Evidence for a modulating effect of transcutaneous auricular vagus nerve stimulation (taVNS) on salivary alpha-amylase as indirect noradrenergic marker: A pooled mega-analysis

Optimizing non-invasive vagus nerve stimulation for treatment in stroke

Stroke remains a leading cause of long-term disability worldwide. There is an unmet need for neuromodulatory therapies that can mitigate against neurovascular injury and potentially promote neurological recovery. Transcutaneous vagus nerve stimulation has been demonstrated to show potential therapeutic effects in both acute and chronic stroke. However, previously published research has only investigated a narrow range of stimulation settings and indications. In this review, we detail the ongoing studies of transcutaneous vagus nerve stimulation in stroke through systematic searches of registered clinical trials. We summarize the upcoming clinical trials of transcutaneous vagus nerve stimulation in stroke, highlighting their indications, parameter settings, scope, and limitations. We further explore the challenges and barriers associated with the implementation of transcutaneous vagus nerve stimulation in acute stroke and stroke rehabilitation, focusing on critical aspects such as stimulation settings, target groups, biomarkers, and integration with rehabilitation interventions.

Ready for translation: non-invasive auricular vagus nerve stimulation inhibits psychophysiological indices of stimulus-specific fear and facilitates responding to repeated exposure in phobic individuals

Recent laboratory research showed that vagus nerve stimulation promotes fear extinction, the inhibitory core mechanism of exposure treatment, presumably via activation of the noradrenergic brain system. However, a translation of this stimulation technique to clinical practice is lacking. We therefore investigated the potential of vagal stimulation to inhibit excessive fear responses and facilitate responding to in-vivo and laboratory exposure in individuals with specific phobia. Spider-phobic participants were subjected to three standardized in-vivo exposures towards a living tarantula, complemented by an exposure in vitro (between exposure in vivo I and II). Transcutaneous auricular vagus nerve stimulation (taVNS) was applied during in-vitro exposure, presenting pictures of the exposed tarantula, other spiders and neutral tools in the laboratory. Fear was assessed by self-reports and behavioral avoidance (in-vivo exposures), and amygdala-mediated autonomic and behavioral fear components (exposure in vitro). Vagal stimulation facilitated the reduction of behavioral avoidance across repeated in-vivo exposures. During laboratory exposure, taVNS inhibited fear tachycardia and corrugator muscle activity specifically in response to pictures of the previously exposed tarantula - an effect that became stronger with increasing stimulation duration. Psychophysiological indices of noradrenergic transmission in the basolateral amygdala were elevated during taVNS and correlated to subsequent attenuation of behavioral avoidance. Our results suggest, that taVNS exerts stimulus-specific and dose-dependent inhibition of multiple automatic response components of excessive fear, highlighting taVNS as a valuable adjunct to exposure-based treatment. A translational mechanism of action is supported, proposing that taVNS exhibits its effects by noradrenergic activation of fear extinction circuitry, particularly targeting the basolateral amygdala.

Transcutaneous Auricular Vagus Nerve Stimulation Enhances Emotional Processing and Long-Term Recognition Memory: Electrophysiological Evidence Across Two Studies

Recently, we found that continuous transcutaneous auricular vagus nerve stimulation (taVNS) facilitates the encoding and later recollection of emotionally relevant information, as indicated by differences in the late positive potential (LPP), memory performance, and late ERP Old/New effect. Here, we aimed to conceptually replicate and extend these findings by investigating the effects of different time-dependent taVNS stimulation protocols. In Study 1, an identical paradigm to our previous study was employed with interval stimulation (30-s on/off). Participants viewed unpleasant and neutral scenes on two consecutive days while receiving taVNS or sham stimulation and completed a recognition test 1 week later. Replicating previous results, unpleasant images encoded under taVNS, compared to sham stimulation, elicited larger amplitudes in an earlier window of the LPP during encoding, as well as more pronounced late Old/New differences. However, no effects of taVNS on memory performance were found. In Study 2, we followed up on these findings by synchronizing the stimulation cycle with image presentation to determine the taVNS effects for images encoded during the on and off cycles. We could replicate the enhancing effects of taVNS on brain potentials (early LPP and late Old/New differences) and found that taVNS improved recollection-based memory performance for both unpleasant and neutral images, independently of the stimulation cycle. Overall, our results suggest that taVNS increases electrophysiological correlates of emotional encoding and retrieval in a time-independent manner, substantiating the vagus nerve's role in emotional processing and memory formation, opening new venues for improving mnemonic processes in both clinical and non-clinical populations.

Does transcutaneous auricular vagus nerve stimulation alter pupil dilation? A living Bayesian meta-analysis

BACKGROUND

Transcutaneous vagus nerve stimulation (tVNS) has emerged as a promising technique to modulate autonomic functions, and pupil dilation has been recognized as a promising biomarker for tVNS-induced monoaminergic release. Nevertheless, studies on the effectiveness of various tVNS protocols have produced heterogeneous results on pupil dilation to date.

METHODS

Here, we synthesize the existing evidence and compare conventional ("continuous") and pulsed stimulation protocols using a Bayesian meta-analysis. To maintain a living version, we developed a Shiny App with the possibility to incorporate newly published studies in the future. Based on a systematic review, we included 18 studies (N = 771) applying either conventional or pulsed stimulation protocols.

RESULTS

Across studies, we found anecdotal evidence for the null hypothesis, showing that taVNS does not increase pupil size (g = 0.15, 95 % CI = [0.03, 0.27], BF01 = 1.0). Separating studies according to conventional vs. pulsed protocols revealed that studies using pulsed taVNS provide strong evidence for the alternative hypothesis(g = 0.36, 95 % CI = [0.19, 0.53], BF10 = 50.8) while conventional taVNS studies provide strong evidence for the null hypothesis (g = 0.002, CI = [-0.14, 0.14], BF01 = 21.9).

CONCLUSION

Our meta-analysis highlights differential effects of conventional and pulsed taVNS protocols on pupil dilation. These findings underscore the relevance of taVNS protocols in optimizing its use for specific applications that may require modulation of tonic vs. phasic monoaminergic responses and might also help to gain mechanistic insights into potential therapeutic effects.

Effects of cervical transcutaneous Vagus Nerve Stimulation (ctVNS) on military cognitive performance during sleep deprivation

Introduction

Maintaining cognitive performance during sleep deprivation is of vital importance in many professions, especially in high-risk professions like the military. It has long been known that sleep deprivation diminishes cognitive performance. To mitigate the negative effects on cognitive performance during crucial military tasks, new interventions are necessary. Non-invasive cervical transcutaneous vagus nerve stimulation (ctVNS) has gained traction as a method to boost alertness and cognitive functioning.

Methods

We investigated the effects of a 2 × 2 minute ctVNS stimulation protocol on three cognitive tasks applied during conditions of sleep-deprivation: a psychomotor vigilance task (PVT), a multitasking task (SynWin), and an inhibitory control task (stop-signal task; SST). In addition, participants also performed a close-quarter-battle (CQB) test in virtual reality (VR) to examine if potential effects of ctVNS translate to operational military contexts. A total of 35 military operators from Special Operations Forces (SOF) and SOF support units participated. They were randomly assigned to an active stimulation group or sham group. Before stimulation at 19:00 h, participants performed baseline tests. Participants stayed awake through the night and performed the cognitive tasks every 3 h. The last round of cognitive tasks was followed by the VR test.

Results

Though sleep deprivation was successfully induced, as evident from a decline in performance on all three cognitive tasks (effect of session: p < 0.001 SynWin; p < 0.001 PVT; p < 0.001 SST; Linear Mixed Model), no significant effects of ctVNS were found on cognitive task performance, as well as on the military operational VR task. However, the influence of stimulation intensity on SynWin performance showed a trend, indicating that higher stimulation intensities could have a negative impact on cognitive performance.

Discussion

A 2 × 2 minute stimulation protocol may not be sufficient to elicit beneficial effects on cognitive-and operational military performance. Moreover, correct stimulation intensity may be critical to induce effects on cognitive performance, as stimulation effects may follow an inverted-u dose-response curve. Stimulation intensities in the current study are higher compared to a similar study that reported beneficial effects of ctVNS, which may explain this null finding. Further research is recommended to optimize stimulation protocols and investigate robustness of effects.

Non-Invasive Auricular Vagus Nerve Stimulation Decreases Heart Rate Variability Independent of Caloric Load

The vagus nerve is crucial in regulating physiological functions, including the cardiovascular system. While heart rate (HR) and its variability (HRV) may provide non-invasive proxies of cardiac vagal activity, transcutaneous auricular vagus nerve stimulation (taVNS) has yielded mixed effects, with limited research on right branch stimulation. In a randomized crossover study with 36 healthy participants, we investigated taVNS effects on HR and HRV indexed by SDRR, RMSSD, HF-HRV, and LF/HF ratio. To assess the impact of the stimulation side (left vs. right ear) on cardiovascular indices and interaction with the physiological state, we recorded electrocardiograms in four sessions per person, covering three session phases: baseline, during stimulation (taVNS vs. sham), and post-milkshake consumption with stimulation. First, we found moderate evidence against taVNS affecting HR (BF10 = 0.21). Second, taVNS decreased HRV (multivariate p = 0.004) independent of physiological state, with strong evidence for RMSSD (BF10 = 15.11) and HF-HRV (BF10 = 11.80). Third, taVNS-induced changes were comparable across sides and stronger than sham, indicating consistent cardiovascular effects independent of the stimulation side. We conclude that taVNS reduces HRV as indexed by RMSSD, HF-HRV, and SDRR without altering HR, contradicting the assumption that taVNS per se increases cardiovagal activity as indexed by increased HRV due to stimulating vagal afferents. Instead, our results support the role of vagal afferent activation in arousal. Crucially, taVNS on both sides can safely modulate the cardiovascular system without increasing the risk of bradycardia or causing adverse events in healthy participants, offering new treatment possibilities.

Can transcutaneous auricular vagus nerve stimulation mitigate vigilance loss? Examining the effects of stimulation at individualized versus constant current intensity

According to the arousal model of vigilance, the locus coeruleus-norepinephrine (LC-NE) system modulates sustained attention over long periods by regulating physiological arousal. Recent research has proposed that transcutaneous auricular vagus nerve stimulation (taVNS) modulates indirect physiological markers of LC-NE activity, although its effects on vigilance have not yet been examined. Aiming to develop a safe and noninvasive procedure to prevent vigilance failures in prolonged tasks, the present study examined whether taVNS can mitigate vigilance loss while modulating indirect markers of LC-NE activity. Following a preregistered protocol (https://osf.io/tu2xy/), 50 participants completed three repeated sessions in a randomized order, in which either active taVNS at individualized intensity set by participant, active taVNS set at 0.5 mA for all participants, or sham taVNS, was delivered while performing an attentional and vigilance task (i.e., ANTI-Vea). Changes in salivary alpha-amylase and cortisol concentrations were measured as markers of LC-NE activity. Self-reports of feelings associated with stimulation and guessing rate of active/sham conditions supported the efficacy of the single-blind procedure. Contrary to our predictions, the observed vigilance decrement was not modulated by active taVNS. Pairwise comparisons showed a mitigation by active taVNS on cortisol reduction across time. Interestingly, Spearman's correlational analyses showed some interindividual effects of taVNS on indirect markers of LC-NE, evidenced by positive associations between changes in salivary alpha-amylase and cortisol in active but not sham taVNS. We highlight the relevance of replicating and extending the present outcomes, investigating further parameters of stimulation and its effects on other indirect markers of LC-NE activity.

Disentangling emotional source memory: a mega-analysis on the effects of emotion on item-context binding in episodic long-term memory

Introduction

It has long been known that highly arousing emotional single items are better recollected than low arousing neutral items. Despite the robustness of this memory advantage, emotional arousing events may not always promote the retrieval of source details (i.e., source memory) or associated neutral information.

Methods

To shed more light on these effects, we pooled data from seven different studies (N = 333) to investigate the role of emotion on item-context binding in episodic memory, as well as potential interacting factors (e.g., encoding instructions, type of retrieval task, or acute stress). In all studies, participants incidentally encoded common neutral objects (i.e., items), overlaid on different pleasant, neutral, or unpleasant background scenes (i.e., contexts). One week later, the encoded objects were presented intermixed with new ones and memory for item and source contextual details was tested, also considering the contribution of recollection and familiarity-based processes.

Results

Linear mixed models revealed a recollection-based retrieval advantage for unpleasant and pleasant source contextual details compared to neutral ones. Bayes hypothesis-testing analysis further indicated decisive evidence in favor of a relevant role of emotional arousal and recollection in source contextual memory. Regarding item memory, linear mixed models revealed enhanced recollection-based memory for items encoded in pleasant contexts compared to their neutral and unpleasant counterparts. However, Bayes analysis revealed strong to moderate evidence for models without affective category (or its interactions), indicating that the affective category of contexts in which objects were paired during encoding had little influence on item memory performance.

Discussion

The present results are discussed in relation to existing evidence and current neurobiological models of emotional episodic memory by also emphasizing the role of predictive processing as a useful conceptual framework to understand the effects of emotion on memory for source details and associated neutral information.

Transcutaneous auricular vagus nerve stimulation as a potential therapy for attention deficit hyperactivity disorder: modulation of the noradrenergic pathway in the prefrontal lobe

Attention deficit hyperactivity disorder (ADHD) is a neurodevelopmental disorder characterized by developmental impairments, inattention, motor hyperactivity, and impulsivity. Currently, there is no effective intervention that can completely cure it. One of the pathogenic mechanisms of ADHD involves abnormalities in the norepinephrine (NE) pathway within the prefrontal cortex (PFC). In recent years, transcutaneous auricular vagus nerve stimulation (taVNS), a non-invasive neuromodulation technique, has demonstrated promising potential in the treatment of neurological and psychiatric disorders. However, its application in the management of ADHD remains relatively unexplored. Previous studies have shown that taVNS exerts therapeutic effects on attention, cognition, arousal, perception, and behavioral regulation primarily through activating the vagus nerve conduction pathway, specifically targeting the nucleus tractus solitarius - locus coeruleus - NE pathway. These findings have led to the hypothesis that taVNS may be an effective intervention for ADHD, with NE and its pathway playing a pivotal role in this context. Therefore, this review comprehensively examines the correlation between NE pathway alterations in the PFC and ADHD, the mechanism of action of taVNS, and the potential role of the NE pathway in treating ADHD with taVNS, aiming to provide a theoretical foundation for clinical applications.

Opposing effects of pre-encoding stress on neural substrates of item and emotional contextual source memory retrieval

Although the mediating role of the stress hormone systems in memory for single- especially emotional- events is well-stablished, less is known about the influence of stress on memory for associated contextual information (source memory). Here, we investigated the impact of acute stress on the neural underpinnings of emotional contextual source memory. Participants underwent a stress or a control manipulation before they encoded objects paired with pleasant, neutral, or unpleasant backgrounds. One week later, item and contextual source memory were tested. Acute stress modulated the neural signature of item and contextual source memory in an opposite fashion: stressed participants showed larger activation in the precuneus and the medial prefrontal cortex (mPFC) during the retrieval of items, while the retrieval of contextual unpleasant information was associated with lower activation in the angular gyrus (AG) and mPFC. Furthermore, as revealed by cross-region representational similarity analyses, stress also reduced the memory reinstatement of the previously encoded visual cortex representations of object/unpleasant background pairings in the AG and mPFC. These results suggest that pre-encoding stress induction increases the activity of memory-related regions for single items but reduces the activity of these regions during the retrieval of contextual unpleasant information. Our findings provide new insights into the dissociative effects of stress on item and contextual source memory which could have clinical relevance for stress-related disorders.

Effects of Transcutaneous Auricular Vagus Nerve Stimulation on the P300: Do Stimulation Duration and Stimulation Type Matter?

Non-invasive transcutaneous auricular vagus nerve stimulation (taVNS) has attracted increasing interest as a neurostimulation tool with potential applications in modulating cognitive processes such as attention and memory, possibly through the modulation of the locus-coeruleus noradrenaline system. Studies examining the P300 brain-related component as a correlate of noradrenergic activity, however, have yielded inconsistent findings, possibly due to differences in stimulation parameters, thus necessitating further investigation. In this event-related potential study involving 61 participants, therefore, we examined how changes in taVNS parameters, specifically stimulation type (interval vs. continuous stimulation) and duration, influence P300 amplitudes during a visual novelty oddball task. Although no effects of stimulation were found over the whole cluster and time window of the P300, cluster-based permutation tests revealed a distinct impact of taVNS on the P300 response for a small electrode cluster, characterized by larger amplitudes observed for easy targets (i.e., stimuli that are easily discernible from standards) following taVNS compared to sham stimulation. Notably, our findings suggested that the type of stimulation significantly modulated taVNS effects on the P300, with continuous stimulation showing larger P300 differences (taVNS vs. sham) for hard targets and standards compared to interval stimulation. We observed no interaction effects of stimulation duration on the target-related P300. While our findings align with previous research, further investigation is warranted to fully elucidate the influence of taVNS on the P300 component and its potential utility as a reliable marker for neuromodulation in this field.

Are the P600 and P3 ERP components linked to the task-evoked pupillary response as a correlate of norepinephrine activity?

During language comprehension, anomalies and ambiguities in the input typically elicit the P600 event-related potential component. Although traditionally interpreted as a specific signal of combinatorial operations in sentence processing, the component has alternatively been proposed to be a variant of the oddball-sensitive, domain-general P3 component. In particular, both components might reflect phasic norepinephrine release from the locus coeruleus (LC/NE) to motivationally significant stimuli. In this preregistered study, we tested this hypothesis by relating both components to the task-evoked pupillary response, a putative biomarker of LC/NE activity. 36 participants completed a sentence comprehension task (containing 25% morphosyntactic violations) and a non-linguistic oddball task (containing 20% oddballs), while the EEG and pupil size were co-registered. Our results showed that the task-evoked pupillary response and the ERP amplitudes of both components were similarly affected by both experimental tasks. In the oddball task, there was also a temporally specific relationship between the P3 and the pupillary response beyond the shared oddball effect, thereby further linking the P3 to NE. Because this link was less reliable in the linguistic context, we did not find conclusive evidence for or against a relationship between the P600 and the pupillary response. Still, our findings further stimulate the debate on whether language-related ERPs are indeed specific to linguistic processes or shared across cognitive domains. However, further research is required to verify a potential link between the two ERP positivities and the LC/NE system as the common neural generator.

Atrophy links lower novelty-related locus coeruleus connectivity to cognitive decline in preclinical AD

INTRODUCTION

Animal research has shown that tau pathology in the locus coeruleus (LC) is associated with reduced norepinephrine signaling, lower projection density to the medial temporal lobe (MTL), atrophy, and cognitive impairment. We investigated the contribution of LC-MTL functional connectivity (FCLC-MTL) on cortical atrophy across Braak stage regions and its impact on cognition.

METHODS

We analyzed functional magnetic resonance imaging and amyloid beta (Aβ) positron emission tomography data from 128 cognitively normal participants, associating novelty-related FCLC-MTL with longitudinal atrophy and cognition with and without Aβ moderation.

RESULTS

Cross-sectionally, lower FCLC-MTL was associated with atrophy in Braak stage II regions. Longitudinally, atrophy in Braak stage 2 to 4 regions related to lower baseline FCLC-MTL at elevated levels of Aβ, but not to other regions. Atrophy in Braak stage 2 regions mediated the relation between FCLC-MTL and subsequent cognitive decline.

DISCUSSION

FCLC-MTL is implicated in Aβ-related cortical atrophy, suggesting that LC-MTL connectivity could confer neuroprotective effects in preclinical AD.

HIGHLIGHTS

Novelty-related functional magnetic resonance imaging (fMRI) LC-medial temporal lobe (MTL) connectivity links to longitudinal Aβ-dependent atrophy. This relationship extended to higher Braak stage regions with increasing Aβ burden. Longitudinal MTL atrophy mediated the LC-MTL connectivity-cognition relationship. Our findings mirror the animal data on MTL atrophy following NE signal dysfunction.

Non-invasive vagus nerve stimulation and the motivation to work for rewards: A replication of Neuser et al. (2020, Nature Communications)

The vagus nerve is thought to be involved in the allostatic regulation of motivation and energy metabolism via gut-brain interactions. A recent study by Neuser and colleagues (2020) provided novel evidence for this process in humans, by reporting a positive effect of transcutaneous auricular vagus nerve stimulation (taVNS) on the invigoration of reward-seeking behaviors, especially for food rewards. We conducted an independent direct replication of Neuser et al. (2020), to assess the robustness of their findings. Following the original study, we used a single-blind, sham-controlled, randomized cross-over design. We applied left-sided taVNS in healthy human volunteers (n = 40), while they performed an effort allocation task in which they had to work for monetary and food rewards. The replication study was purely confirmatory in that it strictly followed the analysis plans and scripts used by Neuser et al. Although, in line with Neuser et al., we found strong effects of task variables on effort invigoration and effort maintenance, we failed to replicate their key finding: taVNS did not increase the strength of invigoration (p = .62); the data were five times more likely (BF10  = 0.19) under the null hypothesis. We also found substantial evidence against an effect of taVNS on effort maintenance (p = .50; BF10  = 0.20). Our results provide evidence against the idea that left-sided taVNS boosts the motivational drive to work for rewards. Our study also highlights the need for direct replications of influential taVNS studies.

Transcutaneous vagus nerve stimulation: a new strategy for Alzheimer's disease intervention through the brain-gut-microbiota axis?

Transcutaneous vagus nerve stimulation (tVNS) is an emerging non-invasive technique designed to stimulate branches of the vagus nerve distributed over the body surface. Studies suggest a correlation between the brain-gut-microbiota (BGM) axis and the pathogenesis of Alzheimer's disease (AD). The BGM axis represents a complex bidirectional communication system, with the vagus nerve being a crucial component. Therefore, non-invasive electrical stimulation of the vagus nerve might have the potential to modify-most of the time probably in a non-physiological way-the signal transmission within the BGM axis, potentially influencing the progression or symptoms of AD. This review explores the interaction between percutaneous vagus nerve stimulation and the BGM axis, emphasizing its potential effects on AD. It examines various aspects, such as specific brain regions, gut microbiota composition, maintenance of intestinal environmental homeostasis, inflammatory responses, brain plasticity, and hypothalamic-pituitary-adrenal (HPA) axis regulation. The review suggests that tVNS could serve as an effective strategy to modulate the BGM axis and potentially intervene in the progression or treatment of Alzheimer's disease in the future.

Transcutaneous auricular vagus nerve stimulation modulates the processing of interoceptive prediction error signals and their role in allostatic regulation

It has recently been suggested that predictive processing principles may apply to interoception, defined as the processing of hormonal, autonomic, visceral, and immunological signals. In the current study, we aimed at providing empirical evidence for the role of cardiac interoceptive prediction errors signals on allostatic adjustments, using transcutaneous auricular vagus nerve stimulation (taVNS) as a tool to modulate the processing of interoceptive afferents. In a within-subject design, participants performed a cardiac-related interoceptive task (heartbeat counting task) under taVNS and sham stimulation, spaced 1-week apart. We observed that taVNS, in contrast to sham stimulation, facilitated the maintenance of interoceptive accuracy levels over time (from the initial, stimulation-free, baseline block to subsequent stimulation blocks), suggesting that vagus nerve stimulation may have helped to maintain engagement to cardiac afferent signals. During the interoceptive task, taVNS compared to sham, produced higher heart-evoked potentials (HEP) amplitudes, a potential readout measure of cardiac-related prediction error processing. Further analyses revealed that the positive relation between interoceptive accuracy and allostatic adjustments-as measured by heart rate variability (HRV)-was mediated by HEP amplitudes. Providing initial support for predictive processing accounts of interoception, our results suggest that the stimulation of the vagus nerve may increase the precision with which interoceptive signals are processed, favoring their influence on allostatic adjustments.

Transcutaneous Auricular Vagus Nerve Stimulation to Improve Emotional State

Emotional experiences are a part of our lives. The maladaptive functioning of an individual's emotional field can lead to emotional disturbances of various kinds, such as anxiety and depression. Currently, there is an increasing prevalence of emotional disorders that cause great human suffering and high socioeconomic costs. Emotional processing has a biological basis. The major neuroscientific theories of emotion are based on biological functioning, and all of them take into account the anatomy and function of the tenth cranial nerve: the vagus nerve. The vagus nerve connects the subdiaphragmatic and supradiaphragmatic areas and modulates emotional processing as the basis of interoceptive functioning. Auricular vagus nerve stimulation is a new and innovative neuromodulation technique based on the function of the vagus nerve. Several interventions have shown that this new neurostimulation technique is a very promising resource for treating emotional disorders. In this paper, we summarise three neuroscientific theories of emotion, explain what transcutaneous auricular nerve stimulation is, and present arguments for its use and continued research.

Current status and prospect of transcutaneous auricular vagus nerve stimulation for disorders of consciousness

Disordered Consciousness (DOC) is among neurological disorders for which there is currently no admitted treatment. The pathogenesis of DOC is still unclear, covering a variety of indistinguishable types of diseases, high misdiagnosis rate and poor prognosis. Most treatments remain to be clarified in the future to provide adequate evidence for clinical guidance. Neuromodulation technology aims to regulate neural circuits to promote awakening more directly. At present, it is confirmed that the potential of transcutaneous auricular vagus nerve stimulation (taVNS) as a therapeutic tool is worth exploring in the context of consciousness disorders, as previously proposed for invasive forms of VNS, in which the means of stimulating the vagus nerve to change the brain areas related to cosciousness have also received widespread attention. In this paper, we review the literature on taVNS and DOC to better understand the current status and development prospect of taVNS treament as a non-invasive neuromodulation method with sensitivity and/or specificity at the single subject.

tVNS in Stroke: A Narrative Review on the Current State and the Future

Ischemic stroke is a leading cause of disability and there is a paucity of therapeutic strategies that promote functional recovery after stroke. Transcutaneous vagus nerve stimulation (tVNS) has shown promising evidence as a tool to reduce infarct size in animal models of hyperacute stroke. In chronic stroke, tVNS paired with limb movements has been shown to enhance neurological recovery. In this review, we summarize the current evidence for tVNS in preclinical models and clinical trials in humans. We highlight the mechanistic pathways involved in the beneficial effects of tVNS. We critically evaluate the current gaps in knowledge and recommend the key areas of research required to translate tVNS into clinical practice in acute and chronic stroke.

Phasic, Event-Related Transcutaneous Auricular Vagus Nerve Stimulation Modifies Behavioral, Pupillary, and Low-Frequency Oscillatory Power Responses

Transcutaneous auricular vagus nerve stimulation (taVNS) has been proposed to activate the locus ceruleus-noradrenaline (LC-NA) system. However, previous studies failed to find consistent modulatory effects of taVNS on LC-NA biomarkers. Previous studies suggest that phasic taVNS may be capable of modulating LC-NA biomarkers such as pupil dilation and alpha oscillations. However, it is unclear whether these effects extend beyond pure sensory vagal nerve responses. Critically, the potential of the pupillary light reflex as an additional taVNS biomarker has not been explored so far. Here, we applied phasic active and sham taVNS in 29 subjects (16 female, 13 male) while they performed an emotional Stroop task (EST) and a passive pupil light reflex task (PLRT). We recorded pupil size and brain activity dynamics using a combined Magnetoencephalography (MEG) and pupillometry design. Our results show that phasic taVNS significantly increased pupil dilation and performance during the EST. During the PLRT, active taVNS reduced and delayed pupil constriction. In the MEG, taVNS increased frontal-midline theta and alpha power during the EST, whereas occipital alpha power was reduced during both the EST and PLRT. Our findings provide evidence that phasic taVNS systematically modulates behavioral, pupillary, and electrophysiological parameters of LC-NA activity during cognitive processing. Moreover, we demonstrate for the first time that the pupillary light reflex can be used as a simple and effective proxy of taVNS efficacy. These findings have important implications for the development of noninvasive neuromodulation interventions for various cognitive and clinical applications.SIGNIFICANCE STATEMENT taVNS has gained increasing attention as a noninvasive neuromodulation technique and is widely used in clinical and nonclinical research. Nevertheless, the exact mechanism of action of taVNS is not yet fully understood. By assessing physiology and behavior in a response conflict task in healthy humans, we demonstrate the first successful application of a phasic, noninvasive vagus nerve stimulation to improve cognitive control and to systematically modulate pupillary and electrophysiological markers of the noradrenergic system. Understanding the mechanisms of action of taVNS could optimize future clinical applications and lead to better treatments for mental disorders associated with noradrenergic dysfunction. In addition, we present a new taVNS-sensitive pupillary measure representing an easy-to-use biomarker for future taVNS studies.

Cognitive Functions following Trigeminal Neuromodulation

Vast scientific effort in recent years have been focused on the search for effective and safe treatments for cognitive decline. In this regard, non-invasive neuromodulation has gained increasing attention for its reported effectiveness in promoting the recovery of multiple cognitive domains after central nervous system damage. In this short review, we discuss the available evidence supporting a possible cognitive effect of trigeminal nerve stimulation (TNS). In particular, we ask that, while TNS has been widely and successfully used in the treatment of various neuropsychiatric conditions, as far as research in the cognitive field is concerned, where does TNS stand? The trigeminal nerve is the largest cranial nerve, conveying the sensory information from the face to the trigeminal sensory nuclei, and from there to the thalamus and up to the somatosensory cortex. On these bases, a bottom-up mechanism has been proposed, positing that TNS-induced modulation of the brainstem noradrenergic system may affect the function of the brain networks involved in cognition. Nevertheless, despite the promising theories, to date, the use of TNS for cognitive empowering and/or cognitive decline treatment has several challenges ahead of it, mainly due to little uniformity of the stimulation protocols. However, as the field continues to grow, standardization of practice will allow for data comparisons across studies, leading to optimized protocols targeting specific brain circuitries, which may, in turn, influence cognition in a designed manner.

The effect of transcutaneous auricular vagus nerve stimulation (taVNS) on cognitive control in multitasking

Current research in brain stimulation suggests transcutaneous auricular vagus nerve stimulation (taVNS) as a promising tool to modulate cognitive functions in healthy populations, such as attention, memory, and executive functions. Empirical evidence in single-task contexts, suggests that taVNS promotes holistic task processing, which strengthens the integration of multiple stimulus features in task processing. However, it is unclear how taVNS might affect performance in multitasking, where the integration of multiple stimuli leads to an overlap in stimulus response translation processes, increasing the risk of between-task interference (crosstalk). In a single-blinded, sham-controlled, within-subject design, participants underwent taVNS while performing a dual task. To assess the effects of taVNS, behavioral (reaction times), physiological (heart rate variability, salivary alpha-amylase), and subjective psychological variables (e.g., arousal) were recorded over three cognitive test blocks. Our results revealed no overall significant effect of taVNS on physiological and subjective psychological variables. However, the results showed a significant increase in between-task interference under taVNS in the first test block, but not in the subsequent test blocks. Our findings therefore suggest that taVNS increased integrative processing of both tasks early during active stimulation.

Transcutaneous Vagus Nerve Stimulation (tVNS) applications in cognitive aging: a review and commentary

Differentiating healthy from pathological aging trajectories is extremely timely, as the global population faces an inversion where older adults will soon outnumber younger 5:1. Many cognitive functions (e.g., memory, executive functions, and processing speed) decline with age, a process that can begin as early as midlife, and which predicts subsequent diagnosis with dementia. Although dementia is a devastating and costly diagnosis, there remains limited evidence for medications, therapies, and devices that improve cognition or attenuate the transition into dementia. There is an urgent need to intervene early in neurodegenerative processes leading to dementia (e.g., depression and mild cognitive impairment). In this targeted review and commentary, we highlight transcutaneous Vagus Nerve Stimulation (tVNS) as a neurostimulation method with unique opportunities for applications in diseases of aging, reviewing recent literature, feasibility of use with remote data collection methods/telehealth, as well as limitations and conflicts in the literature. In particular, small sample sizes, uneven age distributions of participants, lack of standardized protocols, and oversampling of non-representative groups (e.g., older adults with no comorbid diagnoses) limit our understanding of the potential of this method. We offer recommendations for how to improve representativeness, statistical power, and generalizability of tVNS research by integrating remote data collection techniques.

Effects of transcutaneous auricular vagus nerve stimulation paired with tones on electrophysiological markers of auditory perception

BACKGROUND

Transcutaneous auricular vagus nerve stimulation (taVNS) has been introduced as a non-invasive alternative to invasive vagus nerve stimulation (iVNS). While iVNS paired with tones has been highlighted as a potential effective therapy for the treatment of auditory disorders such as tinnitus, there is still scarce data available confirming the efficacy of non-invasive taVNS. Here, we assessed the effect of taVNS paired with acoustic stimuli on sensory-related electrophysiological responses.

METHODS

A total of 22 healthy participants were investigated with a taVNS tone-pairing paradigm using a within-subjects design. In a single session pure tones paired with either active taVNS or sham taVNS were repeatedly presented. Novel tones without electrical stimulation served as control condition. Auditory event related potentials and auditory cortex oscillations were compared before and after the tone pairing procedure between stimulation conditions.

RESULTS

From pre to post pairing, we observed a decrease in the N1 amplitude and in theta power to tones paired with sham taVNS while these electrophysiological measures remained stable for tones paired with active taVNS a pattern mirroring auditory sensory processing of novel, unpaired control tones.

CONCLUSION

Our results demonstrate the efficacy of a short-term application of non-invasive taVNS to modulate auditory processing in healthy individuals and, thereby, have potential implications for interventions in auditory processing deficits.

Short-term transcutaneous vagus nerve stimulation increases pupil size but does not affect EEG alpha power: A replication of Sharon et al. (2021, Journal of Neuroscience)

BACKGROUND

Transcutaneous auricular vagus nerve stimulation (taVNS) has been tested as a potential treatment for pharmaco-resistant epilepsy and depression. Its clinical efficacy is thought to depend on taVNS-induced activation of the locus coeruleus and other neuromodulator systems. However, unlike for invasive VNS in rodents, there is little evidence for an effect of taVNS on noradrenergic activity.

OBJECTIVE

We attempted to replicate recently published findings by Sharon et al. (2021), showing that short bursts of taVNS transiently increased pupil size and decreased EEG alpha power, two correlates of central noradrenergic activity.

METHODS

Following the original study, we used a single-blind, sham-controlled, randomized cross-over design. Human volunteers (n = 29) received short-term (3.4 s) taVNS at the maximum level below the pain threshold, while we collected resting-state pupil-size and EEG data. To analyze the data, we used scripts provided by Sharon and colleagues.

RESULTS

Consistent with Sharon et al. (2021), pupil dilation was significantly larger during taVNS than during sham stimulation (p = .009; Bayes factor supporting the difference = 7.45). However, we failed to replicate the effect of taVNS on EEG alpha power (p = .37); the data were four times more likely under the null hypothesis (BF10 = 0.28).

CONCLUSION

Our findings support the effectiveness of short-term taVNS in inducing transient pupil dilation, a correlate of phasic noradrenergic activity. However, we failed to replicate the recent finding by Sharon et al. (2021) that taVNS attenuates EEG alpha activity. Overall, this study highlights the need for continued research on the neural mechanisms underlying taVNS efficacy and its potential as a treatment option for pharmaco-resistant conditions. It also highlights the need for direct replications of influential taVNS studies.

Auricular Transcutaneous Vagus Nerve Stimulation Specifically Enhances Working Memory Gate Closing Mechanism: A System Neurophysiological Study

Everyday tasks and goal-directed behavior involve the maintenance and continuous updating of information in working memory (WM). WM gating reflects switches between these two core states. Neurobiological considerations suggest that the catecholaminergic and the GABAergic are likely involved in these dynamics. Both of these neurotransmitter systems likely underlie the effects to auricular transcutaneous vagus nerve stimulation (atVNS). We examine the effects of atVNS on WM gating dynamics and their underlying neurophysiological and neurobiological processes in a randomized crossover study design in healthy humans of both sexes. We show that atVNS specifically modulates WM gate closing and thus specifically modulates neural mechanisms enabling the maintenance of information in WM. WM gate opening processes were not affected. atVNS modulates WM gate closing processes through the modulation of EEG alpha band activity. This was the case for clusters of activity in the EEG signal referring to stimulus information, motor response information, and fractions of information carrying stimulus-response mapping rules during WM gate closing. EEG-beamforming shows that modulations of activity in fronto-polar, orbital, and inferior parietal regions are associated with these effects. The data suggest that these effects are not because of modulations of the catecholaminergic (noradrenaline) system as indicated by lack of modulatory effects in pupil diameter dynamics, in the inter-relation of EEG and pupil diameter dynamics and saliva markers of noradrenaline activity. Considering other findings, it appears that a central effect of atVNS during cognitive processing refers to the stabilization of information in neural circuits, putatively mediated via the GABAergic system.SIGNIFICANCE STATEMENT Goal-directed behavior depends on how well information in short-term memory can be flexibly updated but also on how well it can be shielded from distraction. These two functions were guarded by a working memory gate. We show how an increasingly popular brain stimulation techniques specifically enhances the ability to close the working memory gate to shield information from distraction. We show what physiological and anatomic aspects underlie these effects.