Transcranial temporal interference stimulation (tTIS) influences event-related alpha activity during mental rotation.

Non-invasive brain stimulation techniques offer therapeutic potential for neurological and psychiatric disorders. However, current methods are often limited in their stimulation depth. The novel transcranial temporal interference stimulation (tTIS) aims to overcome this limitation by non-invasively targeting deeper brain regions. In this study, we aimed to evaluate the efficacy of tTIS in modulating alpha activity during a mental rotation task. The effects of tTIS were compared with transcranial alternating current stimulation (tACS) and a sham control. Participants were randomly assigned to a tTIS, tACS, or sham group. They performed alternating blocks of resting and mental rotation tasks before, during, and after stimulation. During the stimulation blocks, participants received 20 min of stimulation adjusted to their individual alpha frequency (IAF). We assessed shifts in resting state alpha power, event-related desynchronization (ERD) of alpha activity during mental rotation, as well as resulting improvements in behavioral performance. Our results indicate tTIS and tACS to be effective in modulating cortical alpha activity during mental rotation, leading to an increase in ERD from pre- to poststimulation as well as compared to sham stimulation. However, this increase in ERD was not correlated with enhanced mental rotation performance, and resting state alpha power remained unchanged. Our findings underscore the complex nature of tTIS and tACS efficacy, indicating that stimulation effects are more observable during active cognitive tasks, while their impacts are less pronounced on resting neuronal systems.

Pre-stimulus alpha-band power and phase fluctuations originate from different neural sources and exert distinct impact on stimulus-evoked responses.

Ongoing oscillatory neural activity before stimulus onset influences subsequent visual perception. Specifically, both the power and the phase of oscillations in the alpha-frequency band (9-13 Hz) have been reported to predict the detection of visual stimuli. Up to now, the functional mechanisms underlying pre-stimulus power and phase effects on upcoming visual percepts are debated. Here, we used magnetoencephalography recordings together with a near-threshold visual detection task to investigate the neural generators of pre-stimulus power and phase and their impact on subsequent visual-evoked responses. Pre-stimulus alpha-band power and phase opposition effects were found consistent with previous reports. Source localization suggested clearly distinct neural generators for these pre-stimulus effects: Power effects were mainly found in occipital-temporal regions, whereas phase effects also involved prefrontal areas. In order to be functionally relevant, the pre-stimulus correlates should influence post-stimulus processing. Using a trial-sorting approach, we observed that only pre-stimulus power modulated the Hits versus Misses difference in the evoked response, a well-established post-stimulus neural correlate of near-threshold perception, such that trials with stronger pre-stimulus power effect showed greater post-stimulus difference. By contrast, no influence of pre-stimulus phase effects were found. In sum, our study shows distinct generators for two pre-stimulus neural patterns predicting visual perception, and that only alpha power impacts the post-stimulus correlate of conscious access. This underlines the functional relevance of prestimulus alpha power on perceptual awareness, while questioning the role of alpha phase.

The Role of Working Memory in the Probabilistic Inference of Future Sensory Events.

The ability to represent the emerging regularity of sensory information from the external environment has been thought to allow one to probabilistically infer future sensory occurrences and thus optimize behavior. However, the underlying neural implementation of this process is still not comprehensively understood. Through a convergence of behavioral and neurophysiological evidence, we establish that the probabilistic inference of future events is critically linked to people's ability to maintain the recent past in working memory. Magnetoencephalography recordings demonstrated that when visual stimuli occurring over an extended time series had a greater statistical regularity, individuals with higher working-memory capacity (WMC) displayed enhanced slow-wave neural oscillations in the θ frequency band (4-8 Hz.) prior to, but not during stimulus appearance. This prestimulus neural activity was specifically linked to contexts where information could be anticipated and influenced the preferential sensory processing for this visual information after its appearance. A separate behavioral study demonstrated that this process intrinsically emerges during continuous perception and underpins a realistic advantage for efficient behavioral responses. In this way, WMC optimizes the anticipation of higher level semantic concepts expected to occur in the near future.

Faith and oscillations recovered: On analyzing EEG/MEG signals during tACS.

Despite recent success in analyzing brain oscillations recorded during transcranial alternating current stimulation (tACS), the field still requires further research to establish standards in artifact removal methods. This includes taking a step back from the removal of the tACS artifact and thoroughly characterizing the to-be-removed artifact. A recent study by Noury et al. (2016) contributed importantly to this endeavour by showing the existence of nonlinear artefacts in the tACS signal as seen by MEG and EEG. Unfortunately however this paper conveys the message that current artifact removal attempts have failed altogether and that-based on these available tools-brain oscillations recorded during tACS cannot be analyzed using MEG and EEG. Here we want to balance this overly pessimistic conclusion: In-depth reanalyses of our own data and phantom-head measurements indicate that nonlinearities can occur, but only when technical limits of the stimulator are reached. As such they are part of the "real" stimulation and not a specific MEG analysis problem. Future tACS studies should consider these technical limits to avoid any nonlinear modulations of the tACS artifact. We conclude that even with current approaches, brain oscillations recorded during tACS can be meaningfully studied in many practical cases.

Implicit expectations influence target detection in children and adults.

When a sound occurs at a predictable time, it gets processed more efficiently. Predictability of the temporal structure of acoustic inflow has been found to influence the P3b of event-related potentials in young adults, such that highly predictable compared to less predictable input leads to earlier P3b peak latencies. In our study, we wanted to investigate the influence of predictability on target processing indexed by the P3b in children (10-12 years old) and young adults. To do that, we used an oddball paradigm with two conditions of predictability (high and low). In the High-predictability condition, a high-pitched target tone occurred most of the time in the fifth position of a five-tone pattern (after four low-pitched non-target sounds), whereas in the Low-predictability condition, no such rule was implemented. The target tone occurred randomly following 2, 3, 4, 5, or 6 non-target tones. In both age groups, reaction time to predictable targets was faster than to non-predictable targets. Remarkably, this effect was largest in children. Consistent with the behavioral responses, the onset latency of the P3b response elicited by targets in both groups was earlier in the predictable than the unpredictable conditions. However, only the children had significantly earlier peak latency responses for predictable targets. Our results demonstrate that target stimulus predictability increases processing speed in children and adults even when predictability was only implicitly derived by the stimulus statistics. Children did have larger effects of predictability, seeming to benefit more from predictability for target detection.

Alpha suppression and connectivity modulations in left temporal and parietal cortices index partial awareness of words.

The partial awareness hypothesis is a theoretical proposal that recently provided a reconciling solution to graded and dichotomous accounts of consciousness. It suggests that we can become conscious of distinct properties of an object independently, ranging from low-level features to complex forms of representation. We investigated this hypothesis using classic visual word masking adapted to a near-threshold paradigm. The masking intensity was adjusted to the individual perception threshold, at which individual alphabetical letters, but not words, could be perceived in approximately half of the trials. We confined perception to a pre-lexical stage of word processing that corresponded to a clear condition of partial awareness. At this level of representation, the stimulus properties began to emerge within consciousness, yet they did not escalate to full stimulus awareness. In other words, participants were able to perceive individual letters, while remaining unaware of the whole letter strings presented. Cortical activity measured with MEG was compared between physically identical trials that differed in perception (perceived, not perceived). We found that compared to no awareness, partial awareness of words was characterized by suppression of oscillatory alpha power in left temporal and parietal cortices. The analysis of functional connectivity with seeds based on the power effect in these two regions revealed sparse connections for the parietal seed, and strong connections between the temporal seed and other regions of the language network. We suggest that the engagement of language regions indexed by alpha power suppression is responsible for establishing and maintaining conscious representations of individual pre-lexical units.

Prestimulus Network Integration of Auditory Cortex Predisposes Near-Threshold Perception Independently of Local Excitability.

An ever-increasing number of studies are pointing to the importance of network properties of the brain for understanding behavior such as conscious perception. However, with regards to the influence of prestimulus brain states on perception, this network perspective has rarely been taken. Our recent framework predicts that brain regions crucial for a conscious percept are coupled prior to stimulus arrival, forming pre-established pathways of information flow and influencing perceptual awareness. Using magnetoencephalography (MEG) and graph theoretical measures, we investigated auditory conscious perception in a near-threshold (NT) task and found strong support for this framework. Relevant auditory regions showed an increased prestimulus interhemispheric connectivity. The left auditory cortex was characterized by a hub-like behavior and an enhanced integration into the brain functional network prior to perceptual awareness. Right auditory regions were decoupled from non-auditory regions, presumably forming an integrated information processing unit with the left auditory cortex. In addition, we show for the first time for the auditory modality that local excitability, measured by decreased alpha power in the auditory cortex, increases prior to conscious percepts. Importantly, we were able to show that connectivity states seem to be largely independent from local excitability states in the context of a NT paradigm.

Friends, not foes: Magnetoencephalography as a tool to uncover brain dynamics during transcranial alternating current stimulation.

Brain oscillations are supposedly crucial for normal cognitive functioning and alterations are associated with cognitive dysfunctions. To demonstrate their causal role on behavior, entrainment approaches in particular aim at driving endogenous oscillations via rhythmic stimulation. Within this context, transcranial electrical stimulation, especially transcranial alternating current stimulation (tACS), has received renewed attention. This is likely due to the possibility of defining oscillatory stimulation properties precisely. Also, measurements comparing pre-tACS with post-tACS electroencephalography (EEG) have shown impressive modulations. However, the period during tACS has remained a blackbox until now, due to the enormous stimulation artifact. By means of application of beamforming to magnetoencephalography (MEG) data, we successfully recovered modulations of the amplitude of brain oscillations during weak and strong tACS. Additionally, we demonstrate that also evoked responses to visual and auditory stimuli can be recovered during tACS. The main contribution of the present study is to provide critical evidence that during ongoing tACS, subtle modulations of oscillatory brain activity can be reconstructed even at the stimulation frequency. Future tACS experiments will be able to deliver direct physiological insights in order to further the understanding of the contribution of brain oscillations to cognition and behavior.

Involuntary shifts of spatial attention contribute to distraction-Evidence from oscillatory alpha power and reaction time data.

Imagine you are focusing on the traffic on a busy street to ride your bike safely when suddenly you hear the siren of an ambulance. This unexpected sound involuntarily captures your attention and interferes with ongoing performance. We tested whether this type of distraction involves a spatial shift of attention. We measured behavioral data and magnetoencephalographic alpha power during a cross-modal paradigm that combined an exogenous cueing task and a distraction task. In each trial, a task-irrelevant sound preceded a visual target (left or right). The sound was usually the same animal sound (i.e., standard sound). Rarely, it was replaced by an unexpected environmental sound (i.e., deviant sound). Fifty percent of the deviants occurred on the same side as the target, and 50% occurred on the opposite side. Participants responded to the location of the target. As expected, responses were slower to targets that followed a deviant compared to a standard. Crucially, this distraction effect was mitigated by the spatial relationship between the targets and the deviants: responses were faster when targets followed deviants on the same versus different side, indexing a spatial shift of attention. This was further corroborated by a posterior alpha power modulation that was higher in the hemisphere ipsilateral (vs. contralateral) to the location of the attention-capturing deviant. We suggest that this alpha power lateralization reflects a spatial attention bias. Overall, our data support the contention that spatial shifts of attention contribute to deviant distraction.

Maturation of obligatory auditory responses and their neural sources: evidence from EEG and MEG.

Neural auditory responses are known to change from childhood to adulthood. The most prominent components of the event-related potentials (ERPs) found in children are the P1 and N2, while the P1 and N1 are strongest in adults. Previous dipole localizations showed regions of the auditory cortex (AC) underlying these responses. An N1 in children, however, has only been observed in older age or under certain experimental conditions different than commonly applied in adults. The current study aimed to further elucidate on auditory processing and related components in school-aged children. To do this, MEG and EEG was recorded in adults and 9 to 10year old children, while presenting pure tones either repetitively or randomly among tones of different pitch. Furthermore, the current paradigm was explicitly designed to not only investigate the P1 and N2 in children, but moreover to examine N1 modulations based on different refractory states caused by the two conditions. Our results are clear cut. In adults, P1(m) and N1(m) components were localized in AC regions, with the N1(m) largely attenuated for repetitive tones. The P1(m) and N2(m) components observed in children were also localized in AC regions. Most importantly, ERP modulations in the N1 time window (i.e., larger responses for random than repetitive tones) were remarkably similar for adults and children, both in amplitude and latency. This effect indicates that the N1 sub-component reflecting frequency-specific refractoriness is fully developed in 9 to 10year old children. Thus, previous interpretations on the function and maturation of the N1 need reconsideration.

Transcranial Auricular Vagus Nerve Stimulation (taVNS) and Ear-EEG: Potential for Closed-Loop Portable Non-invasive Brain Stimulation.

No matter how hard we concentrate, our attention fluctuates - a fact that greatly affects our success in completing a current task. Here, we review work from two methods that, in a closed-loop manner, have the potential to ameliorate these fluctuations. Ear-EEG can measure electric brain activity from areas in or around the ear, using small and thus portable hardware. It has been shown to capture the state of attention with high temporal resolution. Transcutaneous auricular vagus nerve stimulation (taVNS) comes with the same advantages (small and light) and critically current research suggests that it is possible to influence ongoing brain activity that has been linked to attention. Following the review of current work on ear-EEG and taVNS we suggest that a combination of the two methods in a closed-loop system could serve as a potential application to modulate attention.

Amplitude modulated transcranial alternating current stimulation (AM-TACS) efficacy evaluation via phosphene induction.

Amplitude modulated transcranial alternating current stimulation (AM-tACS) is a novel method of electrostimulation which enables the recording of electrophysiological signals during stimulation, thanks to an easier removable stimulation artefact compared to classical electrostimulation methods. To gauge the neuromodulatory potential of AM-tACS, we tested its capacity to induce phosphenes as an indicator of stimulation efficacy. AM-tACS was applied via a two-electrode setup, attached on FpZ and below the right eye. AM-tACS waveforms comprised of different carrier (50 Hz, 200 Hz, 1000 Hz) and modulation frequencies (8 Hz, 16 Hz, 28 Hz) were administered with at maximum 2 mA peak-to-peak stimulation strength. TACS conditions in the same frequencies were used as a benchmark for phosphene induction. AM-tACS conditions using a 50 Hz carrier frequency were able to induce phosphenes, but with no difference in phosphene thresholds between modulation frequencies. AM-tACS using a 200 Hz or 1000 Hz carrier frequency did not induce phosphenes. TACS conditions induced phosphenes in line with previous studies. Stimulation effects of AM-tACS conditions were independent of amplitude modulation and instead relied solely on the carrier frequency. A possible explanation may be that AM-tACS needs higher stimulation intensities for its amplitude modulation to have a neuromodulatory effect.

Effects of transcutaneous vagus nerve stimulation (tVNS) on beta and gamma brain oscillations.

Physiological and behavioral effects induced through transcutaneous vagus nerve stimulation (tVNS) are under scrutiny in a growing number of studies, yet its mechanisms of action remain poorly understood. One candidate mechanism is a modulation of γ-aminobutyric acid (GABA) transmission through tVNS. Two recent behavioral studies suggest that such a GABAergic effect might occur in a lateralized fashion, i.e., the GABA modulation might be stronger in the left than in the right brain hemisphere after tVNS applied to the left ear. Using magnetoencephalography (MEG), we tested for GABA-associated modulations in resting and event-related brain oscillations and for a lateralization of those effects in a sample of 41 healthy young adults. Our data provide substantial evidence against all hypotheses, i.e., we neither find effects of tVNS on oscillatory power nor a lateralization of effects.

The modulation of auditory novelty processing by working memory load in school age children and adults: a combined behavioral and event-related potential study.

BACKGROUND: We investigated the processing of task-irrelevant and unexpected novel sounds and its modulation by working-memory load in children aged 9-10 and in adults. Environmental sounds (novels) were embedded amongst frequently presented standard sounds in an auditory-visual distraction paradigm. Each sound was followed by a visual target. In two conditions, participants evaluated the position of a visual stimulus (0-back, low load) or compared the position of the current stimulus with the one two trials before (2-back, high load). Processing of novel sounds were measured with reaction times, hit rates and the auditory event-related brain potentials (ERPs) Mismatch Negativity (MMN), P3a, Reorienting Negativity (RON) and visual P3b. RESULTS: In both memory load conditions novels impaired task performance in adults whereas they improved performance in children. Auditory ERPs reflect age-related differences in the time-window of the MMN as children showed a positive ERP deflection to novels whereas adults lack an MMN. The attention switch towards the task irrelevant novel (reflected by P3a) was comparable between the age groups. Adults showed more efficient reallocation of attention (reflected by RON) under load condition than children. Finally, the P3b elicited by the visual target stimuli was reduced in both age groups when the preceding sound was a novel. CONCLUSION: Our results give new insights in the development of novelty processing as they (1) reveal that task-irrelevant novel sounds can result in contrary effects on the performance in a visual primary task in children and adults, (2) show a positive ERP deflection to novels rather than an MMN in children, and (3) reveal effects of auditory novels on visual target processing.

Pulsed transcranial electric brain stimulation enhances speech comprehension.

BACKGROUND: One key mechanism thought to underlie speech processing is the alignment of cortical brain rhythms to the acoustic input, a mechanism termed entrainment. Recent work showed that transcranial electrical stimulation (tES) in speech relevant frequencies or adapted to the speech envelope can in fact enhance speech processing. However, it is unclear whether an oscillatory tES is necessary, or if transients in the stimulation (e.g., peaks in the tES signal) at relevant times are sufficient. OBJECTIVE: In this study we used a novel pulsed-tES-protocol and tested behaviorally if a transiently pulsed - instead of a persistently oscillating - tES signal, can improve speech processing. METHODS: While subjects listened to spoken sentences embedded in noise, brief electric direct current pulses aligned to speech transients (syllable onsets) were applied to auditory cortex regions to modulate comprehension. Additionally, we modulated the temporal delay between tES-pulses and speech transients to test for periodic modulations of behavior, indicative of entrainment by tES. RESULTS: Speech comprehension was improved when tES-pulses were applied with a delay of 100 ms in respect to the speech transients. Contradictory to previous reports we find no periodic modulation of behavior. However, we find indications that periodic modulations can be spurious results of sampling behavioral data too coarsely. CONCLUSIONS: Subject's speech comprehension benefits from pulsed-tES, yet behavior is not modulated periodically. Thus, pulsed-tES can aid cortical entrainment to speech input, which is especially relevant in a noisy environment. Yet, pulsed-tES does not seem to entrain brain oscillations by itself.

Inconsistent effects of stochastic resonance on human auditory processing.

It has been demonstrated that, while otherwise detrimental, noise can improve sensory perception under optimal conditions. The mechanism underlying this improvement is stochastic resonance. An inverted U-shaped relationship between noise level and task performance is considered as the signature of stochastic resonance. Previous studies have proposed the existence of stochastic resonance also in the human auditory system. However, the reported beneficial effects of noise are small, based on a small sample, and do not confirm the proposed inverted U-shaped function. Here, we investigated in two separate studies whether stochastic resonance may be present in the human auditory system by applying noise of different levels, either acoustically or electrically via transcranial random noise stimulation, while participants had to detect acoustic stimuli adjusted to their individual hearing threshold. We find no evidence for behaviorally relevant effects of stochastic resonance. Although detection rate for near-threshold acoustic stimuli appears to vary in an inverted U-shaped manner for some subjects, it varies in a U-shaped manner or in other manners for other subjects. Our results show that subjects do not benefit from noise, irrespective of its modality. In conclusion, our results question the existence of stochastic resonance in the human auditory system.

Transcutaneous Vagus Nerve Stimulation (tVNS) and the Dynamics of Visual Bistable Perception.

Transcutaneous vagus nerve stimulation (tVNS) is widely used for clinical applications, but its mechanism of action is poorly understood. One candidate pathway that might mediate the effects of tVNS is an increase in GABAergic neurotransmission. In this study, we investigated the effect of tVNS on visual bistable perception, which is highly coupled to GABA. Participants were 34 healthy young subjects. We used a static (Necker cube) and a dynamic (structure from motion) bistable perception task. Each subject underwent tVNS as well as sham (placebo) stimulation for ∼45 min. We analyze effects of tVNS on percept durations by means of Bayesian multilevel regression. We find no evidence for a modulation of bistable perception dynamics through tVNS in either task, but the analyses do not ultimately confirm the null hypothesis either. We discuss different possible implications of our finding and propose that GABAergic effects of tVNS should be further investigated using more direct measures of GABA concentration, and, more generally, that a better understanding of the mechanisms of action of vagus nerve stimulation is needed. Finally, we discuss limitations of our study design, data analysis, and conclusions.

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

Transcranial 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, effectiveness of tACS underlies high variability and dependencies, which, as previous modeling works have suggested, may be mediated by local and network-level brain states. We used magnetoencephalography to record brain activity from 17 healthy participants at rest as they kept their eyes open (EO) or eyes closed (EC) while being stimulated 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 brain activity revealed an interaction of the external stimulation with the endogenous alpha power increase from EO to EC. This interaction was localized to the posterior cingulate, a region remote from occipital cortex. This suggests state-dependent (EO vs. EC) long-range effects of tACS. In a follow-up analysis of this online-tACS effect, we find evidence that this state-dependency effect is mediated by functional network changes: connection strength from the precuneus 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.

Behavioral and electrophysiological evidence for GABAergic modulation through transcutaneous vagus nerve stimulation.

OBJECTIVE: Transcutaneous vagus nerve stimulation (tVNS) has been hypothesized to modulate γ-aminobutyric (GABA) transmission in the human brain. GABA in the motor cortex is highly correlated to measures of automatic motor inhibition that can be obtained in simple response priming paradigms. To test the effects of tVNS on GABA transmission, we measured tVNS-induced alterations in behavioral and electrophysiology during automatic motor inhibition. METHODS: Participants were 16 young, healthy adults (8 female). We combined a subliminal response priming paradigm with tVNS and EEG measurement. In this paradigm, automatic motor inhibition leads to a reversal of the priming effect, a phenomenon referred to as the negative compatibility effect (NCE). We compute the NCE separated by response hands, hypothesizing a modulation of the left-hand NCE. Using EEG we measured readiness potentials, an established electrophysiological index of cortical motor preparation. RESULTS: As hypothesized, for the ipsilateral hand/contralateral hemisphere, compared to sham stimulation, tVNS increased the NCE and modulated the electrophysiological readiness potentials. CONCLUSION: Our results indicate that tVNS is selectively affecting the GABAergic system in the motor system contralateral to the stimulated ear as reflected in a behavioral and electrophysiological modulation. SIGNIFICANCE: We provide first combined behavioral and electrophysiological evidence for direct GABAergic neuromodulation through tVNS.