Biology subjects

Herrmann, C. S.

Publications and source records attributed to Herrmann, C. S..

15 recordsLinked to original sources

bioRxiv2026

Neurofeedback enhances a neural signature of selective attention to speech in cocktail-party settings

Understanding speech in noisy situations is challenging and often fails due to attentional rather than sensory deficits. We report here that neurofeedback can enhance a neural signature of selective attention to speech in a cocktail-party setting. Neural responses to speech were quantified in participants electroencephalogram (EEG) while they attended to one of two audiobooks, presented simultaneously. After each 22s-segment of speech, the participants N1 component was extracted from the temporal response function (TRF). The N1 component is known to be sensitive to attention. During a [~]49-min training session, N1 amplitude values were displayed visually to participants so that they could learn to strengthen their neural responses to target speech and minimise their responses to distracting speech. During neurofeedback training, we found an enhanced N1 component in the response to target audiobooks that was specific to EEG channels used to provide feedback, and not present in a control group that received sham feedback. At right-lateralised fronto-central channels, enhanced N1 components correlated with improvements in a measure of speech comprehension (multiple-choice content questions). These results indicate that neural responses to speech can be regulated through neurofeedback and open up new possibilities to train attentional listening in populations struggling to understand speech in noise.

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bioRxiv2025

Carbonized rubber electrodes can cause a DC-offset in transcranial alternating current stimulation

IntroductionCarbonized rubber electrodes are widely used in non-invasive brain stimulation studies. Due to their polarizable nature, however, they can cause a voltage offset, which might be problematic for concurrent EEG studies. ObjectiveIn this study, we aim to describe the voltage offset and ensure that the offset does not alter the intended waveform of applied stimulation. MethodsUsing data from 2 human studies and phantom measurements, which employed carbonized rubber electrodes, we quantify the magnitude and frequency of DC-offsets and contrast this against pilot-measurements using Ag/AGCl-electrodes. In a further phantom study, we record the offset-voltage that arises from the electrode/electrolyte interface and compare this to the voltage put out by the stimulation device. ResultsA non-zero voltage offset is present in all human and phantom studies employing carbonized rubber electrodes, while the offset using Ag/AgCl electrodes is close to zero. Direct measurements of the stimulator output in the presence of a measurable voltage offset at the stimulation electrodes shows that the offset originates from the electrodes and not from the current provided by the stimulation device. ConclusionUsing carbonized rubber-electrodes for stimulation can result in the emergence of a measurable voltage offset, due to their polarizable nature. We argue that this offset can be problematic in concurrent EEG recordings, as they pose the risk of amplifier saturation and distortions of the recorded stimulation waveform.

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bioRxiv2023

Decomposing the effects of alpha-tACS on brain oscillations and aperiodic 1/f activity

BackgroundAftereffects of transcranial alternating current stimulation (tACS) are usually analyzed with a focus on the individual frequency band, thereby neglecting broadband spectral components. Recently, it was shown that the broadband spectrum, which exhibits a 1/f-like characteristic, is functionally relevant. Objective/HypothesisThe goal of this study was a spectral parameterization of brain activity into oscillatory alpha activity and aperiodic 1/f components before and after tACS and sham stimulation. It was expected that the broadband spectrum will not be differentially influenced by 20-min of tACS at individual -frequency (IAF) in comparison to sham. Additionally, it was expected that the tACS aftereffect on the -band can still be observed, even when controlling for 1/f activity differences. MethodsWe performed a re-analysis of a recently published resting-state tACS-magnetoencephalography (MEG) data set. Parameterization of the frequency spectrum was computed with the fitting-oscillations-and-one-over-F (FOOOF) algorithm. The intercept as well as the slope parameter of the aperiodic 1/f fit was extracted. Data was analyzed in sensor space with a focus on magnetometers. Comparison of changes in -band power and 1/f activity was performed with non-parametric cluster-based random permutation tests. ResultsThe tACS aftereffect survived the 1/f-correction. The previously observed natural rise in alpha oscillations over time independent of experimental conditions could not be replicated. However, differences in aperiodic parameters over time were observed. Especially, the intercept parameter increased from pre to post stimulation to a similar degree in both conditions. ConclusionIt is imperative to correct for the aperiodic 1/f spectral component when analyzing aftereffects of brain stimulation on brain oscillations.

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bioRxiv2023

Personalized alpha-tACS targeting left posterior parietal cortex modulates visuo-spatial attention and posterior evoked EEG activity

BackgroundCovert visuo-spatial attention is marked by the anticipatory lateralization of neuronal alpha activity in the posterior parietal cortex. Previous applications of transcranial alternating current stimulation (tACS) at the alpha frequency, however, were inconclusive regarding the causal contribution of oscillatory activity during visuo-spatial attention. ObjectiveAttentional shifts of behavior and electroencephalography (EEG) after-effects were assessed in a cued visuo-spatial attention paradigm. We hypothesized that parietal alpha-tACS facilitates attention in the ipsilateral visual hemifield. Furthermore, we assumed that modulations of behavior and neurophysiology are related to individual electric field simulations. MethodsWe applied personalized tACS at alpha and gamma frequencies to elucidate the role of oscillatory neuronal activity for visuo-spatial attention. Personalized tACS montages were algorithmically optimized to target individual left and right parietal regions that were defined by an EEG localizer. ResultsBehavioral performance in the left hemifield was specifically increased by alpha-tACS compared to gamma-tACS targeting the left parietal cortex. This hemisphere-specific effect was observed despite the symmetry of simulated electric fields. In addition, visual event-related potential (ERP) amplitudes showed a reduced lateralization over posterior sites induced by left alpha-tACS. Neuronal sources of this effect were localized in the left premotor cortex. Interestingly, accuracy modulations induced by left parietal alpha-tACS were directly related to electric field magnitudes in the left premotor cortex. ConclusionOverall, results corroborate the notion that alpha lateralization plays a causal role in covert visuo-spatial attention and indicate an increased susceptibility of parietal and premotor brain regions of the left dorsal attention network to subtle tACS-neuromodulation.

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bioRxiv2022

Localizing the Sources of Modulations of the P3m Component by Task Difficulty

1The P3m is a late component of the event-related field (ERF) and is assumed to reflect the allocation of resources in various cognitive domains. The amplitude of the P3m can be affected by several task parameters, like stimulus probability, inter-stimulus interval and task difficulty. The task difficulty can alter the P3m amplitude, depending on which mechanisms are involved. The sources of the P3m are located in several brain areas, suggesting a widespread network generating the P3m. Which of these sources of the P3m give rise to the task-difficulty induced amplitude modulation are not well investigated. However, localizing these sources could pave the way for intervention studies with non-invasive brain stimulation methods, which might be suitable to modulate the pathologically altered P3m emerging in various psychiatric and neurodevelopmental disorders. We designed a MEG-study with a visual oddball-like task with two different task difficulties to 1) show decreased P3m amplitudes induced by increased task difficulty and 2) estimate the sources of this P3m modulation. Additionally, the influence of the increased task difficulty on behavioral outcomes were analyzed. The P3m amplitude decreased significantly in the hard condition compared to easy condition. Furthermore, the hit rate for standard trials decreased significantly, while the reaction times increased for both, targets and standards. For this specific visual oddball-like task, the sources of the task difficulty dependent P3m amplitude modulation are located in the centro-parietal regions.

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bioRxiv2021

Electrical neuromodulation biases brightness perception of flickering light

BackgroundVisual phenomena like brightness illusions impressively demonstrate the highly constructive nature of perception. In addition to physical illumination, the subjective experience of brightness is related to temporal neural dynamics in visual cortex. ObjectiveHere, we asked whether biasing the temporal pattern of neural excitability in visual cortex by transcranial alternating current stimulation (tACS) modulates brightness perception of concurrent rhythmic visual stimuli. MethodsParticipants performed a brightness discrimination task of two flickering lights, one of which was targeted by same-frequency electrical stimulation at varying phase shifts. tACS was applied with an occipital and a periorbital active control montage, based on simulations of electrical currents using finite-element head models. ResultsExperimental results reveal that flicker brightness perception is modulated dependent on the phase shift between sensory and electrical stimulation, solely under occipital tACS. Phase-specific modulatory effects by tACS were dependent on flicker-evoked neural phase stability at the tACS-targeted frequency, recorded prior to electrical stimulation. The optimal timing of tACS application leading to enhanced brightness perception was further correlated with the neural phase delay of the cortical flicker response. ConclusionsOur results corroborate the role of temporally coordinated neural activity in visual cortex for brightness perception of rhythmic visual input in humans. Phase-specific behavioral modulations by tACS emphasize its efficacy to transfer perceptually relevant temporal information to the cortex. These findings provide an important step towards understanding the basis of visual perception and further confirm electrical stimulation as a tool for advancing controlled modulations of neural activity and related behavior.

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bioRxiv2020

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

Non-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.

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bioRxiv2020

No evidence for entrainment: endogenous gamma oscillations and responses to rhythmic visual stimulation coexist in visual cortex

Over the past decades, a plethora of studies have linked cortical gamma oscillations ([~]30-100 Hz) to neuro-computational mechanisms. Their functional relevance, however, is still passionately debated. Here, we asked if endogenous gamma oscillations in the human brain can be entrained by a rhythmic photic drive >50 Hz. A noninvasive modulation of endogenous brain rhythms allows conclusions about their causal involvement in neurocognition. To this end, we systematically investigated oscillatory responses to a rapid sinusoidal flicker in the absence and presence of endogenous gamma oscillations using magnetoencephalography (MEG) in combination with a high-frequency projector. The photic drive produced a robust response over visual cortex to stimulation frequencies of up to 80 Hz. Strong, endogenous gamma oscillations were induced using moving grating stimuli as repeatedly done in previous research. When superimposing the flicker and the gratings, there was no evidence for phase or frequency entrainment of the endogenous gamma oscillations by the photic drive. Unexpectedly, we did not observe an amplification of the flicker response around participants individual gamma frequencies; rather, the magnitude of the response decreased monotonically with increasing frequency. Source reconstruction suggests that the flicker response and the gamma oscillations were produced by separate, coexistent generators in visual cortex. The presented findings challenge the notion that cortical gamma oscillations can be entrained by rhythmic visual stimulation. Instead, the mechanism generating endogenous gamma oscillations seems to be resilient to external perturbation. Significance StatementWe aimed to investigate to what extent ongoing, high-frequency oscillations in the gamma band (30-100 Hz) in the human brain can be entrained by a visual flicker. Gamma oscillations have long been suggested to coordinate neuronal firing and enable inter-regional communication. Our results demonstrate that rhythmic visual stimulation cannot hijack the dynamics of ongoing gamma oscillations; rather, the flicker response and the endogenous gamma oscillations coexist in different visual areas. Therefore, while a visual flicker evokes a strong neuronal response even at high frequencies in the gamma-band, it does not entrain endogenous gamma oscillations in visual cortex. This has important implications for interpreting studies investigating the causal and neuroprotective effects of rhythmic sensory stimulation in the gamma band.

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bioRxiv2019

Signal-space projection suppresses the tACS artifact in EEG recordings

1.BackgroundTo probe the functional role of brain oscillations, transcranial alternating current stimulation (tACS) has proven to be a useful neuroscientific tool. Because of the huge tACS-caused artifact 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. New MethodIn 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 a human subject to validate the best-performing data-correction approach. ResultsThe 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 methodsThe best results were achieved with SSP, which outperformed sine subtraction and template subtraction. ConclusionsOur results demonstrate the feasibility of SSP by applying it to human tACS-EEG data.

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bioRxiv2019

Integrating electric field modelling and neuroimaging to explain variability of low intensity tES effects

Understanding variability of transcranial electrical stimulation (tES) effects is one of the major challenges in the brain stimulation community. Promising candidates to explain this variability are individual anatomy and the resulting differences of electric fields inside the brain. We integrated individual simulations of electric fields during tES with source-localization to predict variability of transcranial alternating current stimulation (tACS) aftereffects on -oscillations. In two experiments, participants received 20 minutes of either -tACS (1 mA) or sham stimulation. Magnetoencephalogram was recorded for 10 minutes before and after stimulation. tACS caused a larger power increase in the -band as compared to sham. The variability of this effect was significantly predicted by measures derived from individual electric field modelling. Our results directly link electric field variability to variability of tACS outcomes, stressing the importance of individualizing stimulation protocols and providing a novel approach to analyze tACS effects in terms of dose-response relationships.

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bioRxiv2018

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

Non-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.\n\nSignificanceTranscranial 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.

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bioRxiv2018

Non-linear transfer characteristics of stimulation and recording hardware account for spurious low-frequency artifacts during amplitude modulated transcranial alternating current stimulation (AM-tACS)

Amplitude modulated transcranial alternating current stimulation (AM-tACS) has been recently proposed as a possible solution to overcome the pronounced stimulation artifact encountered when recording brain activity during tACS. In theory, AM-tACS does not entail power at its modulating frequency, thus avoiding the problem of spectral overlap between brain signal of interest and stimulation artifact. However, the current study demonstrates how weak non-linear transfer characteristics inherent in stimulation and recording hardware can reintroduce spurious artifacts at the modulation frequency. The input-output transfer functions (TFs) of different stimulation setups were measured. The setups included basic recordings of signal-generator and stimulator outputs as well as M/EEG phantom measurements. 6th-degree polynomial regression models were fitted to model the input-output TFs of each setup. The resulting TF models were applied to digitally generated AM-tACS signals to predict the location of spurious artifacts in the spectrum. All four setups measured for the study exhibited low-frequency artifacts at the modulation frequency and its harmonics when recording AM-tACS. Fitted TF models showed non-linear contributions significantly different from zero (all p < .05) and successfully predicted the frequency of artifacts observed in AM-signal recordings. Results suggest that even weak non-linearities of stimulation and recording hardware can lead to spurious artifacts at the modulation frequency and its harmonics. Thus, findings emphasize the need for more linear stimulation devices for AM-tACS and careful analysis procedures, which take into account these low-frequency artifacts to avoid confusion with effects of AM-tACS on the brain.

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bioRxiv2017

Transcranial alternating current stimulation modulates auditory temporal resolution in elderly people

Recent research provides evidence for a functional role of brain oscillations for perception. For example, auditory temporal resolution seems to be linked to individual gamma frequency of auditory cortex. Individual gamma frequency not only correlates with performance in between-channel gap detection tasks but can be modulated via auditory transcranial alternating current stimulation. Modulation of individual gamma frequency is accompanied by an improvement in gap detection performance. Aging changes electrophysiological frequency components and sensory processing mechanisms. Therefore, we conducted a study to investigate the link between individual gamma frequency and gap detection performance in elderly people using auditory transcranial alternating current stimulation. In a within-subject design, nine participants were electrically stimulated with two individualized transcranial alternating current stimulation frequencies: 3 Hz above their individual gamma frequency (experimental condition) and 4 Hz below their individual gamma frequency (control condition) while they were performing a between-channel gap detection task. As expected, individual gamma frequencies correlated significantly with gap detection performance at baseline and in the experimental condition, transcranial alternating current stimulation modulated gap detection performance. In the control condition, stimulation did not modulate gap detection performance. In addition, in elderly, the effect of transcranial alternating current stimulation on auditory temporal resolution seems to be dependent on endogenous frequencies in auditory cortex: elderlies with slower individual gamma frequencies and lower auditory temporal resolution profit from auditory transcranial alternating current stimulation and show increased gap detection performance during stimulation. Our results strongly suggest individualized transcranial alternating current stimulation protocols for successful modulation of performance.

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bioRxiv2017

Temporal expectation modulates cortical dynamics of sensory memory

Increased memory load is often signified by enhanced neural oscillatory power in the alpha range (8-13 Hz), taken to reflect inhibition of task-irrelevant brain regions. The corresponding neural correlates of memory decay, however, are not yet well-understood. Here, we investigated auditory sensory memory decay using a delayed matching-to-sample task with pure-tone sequences. First, in a behavioral experiment we modeled memory behavior over six different delay-phase durations. Second, in a magnetoencephalography (MEG) experiment, we assessed alpha-power modulations over three different delay-phase durations. In both experiments, the temporal expectation for the to-be-remembered sound was manipulated, so that it was either temporally expected or not. In both studies, memory performance declined over time but this decline was less strong under a more precise temporal expectation. Similarly, patterns of alpha power in and alpha-tuned connectivity between sensory cortices changed parametrically with delay duration (i.e., decrease in occipito-parietal regions, increase in temporal regions). Notably, temporal expectation counteracted alpha-power decline in heteromodal brain areas (i.e., supramarginal gyrus), in line with its memory-decay counteracting effect on performance. Correspondingly, temporal expectation also boosted alpha connectivity within attention networks known to play an active role during memory maintenance. The present data outline how patterns of alpha power orchestrate sensory memory decay, and encourage a refined perspective on alpha power and its inhibitory role across brain space and time.\n\nSignificance StatementOur sensory memories of the physical world fade quickly. We show here that this decay of sensory memory can be counteracted by so-called temporal expectation, that is, knowledge of when to expect the to-be-remembered sensory event (here, brief sound patterns). We also show that distinct patterns and modulations of neural oscillations in the \"alpha\" (8-13 Hz) range index both, the degree of memory decay, and any benefit from temporal expectation, both of which affect memory performance. Critically, spatially distributed cortical patterns of alpha power, with opposing effects in auditory vs. visual sensory cortices and alpha-tuned connectivity changes within supramodal attention networks, reflect the allocation of neural resources as sensory memory representations fade.

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bioRxiv2017

Envelope-tACS modulates intelligibility of speech in noise

Cortical entrainment of the auditory cortex to the broad-band temporal envelope of a speech signal is crucial for speech comprehension. This entrainment results in phases of high and low neural excitability which structure and decode the incoming speech signal. Entrainment to speech is strongest in the theta frequency range (4-8 Hz), the average frequency of the speech envelope. If a speech signal is degraded, for example masked by irrelevant information such as noise, entrainment to the speech envelope is weaker and speech intelligibility declines.\n\nBesides perceptually evoked cortical entrainment, transcranial alternating current stimulation (tACS) can entrain neural oscillations by applying an electric signal to the brain. Accordingly, tACS-induced entrainment in auditory cortex has been shown to improve auditory perception. The aim of the current study was to externally modulate speech intelligibility by means of tACS such that the electric current corresponds to the envelope of the presented speech stream.\n\nParticipants performed the Oldenburg sentence test with sentences presented in noise in combination with tACS. Critically, the time lag between sentence presentation and tACS was manipulated from 0 to 250 ms in 50-ms steps (auditory stimuli were simultaneous to or preceded tACS).\n\nFirst, we were able to show that envelope-tACS modulated sentence comprehension such that on average sentence comprehension at the time lag of the best performance was significantly better than sentence comprehension of the worst performance. Second, sentence comprehension across time lags was modulated sinusoidally.\n\nIn sum, envelope tACS modulates intelligibility of speech in noise presumably by enhancing (time lag with in-phase stimulation) and disrupting (time lag with out-of-phase stimulation) cortical entrainment to the speech envelope in auditory cortex.

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