of SSVEP amplitudes across all three conditions are presented. Moreover, data is provided for assessing nonlinear modulations of the stimulation artifact in both time and frequency domains due to heartbeats. Finally, the dataset includes eigenvalue spectra and spatial patterns of signal components that were identified and removed by SASS for stimulation artefact suppression at the target frequency. Besides providing a valuable resource to assess properties of AM tACS artifacts in the EEG, this dataset allows for testing different artifact rejection methods and offers in depth insights into the workings of SASS.

title: Parietal tACS at beta frequency improves vision in a crowding regime
abstract: Visual crowding is the inability to discriminate objects when presented with nearby flankers and sets a fundamental limit for conscious perception. Beta oscillations in the parietal cortex were found to be associated to crowding, with higher beta amplitude related to better crowding resilience. An open question is whether beta activity directly and selectively modulates crowding. We employed transcranial alternating current stimulation (tACS) in the beta band (18 Hz) in the alpha band (10 Hz) or in a sham regime, asking whether 18 Hz tACS would selectively improve the perception of crowded stimuli by increasing parietal beta activity. Resting electroencephalography (EEG) was measured before and after stimulation to test the influence of tACS on endogenous oscillations. Consistently with our predictions, we found that 18 Hz tACS, as compared to 10 Hz tACS and sham stimulation, reduced crowding. This improvement was found specifically in the contralateral visual hemifield and was accompanied by an increased amplitude of EEG beta oscillations, confirming an effect on endogenous brain rhythms. These results support a causal relationship between parietal beta oscillations and visual crowding and provide new insights into the precise oscillatory mechanisms involved in human vision.

title: Transcranial Alternating Current Stimulation (tACS) as a Tool to Modulate P300 Amplitude in Attention Deficit Hyperactivity Disorder (ADHD) Preliminary Findings
abstract: Studies examining event related potentials (ERP) in patients affected by attention deficit/hyperactivity disorder (ADHD) have found considerable evidence of reduced target P300 amplitude across different perceptual modalities. P300 amplitude has been related to attention driven context comparison and resource allocation processes. Altered P300 amplitude in ADHD can be reasonably assumed to be related to ADHD typical cognitive performance deficits. Transcranial alternating current stimulation (tACS) can increase the amplitude of endogenous brain oscillations Because ERP components can be viewed as event related oscillations (EROs) with P300 translating into the delta (0 4 Hz) and theta (4 8 Hz) frequency range, an increase of delta and theta ERO amplitudes by tACS should result in an increase of P300 amplitudes in ADHD patients. In this pilot study, 18 adult ADHD patients (7 female) performed three consecutive blocks of a visual oddball task while the electroencephalogram (EEG) was recorded. Patients received either 20 min of tACS or sham stimulation at a stimulation intensity of 1 mA. Individual stimulation frequency was determined using a time frequency decomposition of the P300. Our preliminary results demonstrate a significant increase in P300 amplitude in the stimulation group which was accompanied by a decrease in omission errors pre to post tACS. However, studies including larger sample sizes are advised.

title: Transient amplitude modulation of alpha band oscillations by short time intermittent closed loop tACS
abstract: Non 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.