Transcutaneous Vagus Nerve Stimulation Does Not Affect Verbal Memory Performance in Healthy Volunteers

Vagus nerve stimulation as a predictive coding modulator that enhances feedforward over feedback transmission

Vagus nerve stimulation (VNS) has emerged as a promising therapeutic intervention across various neurological and psychiatric conditions, including epilepsy, depression, and stroke rehabilitation; however, its mechanisms of action on neural circuits remain incompletely understood. Here, we present a novel theoretical framework based on predictive coding that conceptualizes VNS effects through differential modulation of feedforward and feedback neural circuits. Based on recent evidence, we propose that VNS shifts the balance between feedforward and feedback processing through multiple neuromodulatory systems, resulting in enhanced feedforward signal transmission. This framework integrates anatomical pathways, receptor distributions, and physiological responses to explain the influence of the VNS on neural dynamics across different spatial and temporal scales. Vagus nerve stimulation may facilitate neural plasticity and adaptive behavior through acetylcholine and noradrenaline (norepinephrine), which differentially modulate feedforward and feedback signaling. This mechanistic understanding serves as a basis for interpreting the cognitive and therapeutic outcomes across different clinical conditions. Our perspective provides a unified theoretical framework for understanding circuit-specific VNS effects and suggests new directions for investigating their therapeutic mechanisms.

Tonic and Event-Related Phasic Transcutaneous Auricular Vagus Nerve Stimulation Alters Pupil Responses in the Change-Detection Task

BACKGROUND

Transcutaneous auricular vagus nerve stimulation (taVNS) has emerged as a potential modulator of cognitive behavior that activates the locus coeruleus-noradrenaline (LC-NA) system. Previous studies explored both phasic and tonic taVNS by investigating their impact on LC-NA markers such as pupil dilation and heart rate variability (HRV).

OBJECTIVE

Inconsistencies persist in the identification of reliable markers for assessing the effects of taVNS on noradrenergic activity. Furthermore, it remains unclear whether the effects of taVNS extend beyond pure vagal nerve responses, particularly in specific cognitive domains such as working memory. In the present study, we investigated the effects of taVNS on working memory capacity and LC-NA markers using a change-detection task.

MATERIALS AND METHODS

Twenty-two healthy, right-handed university students participated in a sham-controlled, randomized cross-over experiment with four sessions. We applied two types of phasic and event-related stimulation (Pre-event and Event-synchronous), tonic stimulation (Pre-task), and sham stimulation across different sessions. Pupil size and electrocardiogram data were recorded during the tasks.

RESULTS

taVNS did not significantly modulate behavioral performance on the change-detection task, specifically working memory capacity. However, both tonic and event-related phasic taVNS significantly influenced the pupillary response during the task. In addition, the Pre-task condition of the taVNS affected the low-frequency parameter of HRV.

CONCLUSIONS

Our findings suggest that tonic and event-related phasic taVNS may modulate noradrenergic activity, as evidenced by pupil responses and HRV changes during the change-detection task. This study provides new evidence regarding the impact of taVNS on cognitive tasks, thus supporting the development of noninvasive neuromodulation interventions.

A pooled analysis of the side effects of non-invasive Transcutaneous Auricular Vagus Nerve Stimulation (taVNS)

Introduction

Transcutaneous auricular vagus nerve stimulation (taVNS) is a promising technique for modulating vagal afferent fibers non-invasively and has shown therapeutic potential in neurological, cognitive, and affective disorders. While previous research highlights its efficacy, the safety profile of taVNS has been less extensively examined.

Methods

This study therefore aimed to systematically investigate side effects of taVNS in a large pooled dataset consisting of n = 488 participants, utilizing a standardized questionnaire to assess ten reported side effects. Analyses included effects of stimulation type (interval vs. continuous), stimulation duration, stimulation intensity and participant characteristics (age and gender) as potential modulators.

Results

The findings support the safety of taVNS, with minimal and mild side effects reported across participants (M = 1.86, SD = 1.36). Although participants receiving sham stimulation were 32.4% less likely to report unpleasant feelings compared to participants receiving taVNS, this effect was driven primarily by low-end ratings (specifically, a rating of 1, indicating not at all when experiencing the corresponding side effect), thus suggesting limited clinical relevance. Interval stimulation notably reduced the likelihood of some side effects, particularly for neck pain, dizziness and unpleasant feelings, suggesting potential for optimizing taVNS protocols. Stimulation intensity and duration showed few statistically significant, but clinically minimal (i.e., very small) effects.

Conclusion

Overall, these findings demonstrate a favorable safety profile of taVNS, with mostly mild and transient effects, supporting its use as a suitable non-invasive tool in both research and clinical applications.

Vagus nerve stimulation: a physical therapy with promising potential for central nervous system disorders

The diseases of the central nervous system (CNS) often cause irreversible damage to the human body and have a poor prognosis, posing a significant threat to human health. They have brought enormous burdens to society and healthcare systems. However, due to the complexity of their causes and mechanisms, effective treatment methods are still lacking. Vagus nerve stimulation (VNS), as a physical therapy, has been utilized in the treatment of various diseases. VNS has shown promising outcomes in some CNS diseases and has been approved by the Food and Drug Administration (FDA) in the United States for epilepsy and depression. Moreover, it has demonstrated significant potential in the treatment of stroke, consciousness disorders, and Alzheimer's disease. Nevertheless, the exact efficacy of VNS, its beneficiaries, and its mechanisms of action remain unclear. This article discusses the current clinical evidence supporting the efficacy of VNS in CNS diseases, providing updates on the progress, potential, and potential mechanisms of action of VNS in producing effects on CNS diseases.

Advances in VNS efficiency and mechanisms of action on cognitive functions

Objective

This systematic review aims to comprehensively analyze the efficacy and underlying mechanisms of vagus nerve stimulation (VNS) in enhancing cognitive functions and its therapeutic potential for various cognitive impairments. The review focuses on the impact of VNS on emotional processing, executive functions, learning, memory, and its clinical applications in conditions such as epilepsy, depression, Alzheimer's disease, and other neurological disorders.

Methods

A systematic search of electronic databases (PubMed, Scopus, Web of Science) was conducted using the keywords "vagus nerve stimulation," "cognitive enhancement," "emotional processing," "executive function," "learning and memory," "epilepsy," "depression," "Alzheimer's disease," "neurological disorders," "attention-deficit/hyperactivity disorder," "sleep disorders," and "long COVID." The inclusion criteria encompassed controlled trials, longitudinal studies, and meta-analyses published in English between 2000 and July 2024.

Results

A comprehensive review of 100 articles highlighted the cognitive effects of Vagus Nerve Stimulation (VNS). Studies show that VNS, especially through transcutaneous auricular VNS (taVNS), enhances emotional recognition, particularly for facial expressions, and improves selective attention under high cognitive demands. Additionally, VNS enhances learning and memory, including associative memory and spatial working memory tasks. In clinical applications, VNS exhibits promising benefits for improving cognitive functions in treatment-resistant epilepsy, depression, and Alzheimer's disease.

Conclusion

VNS represents a promising therapeutic approach for enhancing cognitive function across diverse patient populations. The reviewed evidence highlights its efficacy in modulating cognitive domains in healthy individuals and improving cognition in neurological conditions. However, the comparative effectiveness of different VNS modalities and the differential effects of online versus offline VNS on cognitive psychology require further investigation. Future research should focus on optimizing VNS protocols and elucidating specific cognitive domains that benefit most from VNS interventions. This ongoing exploration is essential for maximizing the therapeutic potential of VNS in clinical practice.

Vagus nerve stimulation in dementia: A scoping review of clinical and pre-clinical studies

Background

Dementia is a prevalent, progressive, neurodegenerative condition with multifactorial causes. Due to the lack of effective pharmaceutical treatments for dementia, there are growing clinical and research interests in using vagus nerve stimulation (VNS) as a potential non-pharmacological therapy for dementia. However, the extent of the research volume and nature into the effects of VNS on dementia is not well understood. This study aimed to examine the extent and nature of research activities in relation to the use of VNS in dementia and disseminate research findings for the potential utility in dementia care.

Methods

We performed a scoping review of literature searches in PubMed, HINARI, Google Scholar, and the Cochrane databases from 1980 to November 30th, 2023, including the reference lists of the identified studies. The following search terms were utilized: brain stimulation, dementia, Alzheimer's disease, vagal stimulation, memory loss, Deme*, cognit*, VNS, and Cranial nerve stimulation. The included studies met the following conditions: primary research articles pertaining to both humans and animals for both longitudinal and cross-sectional study designs and published in English from January 1st, 1980, to November 30th, 2023; investigated VNS in either dementia or cognitive impairment; and were not case studies, conference proceedings/abstracts, commentaries, or ordinary review papers.

Findings and conclusions

We identified 8062 articles, and after screening for eligibility (sequentially by titles, abstracts and full text reading, and duplicate removal), 10 studies were included in the review. All the studies included in this literature review were conducted over the last three decades in high-income geographical regions (i.e., Europe, the United States, the United Kingdom, and China), with the majority of them (7/10) being performed in humans. The main reported outcomes of VNS in the dementia cases were enhanced cognitive functions, an increased functional connectivity of various brain regions involved in learning and memory, microglial structural modifications from neurodestructive to neuroprotective configurations, a reduction of cerebral spinal fluid tau-proteins, and significant evoked brain tissue potentials that could be utilized to diagnose neurodegenerative disorders. The study outcomes highlight the potential for VNS to be used as a non-pharmacological therapy for cognitive impairment in dementia-related diseases such as Alzheimer's disease.

Influence of a 2-week transcutaneous auricular vagus nerve stimulation on memory: findings from a randomized placebo controlled trial in non-clinical adults

PURPOSE

Memory plays an essential role in daily life and is one of the first functions to deteriorate in cognitive impairment and dementia. Transcutaneous vagus nerve stimulation (tVNS) is a promising therapeutic method; however, its ability to enhance memory is underexplored, especially considering long-term stimulation. We aimed to investigate the effect of a 2-week course of auricular tVNS (taVNS) on memory in a non-clinical population.

METHODS

This single-blind randomized placebo-wait-list controlled trial recruited 76 participants (30 men; mean age 48.32 years) and randomized them into four groups: early active/sham taVNS and late active/sham taVNS. Participation in the study lasted 4 weeks; early groups underwent 2 weeks intervention immediately following the first study site visit (days 0-13) and late groups 2 weeks after the first study site visit (days 14-27). Active and sham taVNS included 2 weeks of daily 4-h neurostimulation at the tragus or earlobe, respectively. To assess memory, we used the Rey Auditory Verbal Learning Test.

RESULTS

Two weeks of active taVNS, but not sham taVNS, improved immediate recall and short-term memory score both in early and late groups. Furthermore, the improvements persisted over subsequent follow-up in early active taVNS. Importantly, the effect of active taVNS was superior to sham for immediate recall in both early and late groups. There were no statistical differences in delayed recall.

CONCLUSION

Our findings suggest that taVNS has potential to improve memory, particularly immediate recall, and may be an effective method in preventing memory loss and mitigating cognitive aging.

Transcutaneous cervical vagus nerve stimulation enhances second-language vocabulary acquisition while simultaneously mitigating fatigue and promoting focus

Transcutaneous vagus nerve stimulation (tVNS) is a promising technique for enhancing cognitive performance and skill acquisition. Yet, its efficacy for enhancing learning rate and long-term retention in an ecologically valid learning environment has not been demonstrated. We conducted two double-blind sham-controlled experiments examining the efficacy of auricular tVNS (taVNS: Experiment (1) and cervical tVNS (tcVNS: Experiment (2), on a 5 day second-language vocabulary acquisition protocol among highly selected career linguists at the US Department of Defense's premier language school. tcVNS produced accelerated recall performance during training (Day 2-4), benefits of which were maintained across a 24 h retention interval with no stimulation at the final test. Consistent with prior work, tcVNS also produced fatigue-mitigating and focus-promoting effects as measured by the Air Force Research Laboratory Mood Questionnaire. Based on the current and the previous findings supporting tVNS' efficacy on performance, training enhancement, and fatigue mitigation, we believe tcVNS to be an effective learning acceleration tool that can be utilized at language-teaching and other institutions focused on intensive training of cognitive skills.

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.

Non-invasive vagus nerve stimulation conditions increased invigoration and wanting in depression

BACKGROUND

Major depressive disorder (MDD) is often marked by impaired motivation and reward processing, known as anhedonia. Many patients do not respond to first-line treatments, and improvements in motivation can be slow, creating an urgent need for rapid interventions. Recently, we demonstrated that transcutaneous auricular vagus nerve stimulation (taVNS) acutely boosts effort invigoration in healthy participants, but its effects on depression remain unclear.

OBJECTIVE

To assess the impact of taVNS on effort invigoration and maintenance in a sample that includes patients with MDD, evaluating the generalizability of our findings.

METHODS

We used a single-blind, randomized crossover design in 30 patients with MDD and 29 matched (age, sex, and BMI) healthy control participants (HCP).

RESULTS

Consistent with prior findings, taVNS increased effort invigoration for rewards in both groups during Session 1 (p = .040), particularly for less wanted rewards in HCP (pboot < 0.001). However, invigoration remained elevated in all participants, and no acute changes were observed in Session 2 (Δinvigoration = 3.3, p = .12). Crucially, throughout Session 1, we found taVNS-induced increases in effort invigoration (pboot = 0.008) and wanting (pboot = 0.010) in patients with MDD, with gains in wanting maintained across sessions (Δwanting = 0.06, p = .97).

CONCLUSIONS

Our study replicates the invigorating effects of taVNS in Session 1 and reveals its generalizability to depression. Furthermore, we expand upon previous research by showing taVNS-induced conditioning effects on invigoration and wanting within Session 1 in patients that were largely sustained. While enduring motivational improvements present challenges for crossover designs, they are highly desirable in interventions and warrant further follow-up research.

Mechanisms for survival: vagal control of goal-directed behavior

Survival is a fundamental physiological drive, and neural circuits have evolved to prioritize actions that meet the energy demands of the body. This fine-tuning of goal-directed actions based on metabolic states ('allostasis') is deeply rooted in our brain, and hindbrain nuclei orchestrate the vital communication between the brain and body through the vagus nerve. Despite mounting evidence for vagal control of allostatic behavior in animals, its broader function in humans is still contested. Based on stimulation studies, we propose that the vagal afferent pathway supports transitions between survival modes by gating the integration of ascending bodily signals, thereby regulating reward-seeking. By reconceptualizing vagal signals as catalysts for goal-directed behavior, our perspective opens new avenues for theory-driven translational work in mental disorders.

The effects of transcutaneous auricular vagus nerve stimulation on visual memory performance and fatigue

Objectives

This study aims to investigate the effects of transcutaneous auricular vagus nerve stimulation (taVNS) on visual memory performance and fatigue in healthy individuals.

Patients and methods

Between April 10, 2022 and May 25, 2022, a total of 60 physical therapy and rehabilitation students (27 males, 33 females; mean age: 20.6±1.6 years; range, 18 to 24 years) were included in the study. The individuals were divided into two groups as the experimental group (n=30) and the control group (n=30). The experimental group received taVNS, mobile device supported games, and low-medium intensity aerobic exercises, while the control group received mobile device supported games and aerobic exercises. The personal information form was applied to all participants. The level of fatigue was measured using a computer-based evaluation and Fatigue Severity Scale (FSS) to analyze the visual memory performance.

Results

All parameters used to evaluate visual memory performance showed a significant difference, while the FSS scores showed no significant difference (p>0.05). Only one sub-parameter in the control group was significantly different, while none of the other sub-parameters or FSS scores were significantly different (p>0.05). There was a significant difference between the two groups in terms of two of the visual memory sub-parameters, although no significant difference was found for the results of one parameter and the FSS (p>0.05).

Conclusion

Our study results show that taVNS can produce positive effects on visual memory performance, although it does not apparently affect fatigue.

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.

Vagus nerve stimulation: mechanisms and factors involved in memory enhancement

Vagus nerve stimulation (VNS) has been recognized as a useful neuromodulation tool to target the central nervous system by electrical stimulation of peripheral nerves. Activation of the nucleus of the solitary tract (NTS) in the brainstem by vagal afferent nerve fibers allows for modulation of various higher order brain regions, including limbic and cerebral cortex structures. Along with neurological and psychiatric indications, clinical and preclinical studies suggest that VNS can improve memory. While the underlying mechanisms to improve memory with VNS involve brain areas, such as the prefrontal cortex and processes including alertness and arousal, here we focus on VNS-induced memory improvements related to the hippocampus, the main area implicated in memory acquisition. In addition, we detail research demonstrating that a targeted approach to VNS can modify memory outcomes and delve into the molecular mechanisms associated with these changes. These findings indicate that a greater understanding of VNS mechanisms while also considering stimulation parameters, administration site, timing in relation to training, and sex-specific factors, may allow for optimal VNS application to enhance memory.

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.

Priorities for research on neuromodulatory subcortical systems in Alzheimer's disease: Position paper from the NSS PIA of ISTAART

The neuromodulatory subcortical system (NSS) nuclei are critical hubs for survival, hedonic tone, and homeostasis. Tau-associated NSS degeneration occurs early in Alzheimer's disease (AD) pathogenesis, long before the emergence of pathognomonic memory dysfunction and cortical lesions. Accumulating evidence supports the role of NSS dysfunction and degeneration in the behavioral and neuropsychiatric manifestations featured early in AD. Experimental studies even suggest that AD-associated NSS degeneration drives brain neuroinflammatory status and contributes to disease progression, including the exacerbation of cortical lesions. Given the important pathophysiologic and etiologic roles that involve the NSS in early AD stages, there is an urgent need to expand our understanding of the mechanisms underlying NSS vulnerability and more precisely detail the clinical progression of NSS changes in AD. Here, the NSS Professional Interest Area of the International Society to Advance Alzheimer's Research and Treatment highlights knowledge gaps about NSS within AD and provides recommendations for priorities specific to clinical research, biomarker development, modeling, and intervention. HIGHLIGHTS: Neuromodulatory nuclei degenerate in early Alzheimer's disease pathological stages. Alzheimer's pathophysiology is exacerbated by neuromodulatory nuclei degeneration. Neuromodulatory nuclei degeneration drives neuropsychiatric symptoms in dementia. Biomarkers of neuromodulatory integrity would be value-creating for dementia care. Neuromodulatory nuclei present strategic prospects for disease-modifying therapies.

Motor and Cognitive Modulation of a Single Session of Transcutaneous Auricular Vagus Nerve Stimulation in Post Stroke Patients: A Pilot Study

Objective: The aim of the present study is to explore whether a single session of transcutaneous Vagus Nerve Stimulation (tVNS) can enhance the ipsilesional, and contralesional upper limb motor functions as well as cognitive functions in stroke patients. The effects of the stimulation were evaluated through two different tasks: the box and blocks test (BB), indexing manual dexterity, and the Go/No-go task, a visuomotor paradigm used to assess both motor readiness and response inhibition. Tests were administered without tVNS, during tVNS and during sham tVNS. Results: The BB showed a statistical difference for both contralesional side (p = 0.05) between Basal-Real condition (p = 0.042) and ipsilesional side (p = 0.001) between Basal-Real (p = 0.008) and for Real-Sham (p = 0.005). Any statistical difference was found for the mean latencies in the three conditions of the Go/No-go test. Conclusion: A single session of tVNS seems to improve upper limb motor functions but not cognitive functions in post-stroke patients, despite a positive trend was detected.

Immediate effects and duration of a short and single application of transcutaneous auricular vagus nerve stimulation on P300 event related potential

Introduction

Transcutaneous auricular vagus nerve stimulation (taVNS) is a neuromodulatory technique that stimulates the auricular branch of the vagus nerve. The modulation of the locus coeruleus-norepinephrine (LC-NE) network is one of the potential working mechanisms of this method. Our aims were 1-to investigate if short and single applications of taVNS can modulate the P300 cognitive event-related potential (ERP) as an indirect marker that reflects NE brain activation under control of the LC, and 2-to evaluate the duration of these changes.

Methods

20 healthy volunteers executed an auditory oddball paradigm to obtain P300 and reaction time (RT) values. Then a 7 min active or sham taVNS period was initiated and simultaneously a new P300 paradigm was performed. We successively repeated the paradigm on 4 occasions with different time intervals up to 56 min after the stimulation onset.

Results

During active taVNS an immediate and significant effect of increasing the amplitude and reducing the latency of P300, as well as a shortening in the RT was observed. This effect was prolonged in time up to 28 min. The values then returned to pre-stimulation levels. Sham stimulation did not generate changes.

Discussion

Our results, demonstrate differential facilitating effects in a concrete time window after taVNS. Literature about the modulatory effect of taVNS over P300 ERP shows a wide spread of results. There is not a standardized system for taVNS and currently the great heterogeneity of stimulation approaches concerning targets and parameters, make it difficult to obtain conclusions about this relationship. Our study was designed optimizing several stimulation settings, such as a customized earbud stimulator, enlarged stimulating surface, simultaneous stimulation over the cymba and cavum conchae, a Delayed Biphasic Pulse Burst and current controlled stimulation that adjusted the output voltage and guaranteed the administration of a preset electrical dose. Under our stimulation conditions, targeting vagal nerve fibers via taVNS modulates the P300 in healthy participants. The optimal settings of modulatory function of taVNS on P300, and their interdependency is insufficiently studied in the literature, but our data provides several easily optimizable parameters, that will produce more robust results in future.

Short-term transcutaneous trigeminal nerve stimulation does not affect visual oddball task and paired-click paradigm ERP responses in healthy volunteers

Recent research suggests that transcutaneous trigeminal nerve stimulation (TNS) may positively affect cognitive function. However, no clear-cut evidence is available yet, since the majority of it derives from clinical studies, and the few data on healthy subjects show inconsistent results. In this study, we report the effects of short-term TNS on event-related potentials (ERP) recorded during the administration of a simple visual oddball task and a paired-click paradigm, both considered useful for studying brain information processing functions. Thirty-two healthy subjects underwent EEG recording before and after 20 min of sham- or real-TNS, delivered bilaterally to the infraorbital nerve. The amplitude and latency of P200 and P300 waves in the simple visual oddball task and P50, N100 and P200 waves in the paired-click paradigm were measured before and after treatment. Our results show that short-term TNS did not alter any of the ERP parameters measured, suggesting that in healthy subjects, short-term TNS may not affect brain processes involved in cognitive functions such as pre-attentional processes, early allocation of attention and immediate memory. The perspective of having an effective, non-pharmacological, non-invasive, and safe treatment option for cognitive decline is particularly appealing; therefore, more research on the positive effects on cognition of TNS is definitely needed.

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

BACKGROUND

Non-invasive transcutaneous auricular vagus nerve stimulation (taVNS) has received tremendous attention as a potential neuromodulator of cognitive and affective functions, which likely exerts its effects via activation of the locus coeruleus-noradrenaline (LC-NA) system. Reliable effects of taVNS on markers of LC-NA system activity, however, have not been demonstrated yet.

METHODS

The aim of the present study was to overcome previous limitations by pooling raw data from a large sample of ten taVNS studies (371 healthy participants) that collected salivary alpha-amylase (sAA) as a potential marker of central NA release.

RESULTS

While a meta-analytic approach using summary statistics did not yield any significant effects, linear mixed model analyses showed that afferent stimulation of the vagus nerve via taVNS increased sAA levels compared to sham stimulation (b = 0.16, SE = 0.05, p = 0.001). When considering potential confounders of sAA, we further replicated previous findings on the diurnal trajectory of sAA activity.

CONCLUSION(S)

Vagal activation via taVNS increases sAA release compared to sham stimulation, which likely substantiates the assumption that taVNS triggers NA release. Moreover, our results highlight the benefits of data pooling and data sharing in order to allow stronger conclusions in research.

Understanding the roles of central and autonomic activity during sleep in the improvement of working memory and episodic memory

The last decade has seen significant progress in identifying sleep mechanisms that support cognition. Most of these studies focus on the link between electrophysiological events of the central nervous system during sleep and improvements in different cognitive domains, while the dynamic shifts of the autonomic nervous system across sleep have been largely overlooked. Recent studies, however, have identified significant contributions of autonomic inputs during sleep to cognition. Yet, there remain considerable gaps in understanding how central and autonomic systems work together during sleep to facilitate cognitive improvement. In this article we examine the evidence for the independent and interactive roles of central and autonomic activities during sleep and wake in cognitive processing. We specifically focus on the prefrontal-subcortical structures supporting working memory and mechanisms underlying the formation of hippocampal-dependent episodic memory. Our Slow Oscillation Switch Model identifies separate and competing underlying mechanisms supporting the two memory domains at the synaptic, systems, and behavioral levels. We propose that sleep is a competitive arena in which both memory domains vie for limited resources, experimentally demonstrated when boosting one system leads to a functional trade-off in electrophysiological and behavioral outcomes. As these findings inevitably lead to further questions, we suggest areas of future research to better understand how the brain and body interact to support a wide range of cognitive domains during a single sleep episode.

Non-invasive vagus nerve stimulation in epilepsy patients enhances cooperative behavior in the prisoner's dilemma task

The vagus nerve constitutes a key link between the autonomic and the central nervous system. Previous studies provide evidence for the impact of vagal activity on distinct cognitive processes including functions related to social cognition. Recent studies in animals and humans show that vagus nerve stimulation is associated with enhanced reward-seeking and dopamine-release in the brain. Social interaction recruits similar brain circuits to reward processing. We hypothesize that vagus nerve stimulation (VNS) boosts rewarding aspects of social behavior and compare the impact of transcutaneous VNS (tVNS) and sham stimulation on social interaction in 19 epilepsy patients in a double-blind pseudo-randomized study with cross-over design. Using a well-established paradigm, i.e., the prisoner’s dilemma, we investigate effects of stimulation on cooperative behavior, as well as interactions of stimulation effects with patient characteristics. A repeated-measures ANOVA and a linear mixed-effects model provide converging evidence that tVNS boosts cooperation. Post-hoc correlations reveal that this effect varies as a function of neuroticism, a personality trait linked to the dopaminergic system. Behavioral modeling indicates that tVNS induces a behavioral starting bias towards cooperation, which is independent of the decision process. This study provides evidence for the causal influence of vagus nerve activity on social interaction.

“The Wandering Nerve Linking Heart and Mind” – The Complementary Role of Transcutaneous Vagus Nerve Stimulation in Modulating Neuro-Cardiovascular and Cognitive Performance

The vagus nerve is the longest nerve in the human body, providing afferent information about visceral sensation, integrity and somatic sensations to the CNS via brainstem nuclei to subcortical and cortical structures. Its efferent arm influences GI motility and secretion, cardiac ionotropy, chonotropy and heart rate variability, blood pressure responses, bronchoconstriction and modulates gag and cough responses via palatine and pharyngeal innervation. Vagus nerve stimulation has been utilized as a successful treatment for intractable epilepsy and treatment-resistant depression, and new non-invasive transcutaneous (t-VNS) devices offer equivalent therapeutic potential as invasive devices without the surgical risks. t-VNS offers exciting potential as a therapeutic intervention in cognitive decline and aging populations, classically affected by reduced cerebral perfusion by modulating both limbic and frontal cortical structures, regulating cerebral perfusion and improving parasympathetic modulation of the cardiovascular system. In this narrative review we summarize the research to date investigating the cognitive effects of VNS therapy, and its effects on neurocardiovascular stability.

Vagus Nerve Stimulation as a Potential Therapy in Early Alzheimer's Disease: A Review

Cognitive dysfunction in Alzheimer's disease (AD) is caused by disturbances in neuronal circuits of the brain underpinned by synapse loss, neuronal dysfunction and neuronal death. Amyloid beta and tau protein cause these pathological changes and enhance neuroinflammation, which in turn modifies disease progression and severity. Vagal nerve stimulation (VNS), via activation of the locus coeruleus (LC), results in the release of catecholamines in the hippocampus and neocortex, which can enhance synaptic plasticity and reduce inflammatory signalling. Vagal nerve stimulation has shown promise to enhance cognitive ability in animal models. Research in rodents has shown that VNS can have positive effects on basal synaptic function and synaptic plasticity, tune inflammatory signalling, and limit the accumulation of amyloid plaques. Research in humans with invasive and non-invasive VNS devices has shown promise for the modulation of cognition. However, the direct stimulation of the vagus nerve afforded with the invasive procedure carries surgical risks. In contrast, non-invasive VNS has the potential to be a broadly available therapy to manage cognitive symptoms in early AD, however, the magnitude and specificity of its effects remains to be elucidated, and the non-inferiority of the effects of non-invasive VNS as compared with invasive VNS still needs to be established. Ongoing clinical trials with healthy individuals and patients with early AD will provide valuable information to clarify the potential benefits of non-invasive VNS in cognition and AD. Whether invasive or non-invasive VNS can produce a significant improvement on memory function and whether its effects can modify the progression of AD will require further investigation.

Toward Diverse or Standardized: A Systematic Review Identifying Transcutaneous Stimulation of Auricular Branch of the Vagus Nerve in Nomenclature

OBJECTIVES

After 20 years of development, there is confusion in the nomenclature of transcutaneous stimulation of the auricular branch of the vagus nerve (ABVN). We performed a systematic review of transcutaneous stimulation of ABVN in nomenclature.

MATERIALS AND METHODS

A systematic search of the literature was carried out, using the bibliographic search engine PubMed. The search covered articles published up until June 11, 2020. We recorded the full nomenclature and abbreviated nomenclature same or similar to transcutaneous stimulation of ABVN in the selected eligible studies, as well as the time and author information of this nomenclature.

RESULTS

From 261 studies, 67 full nomenclatures and 27 abbreviated nomenclatures were finally screened out, transcutaneous vagus nerve stimulation and tVNS are the most common nomenclature, accounting for 38.38% and 42.06%, respectively. In a total of 97 combinations of full nomenclatures and abbreviations, the most commonly used nomenclature for the combination of transcutaneous vagus nerve stimulation and tVNS, accounting for 30.28%. Interestingly, the combination of full nomenclatures and abbreviations is not always a one-to-one relationship, there are ten abbreviated nomenclatures corresponding to transcutaneous vagus nerve stimulation, and five full nomenclatures corresponding to tVNS. In addition, based on the analysis of the usage habits of nomenclature in 21 teams, it is found that only three teams have fixed habits, while other different teams or the same team do not always use the same nomenclature in their paper.

CONCLUSIONS

The phenomenon of confusion in the nomenclature of transcutaneous stimulation of ABVN is obvious and shows a trend of diversity. The nomenclature of transcutaneous stimulation of ABVN needs to become more standardized in the future.

Effects of Noninvasive Cervical Vagal Nerve Stimulation on Cognitive Performance But Not Brain Activation in Healthy Adults

OBJECTIVES

While preliminary evidence suggests that noninvasive vagal nerve stimulation (nVNS) may enhance cognition, to our knowledge, no study has directly assessed the effects of nVNS on brain function and cognitive performance in healthy individuals. The aim of this study was therefore to assess whether nVNS enhances complex visuospatial problem solving in a normative sample. Functional magnetic resonance imaging (fMRI) was used to examine underlying neural substrates.

MATERIAL AND METHODS

Participants received transcutaneous cervical nVNS (N = 15) or sham (N = 15) stimulation during a 3 T fMRI scan. Stimulation lasted for 2 min at 24 V for nVNS and at 4.5 V for sham. Subjects completed a matrix reasoning (MR) task in the scanner and a forced-choice recognition task outside the scanner. An analysis of variance (ANOVA) was used to assess group differences in cognitive performance. And linear mixed effects (LMEs) regression analysis was used to assess main and interaction effects of experimental groups, level of MR task difficulty, and recall accuracy on changes in blood oxygen level-dependent (BOLD) signal.

RESULTS

Subjects who received nVNS showed higher accuracy for both easy (p = 0.017) and hard (p = 0.013) items of the MR task, slower reaction times for hard items (p = 0.014), and fewer false negative errors during the forced-choice recognition task (p = 0.047). MR task difficulty related to increased activation in frontoparietal regions (p < 0.001). No difference between nVNS and sham stimulation was found on BOLD response during performance of the MR task.

CONCLUSIONS

We hypothesize that nVNS increased attention compared to sham, and that this effect led to enhanced executive functions, and consequently to better performance on visuospatial reasoning and recognition tasks. Results provide initial support that nVNS may be a low-risk, low-cost treatment for cognitive disorders.

The potential of invasive and non-invasive vagus nerve stimulation to improve verbal memory performance in epilepsy patients

It has been demonstrated that acute vagus nerve stimulation (VNS) improves word recognition memory in epilepsy patients. Transcutaneous auricular vagus nerve stimulation (taVNS) has gained interest as a non-invasive alternative to improve cognition. In this prospective randomized cross-over study, we investigated the effect of both invasive VNS and taVNS on verbal memory performance in 15 patients with drug-resistant epilepsy. All patients conducted a word recognition memory paradigm in 3 conditions: VNS ON, VNS OFF and taVNS (3-period 3-treatment cross-over study design). For each condition, patients memorized 21 highlighted words from text paragraphs. Afterwards, the intervention was delivered for 30 s. Immediate recall and delayed recognition scores were obtained for each condition. This memory paradigm was repeated after 6 weeks of VNS therapy in 2 conditions: VNS ON and VNS OFF (2-period 2-treatment cross-over study design). Acute VNS and taVNS did not improve verbal memory performance. Immediate recall and delayed recognition scores were significantly improved after 6 weeks of VNS treatment irrespective of the acute intervention. We can conclude that the previously described positive effects of invasive VNS on verbal memory performance could not be replicated with invasive VNS and taVNS. An improved verbal memory performance was seen after 6 weeks of VNS treatment, suggesting that longer and more repetitive stimulation of the vagal pathway is required to modulate verbal memory performance.Clinical trial registration number: NCT05031208.

Transcutaneous Auricular Vagus Nerve Stimulation Improves Spatial Working Memory in Healthy Young Adults

Working memory (WM) is one of the core components of higher cognitive functions. There exists debate regarding the extent to which current techniques can enhance human WM capacity. Here, we examined the WM modulation effects of a previously less studied technique, transcutaneous auricular vagus nerve stimulation (taVNS). In experiment 1, a within-subject study, we aimed to investigate whether and which stimulation protocols of taVNS can modulate spatial WM performance in healthy adults. Forty-eight participants performed baseline spatial n-back tasks (1, 3-back) and then received online taVNS, offline taVNS, or sham stimulation before or during (online group) the posttest of spatial n-back tasks in random order. Results showed that offline taVNS could significantly increase hits in spatial 3-back task, whereas no effect was found in online taVNS or sham group. No significant taVNS effects were found on correct rejections or reaction time of accurate trials (aRT) in both online and offline protocols. To replicate the results found in experiment 1 and further investigate the generalization effect of offline taVNS, we carried out experiment 2. Sixty participants were recruited and received offline taVNS or offline earlobe stimulation in random order between baseline and posttests of behavioral tests (spatial/digit 3-back tasks). Results replicated the findings; offline taVNS could improve hits but not correct rejections or aRT in spatial WM performance, which were found in experiment 1. However, there were no significant stimulation effects on digit 3-back task. Overall, the findings suggest that offline taVNS has potential on modulating WM performance.

Transcutaneous auricular vagus nerve stimulation cannot modulate the P3b event-related potential in healthy volunteers

OBJECTIVE

The release of cortical norepinephrine is one of the possible mechanisms of action of vagus nerve stimulation (VNS), a neuromodulatory treatment currently under investigation for cognitive impairment. Transcutaneous auricular VNS (taVNS) may be able to activate vagal nerve branches ending in the brainstem's locus coeruleus (LC) non-invasively. The aim was to investigate if acute taVNS can modulate the P3b, a cognitive event-related potential (ERP) reflecting noradrenergic brain activation under control of the LC.

METHODS

Thirty-nine healthy volunteers performed an auditory oddball task during no stimulation, sham stimulation and taVNS in a randomized order. P3b amplitude, latency and behavioral outcome parameters were compared between conditions using linear mixed models.

RESULTS

P3b amplitude and latency during taVNS did not differ significantly from sham or control. Reaction time shortened and P3b latency prolonged with repetition of the oddball task.

CONCLUSIONS

We were unable to modulate cognitive ERPs by means of acute taVNS in a large group of healthy volunteers.

SIGNIFICANCE

Targeting vagal nerve fibres via a transcutaneous approach did not alter the P3b in healthy participants. The stimulation parameters used and transient delivery of taVNS might be insufficient to adequately modulate the LC. Also, a disbalanced locus coeruleus - norepinephrine system in patients may be more prone for improvement.

Vagus nerve afferent stimulation: Projection into the brain, reflexive physiological, perceptual, and behavioral responses, and clinical relevance

The afferent vagus nerves project to diverse neural networks within the brainstem and forebrain, based on neuroanatomical, neurophysiological, and functional (fMRI) brain imaging evidence. In response to afferent vagal stimulation, multiple homeostatic visceral reflexes are elicited. Physiological stimuli and both invasive and non-invasive electrical stimulation that activate the afferent vagus elicit perceptual and behavioral responses that are of physiological and clinical significance. In the present review, we address these multiple roles of the afferent vagus under normal and pathological conditions, based on both animal and human evidence.

Establishment of Emotional Memories Is Mediated by Vagal Nerve Activation: Evidence from Noninvasive taVNS

Emotional memories are better remembered than neutral ones, but the mechanisms leading to this memory bias are not well understood in humans yet. Based on animal research, it is suggested that the memory-enhancing effect of emotion is based on central noradrenergic release, which is triggered by afferent vagal nerve activation. To test the causal link between vagus nerve activation and emotional memory in humans, we applied continuous noninvasive transcutaneous auricular vagus nerve stimulation (taVNS) during exposure to emotional arousing and neutral scenes and tested subsequent, long-term recognition memory after 1 week. We found that taVNS, compared with sham, increased recollection-based memory performance for emotional, but not neutral, material. These findings were complemented by larger recollection-related brain potentials (parietal ERP Old/New effect) during retrieval of emotional scenes encoded under taVNS, compared with sham. Furthermore, brain potentials recorded during encoding also revealed that taVNS facilitated early attentional discrimination between emotional and neutral scenes. Extending animal research, our behavioral and neural findings confirm a modulatory influence of the vagus nerve in emotional memory formation in humans.SIGNIFICANCE STATEMENT Emotionally relevant information elicits stronger and more enduring memories than nonrelevant information. Animal research has shown that this memory-enhancing effect of emotion is related to the noradrenergic activation in the brain, which is triggered by afferent fibers of the vagus nerve (VN). In the current study, we show that noninvasive transcutaneous auricular VN stimulation enhances recollection-based memory formation specifically for emotionally relevant information as indicated by behavioral and electrophysiological indices. These human findings give novel insights into the mechanisms underlying the establishment of emotional episodic memories by confirming the causal link between the VN and memory formation which may help understand the neural mechanisms underlying disorders associated with altered memory functions and develop treatment options.

Vagus nerve stimulation in brain diseases: Therapeutic applications and biological mechanisms

Brain diseases, including neurodegenerative, cerebrovascular and neuropsychiatric diseases, have posed a deleterious threat to human health and brought a great burden to society and the healthcare system. With the development of medical technology, vagus nerve stimulation (VNS) has been approved by the Food and Drug Administration (FDA) as an alternative treatment for refractory epilepsy, refractory depression, cluster headaches, and migraines. Furthermore, current evidence showed promising results towards the treatment of more brain diseases, such as Parkinson's disease (PD), autistic spectrum disorder (ASD), traumatic brain injury (TBI), and stroke. Nonetheless, the biological mechanisms underlying the beneficial effects of VNS in brain diseases remain only partially elucidated. This review aims to delve into the relevant preclinical and clinical studies and update the progress of VNS applications and its potential mechanisms underlying the biological effects in brain diseases.

Non-invasive vagal nerve stimulation enhances cognitive emotion regulation

Transcutaneous auricular vagus nerve stimulation (taVNS) has been proposed as a potential new tool in the treatment of major depressive disorder. Prior studies have demonstrated that taVNS enhances cognitive control and is able to modulate brain activity in key regions involved in cognitive emotion regulation, such as the anterior cingulate and medial prefrontal cortex, which is known to be impaired in depressed patients. Preclinical studies are lacking but may provide important insights into the working mechanisms of taVNS on cognitive emotion regulatory processes. In this between-subject study, 83 healthy subjects underwent a single-session of active taVNS or sham stimulation, after which cognitive reappraisal was examined using a computer-based cognitive emotion regulation task. Our results indicate that participants receiving active taVNS, compared to sham, were better at using cognitive reappraisal and rated their response to emotion-eliciting pictures as less intense. Yet, even though we found significant differences in behavioral measures of cognitive emotion regulation, no differences between groups were found in terms of physiological responses to the emotional stimuli. Overall, these findings suggest a positive effect of taVNS on the cognitive reappraisal of emotions, but future studies assessing objective measures of neural activity during cognitive emotion regulation following taVNS are warranted.

Transcutaneous auricular vagus nerve stimulation induces stabilizing modifications in large-scale functional brain networks: towards understanding the effects of taVNS in subjects with epilepsy

Transcutaneous auricular vagus nerve stimulation (taVNS) is a novel non-invasive brain stimulation technique considered as a potential supplementary treatment option for subjects with refractory epilepsy. Its exact mechanism of action is not yet fully understood. We developed an examination schedule to probe for immediate taVNS-induced modifications of large-scale epileptic brain networks and accompanying changes of cognition and behaviour. In this prospective trial, we applied short-term (1 h) taVNS to 14 subjects with epilepsy during a continuous 3-h EEG recording which was embedded in two standardized neuropsychological assessments. From these EEG, we derived evolving epileptic brain networks and tracked important topological, robustness, and stability properties of networks over time. In the majority of investigated subjects, taVNS induced measurable and persisting modifications in network properties that point to a more resilient epileptic brain network without negatively impacting cognition, behaviour, or mood. The stimulation was well tolerated and the usability of the device was rated good. Short-term taVNS has a topology-modifying, robustness- and stability-enhancing immediate effect on large-scale epileptic brain networks. It has no detrimental effects on cognition and behaviour. Translation into clinical practice requires further studies to detail knowledge about the exact mechanisms by which taVNS prevents or inhibits seizures.

International Consensus Based Review and Recommendations for Minimum Reporting Standards in Research on Transcutaneous Vagus Nerve Stimulation (Version 2020)

Given its non-invasive nature, there is increasing interest in the use of transcutaneous vagus nerve stimulation (tVNS) across basic, translational and clinical research. Contemporaneously, tVNS can be achieved by stimulating either the auricular branch or the cervical bundle of the vagus nerve, referred to as transcutaneous auricular vagus nerve stimulation(VNS) and transcutaneous cervical VNS, respectively. In order to advance the field in a systematic manner, studies using these technologies need to adequately report sufficient methodological detail to enable comparison of results between studies, replication of studies, as well as enhancing study participant safety. We systematically reviewed the existing tVNS literature to evaluate current reporting practices. Based on this review, and consensus among participating authors, we propose a set of minimal reporting items to guide future tVNS studies. The suggested items address specific technical aspects of the device and stimulation parameters. We also cover general recommendations including inclusion and exclusion criteria for participants, outcome parameters and the detailed reporting of side effects. Furthermore, we review strategies used to identify the optimal stimulation parameters for a given research setting and summarize ongoing developments in animal research with potential implications for the application of tVNS in humans. Finally, we discuss the potential of tVNS in future research as well as the associated challenges across several disciplines in research and clinical practice.