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)

Effects of off-line auricular transcutaneous vagus nerve stimulation (taVNS) on a short-term memory task: a pilot study

Introduction

One of the commonly used indices of short-term memory (STM) is the digit span task. Prior studies have proposed pupil dilation as a measure of task engagement and as a promising biomarker of vagal activation. Transcutaneous auricular vagus nerve stimulation (taVNS) is a novel non-invasive brain stimulation technique which might be used to improve cognition and modulate pupil size through its effects on the noradrenergic release in the locus coeruleus. No previous study has investigated the effects of off-line taVNS on a digit span task. With this single-blind, sham-controlled, crossover design trial, we aimed to assess whether taVNS was able to improve the digit span score, as well as to modulate the pupillary response to cognitive load in a sample of 18 elderly Japanese volunteers with no self-reported cognitive impairments.

Results

Subjects were randomized to receive either real or sham taVNS during a digit span task while recording the pupil size, and then switched over to the other treatment group. We found that real stimulation significantly reduced the mean number of errors performed at span length 7, 8, and 9 (-0.83, -0.90, and -0.39, respectively compared to pre-stimulation values, and -0.71, -1.08, and -0.79, respectively, compared to sham stimulation). Additionally, real taVNS stimulation slightly but significantly increased the pupil size at all span lengths during the encoding period of the task, with larger effects for span 7-10 compared to pre-stimulation, and for span 5-10 compared to sham. No effect over the pupil size was found during the recall period.

Discussion

Our results suggest that taVNS might selectively improve the cognitive performance during the encoding phase of the task. Although further studies are needed to better clarify the optimal stimulation parameters, findings from this study could support the use of taVNS as a safe neuromodulation technique to improve cognitive function.

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.

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.

Continuous Transcutaneous Auricular Vagus Nerve Stimulation Increases Long-Latency Neural Processing in Multiple Sensory Modalities

Transcutaneous auricular vagus nerve stimulation (taVNS) is a noninvasive technique stimulating vagal afferent fibers, showing promise in treating neurological and mental disorders. taVNS is believed to activate the locus coeruleus (LC), promoting noradrenergic activation (NA), which enhances arousal and attention. However, evidence for the LC-NA hypothesis is mixed, and investigations in different sensory modalities are lacking. This study investigated whether continuous taVNS enhances standard NA markers along with neural processing in three sensory modalities (auditory, respiratory, and somatosensory). In a 2-day Sham-controlled crossover protocol, 45 healthy adults received taVNS at the cymba concha and Sham stimulation at the earlobe. During stimulation, participants experienced paired auditory clicks, inspiratory occlusions, and electrocutaneous stimuli, while EEG was acquired. Salivary alpha-amylase (sAA) and subjective experienced arousal were measured at pre-/end-stimulation. Resting-state EEG was measured pre-/poststimulation to assess alpha-band (8-13 Hz) oscillation power, and participants rated the intensity and unpleasantness of all stimuli. Auditory-, respiratory-related-, and somatosensory evoked potentials were measured, specifically P50, N1, and P2 components, as well as the P50/N1 amplitude difference of the second and the first stimulus in the pair (neural gating; S2-S1). Although no effects in P50 or N1 amplitudes were observed, P2 amplitudes in auditory and somatosensory blocks increased during taVNS. Self-reported arousal increased in the taVNS condition, with no effects on neural gating, sAA concentration, or resting-state alpha power. taVNS had no effect on self-reported intensity/unpleasantness of stimuli. These results highlight certain limitations posed by combining taVNS and EEG and underline the need for further mechanistic taVNS research.

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.

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 by activating the locus ceruleus-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.

Stimulate to Remember? The Effects of Short Burst of Transcutaneous Vagus Nerve Stimulation (taVNS) on Memory Performance and Pupil Dilation

The decline in noradrenergic (NE) locus coeruleus (LC) function in aging is thought to be implicated in episodic memory decline. Transcutaneous auricular vagus nerve stimulation (taVNS), which supports LC function, might serve to preserve or improve memory function in aging. However, taVNS effects are generally very heterogeneous, and it is currently unclear whether taVNS has an effect on memory. In this study, an emotional memory task with negative events involving the LC-NE system was combined with the short burst of event-related taVNS (3 s) in younger adults (N = 24). The aim was to investigate taVNS-induced changes in pupil dilation during encoding and possible taVNS-induced improvements in (emotional) memory performance for early and delayed (24 h) recognition. Negative events were associated with increased pupil dilation and better memory performance. Additionally, real as compared to sham or no stimulation selectively increased memory for negative events. Short bursts of stimulation, whether real or sham, led to an increase in pupil dilation and an improvement in memory performance over time, likely due to the attention-inducing sensory modulation of electrical stimulation.

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.

Transcutaneous Auricular Vagus Nerve Stimulation Does Not Accelerate Fear Extinction: A Randomized, Sham-Controlled Study

Transcutaneous auricular vagus nerve stimulation (taVNS) has been tested as a strategy to facilitate fear extinction learning based on the hypothesis that taVNS increases central noradrenergic activity. Four studies out of six found taVNS to enhance extinction learning especially at the beginning of extinction. Facilitatory effects of taVNS were mainly observed in US expectancy, less in fear-potentiated startle (FPS), and not in the skin conductance response (SCR). Suboptimal stimulation parameters may explain the reported mixed results. Also, variability in selected fear conditioning paradigms and statistical power impedes the comparability between studies. This study sought to further test whether taVNS accelerates fear extinction learning as indexed by US expectancy, FPS, and SCR. Similar to most previous studies, we employed a differential fear conditioning paradigm. The left ear of 79 healthy participants was stimulated with either sham (earlobe) or taVNS (cymba concha) during extinction learning. To maximize the beneficial effects of taVNS, the stimulation of the left cymba concha was administered continuously at the maximum level below the pain threshold. Results of the pre-registered frequentist and exploratory Bayesian analyses indicate that taVNS did not accelerate extinction learning in any of the outcomes. The null results indicate that taVNS with commonly used stimulation parameters does not reliably optimize fear extinction learning. More research is needed to test if the stimulation protocol determines the efficacy of taVNS in optimizing fear extinction learning.

Earable Multimodal Sensing and Stimulation: A Prospective Towards Unobtrusive Closed-Loop Biofeedback

The human ear has emerged as a bidirectional gateway to the brain's and body's signals. Recent advances in around-the-ear and in-ear sensors have enabled the assessment of biomarkers and physiomarkers derived from brain and cardiac activity using ear-electroencephalography (ear-EEG), photoplethysmography (ear-PPG), and chemical sensing of analytes from the ear, with ear-EEG having been taken beyond-the-lab to outer space. Parallel advances in non-invasive and minimally invasive brain stimulation techniques have leveraged the ear's access to two cranial nerves to modulate brain and body activity. The vestibulocochlear nerve stimulates the auditory cortex and limbic system with sound, while the auricular branch of the vagus nerve indirectly but significantly couples to the autonomic nervous system and cardiac output. Acoustic and current mode stimuli delivered using discreet and unobtrusive earables are an active area of research, aiming to make biofeedback and bioelectronic medicine deliverable outside of the clinic, with remote and continuous monitoring of therapeutic responsivity and long-term adaptation. Leveraging recent advances in ear-EEG, transcutaneous auricular vagus nerve stimulation (taVNS), and unobtrusive acoustic stimulation, we review accumulating evidence that combines their potential into an integrated earable platform for closed-loop multimodal sensing and neuromodulation, towards personalized and holistic therapies that are near, in- and around-the-ear.

Impact of Stimulation Duration in taVNS-Exploring Multiple Physiological and Cognitive Outcomes

Transcutaneous auricular vagus nerve stimulation (taVNS) is a non-invasive neuromodulation technique that modulates the noradrenergic activity of the locus coeruleus (LC). Yet, there is still uncertainty about the most effective stimulation and reliable outcome parameters. In a double blind, sham-controlled study including a sample of healthy young individuals (N = 29), we compared a shorter (3.4 s) and a longer (30 s) stimulation duration and investigated the effects of taVNS (real vs. sham) on saliva samples (alpha amylase and cortisol concentration), pupil (pupillary light reflex and pupil size at rest) and EEG data (alpha and theta activity at rest, ERPs for No-Go signals), and cognitive tasks (Go/No-Go and Stop Signal Tasks). Salivary alpha amylase concentration was significantly increased in the real as compared to sham stimulation for the 30 s stimulation condition. In the 3.4 s stimulation condition, we found prolonged reaction times and increased error rates in the Go/No-Go task and increased maximum acceleration in the pupillary light reflex. For the other outcomes, no significant differences were found. Our results show that prolonged stimulation increases salivary alpha-amylase, which was expected from the functional properties of the LC. The finding of longer response times to short taVNS stimulation was not expected and cannot be explained by an increase in LC activity. We also discuss the difficulties in assessing pupil size as an expression of taVNS-mediated LC functional changes.

Tonic and phasic transcutaneous auricular vagus nerve stimulation (taVNS) both evoke rapid and transient pupil dilation

BACKGROUND

Transcutaneous auricular vagus nerve stimulation (tVNS or taVNS) is a non-invasive method of electrical stimulation of the afferent pathway of the vagus nerve, suggested to drive changes in putative physiological markers of noradrenergic activity, including pupil dilation.

OBJECTIVE

However, it is unknown whether different taVNS modes can map onto the phasic and tonic modes of noradrenergic activity. The effects of taVNS on pupil dilation in humans are inconsistent, largely due to differences in stimulation protocols. Here, we attempted to address these issues.

METHODS

We investigated pupil dilation under phasic (1 s) and tonic (30 s) taVNS, in a pre-registered, single-blind, sham-controlled, within-subject cross-over design, in the absence of a behavioural task.

RESULTS

Phasic taVNS induced a rapid increase in pupil size over baseline, significantly greater than under sham stimulation, which rapidly declined after stimulation offset. Tonic taVNS induced a similarly rapid (and larger than sham) increase in pupil size over baseline, returning to baseline within 5 s, despite the ongoing stimulation. Thus, both active and sham tonic modes closely resembled the phasic effect. There were no differences in tonic baseline pupil size, and no sustained effects of stimulation on tonic baseline pupil size.

CONCLUSIONS

These results suggest that both phasic- and tonic-like taVNS under the standard stimulation parameters may modulate primarily the phasic mode of noradrenergic activity, as indexed by evoked pupil dilation, over and above somatosensory effects. This result sheds light on the temporal profile of phasic and tonic stimulation, with implications for their applicability in further research.

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.

Pupillary response to percutaneous auricular vagus nerve stimulation in alcohol withdrawal syndrome: A pilot trial

BACKGROUND

Autonomic symptoms in alcohol withdrawal syndrome (AWS) are associated with a sympathetic-driven imbalance of the autonomic nervous system. To restore autonomic balance in AWS, novel neuromodulatory approaches could be beneficial. We conducted a pilot trial with percutaneous auricular vagus nerve stimulation (pVNS) in AWS and hypothesized that pVNS will enhance the parasympathetic tone represented by a reduction of pupillary dilation in a parasympatholytic pharmacological challenge.

METHODS

Thirty patients suffering from alcohol use disorder, undergoing AWS, and stable on medication, were recruited in this open-label, single-arm pilot trial with repeated-measure design. Peripheral VNS (monophasic volt impulses of 1 msec, alternating polarity, frequency 1 Hz, amplitude 4 mV) was administered at the left cymba conchae for 72 h, followed by pupillometry under a tropicamide challenge. We assessed craving with a visual analog scale. We used pupillary mean as the dependent variable in a repeated-measures ANOVA (rmANOVA).

RESULTS

A repeated-measures ANOVA resulted in a significant difference for pupillary diameter across time and condition (F(2,116) = 27.97, p < .001, ηp2 > .14). Tukey-adjusted post hoc analysis revealed a significant reduction of pupillary diameter after pVNS. Alcohol craving was significantly reduced after pVNS (p < .05, Cohen's d = 1.27).

CONCLUSION

Our study suggests that pVNS activates the parasympathetic nervous system in patients with acute AWS, and that this activation is measurable by pupillometry. To this end, pVNS could be beneficial as a supportive therapy for AWS. Potential confounding effects of anti-craving treatment should be kept in mind.