Why Does taVNS Usually Use the Left Ear？From Cardiac Safety Concerns to the Evidence on Stimulation Laterality.
Place an electrode on the ear and most people expect it to be on the left. That habit has a history, but it is easy to mistake history for proof. A 2024 scoping review found that among 84 clinical studies reporting stimulation laterality, about 62% used the left ear, 11% used the right ear, and 27% used both ears.
Why is the left side so common? Is the right ear risky for the heart? And has the left ear actually been shown to work better? The simplest explanation is a cautious convention. Once an early protocol seemed safe and workable, later studies often kept the same side, leaving less incentive to test alternatives.
Reviews published in 2026 point in the same direction. They describe left, right, and bilateral stimulation as accepted approaches, but they do not treat the number of left-sided studies as a ranking of efficacy. In cognitive and stroke-rehabilitation reviews, left-ear protocols remain dominant, while the optimal side and stimulation parameters are still unsettled.
The left-ear convention began in the neck
Implanted vagus nerve stimulation came first. Its electrode is usually placed on the left cervical vagus nerve. The reason is cardiac physiology. The right vagus has a stronger influence on the sinoatrial node, which sets the heartbeat, while the left side has a stronger influence on atrioventricular conduction. Avoiding direct effects on heart rate made the left side the conservative choice.
The division is not absolute. Classic animal work showed that strong stimulation on either side can slow the heart or alter conduction. Left-sided stimulation was a cautious starting point, not a guarantee that the other side could never matter.
When researchers moved from the neck to the ear, the old convention came with them. The electrode was now contacting a different set of fibers, but the protocol already felt familiar.
Ear stimulation follows a different route
Cervical stimulation can affect both incoming sensory fibers and outgoing autonomic fibers in the vagus nerve. taVNS mainly targets the auricular branch of the vagus nerve, a sensory pathway in parts of the outer ear. Signals travel to the brainstem and then engage wider networks that can influence the body.
In a 2015 human imaging study, Frangos and colleagues stimulated the left cymba conchae in 12 healthy adults. Compared with earlobe control, the stimulation changed activity in regions that included the nucleus of the solitary tract, a brainstem hub for visceral sensation and autonomic regulation.
Figure 1. Electrode placement for taVNS at the concha and external ear canal. Adapted from Ng et al. (2024).
The cardiac concern linked to right-sided neck stimulation therefore cannot be transferred directly to the right ear. Ear stimulation is still capable of influencing cardiovascular measures, but the size and direction of the response depend on the site and parameters.
The right ear is already being tested clinically
The right ear has not been excluded from clinical research. In the TREAT-AF trial, 53 people with paroxysmal atrial fibrillation received either one hour of right-ear tragus stimulation each day or earlobe control for six months. The active group had a lower atrial-fibrillation burden, and no device-related adverse effects were observed. This showed that right-ear stimulation could be tested in a selected clinical population, but the trial did not establish that right-ear stimulation was safer or more effective than left-ear stimulation.
A 2022 safety review reached a similar practical conclusion. It included 177 studies and 6,322 participants. Only five studies used right-sided stimulation, so the evidence base was uneven. Still, the available data did not show that right-ear taVNS carries a distinctive long-term cardiac risk.
Laterality depends on what you want to change
“Effective” can mean a change in heart-rate variability, a shift in brain connectivity, or an improvement in a symptom. A side that produces a signal in one measure may not be the best side for another.
De Couck and colleagues compared ten minutes of left-ear, right-ear, and no-current stimulation in the same participants. Only right-ear stimulation produced a significant increase in SDNN, one measure of heart-rate variability. The left ear did not show the same significant change. That finding shows that a positive cardiovascular signal can occur on the right, but it does not prove a general right-ear advantage.
Ng and colleagues provided a more visual comparison in a 2024 cough study. Under the same 80 Hz ear-canal condition, the average natural cough threshold fell from 0.28 to 0.19 mol/L on the left, while it rose from 0.25 to 0.28 mol/L on the right. The directions were opposite. The study was small and its formal significance test compared two left-ear conditions, so these numbers are a clue rather than a prescription.
Figure 2. Changes in natural cough threshold across left- and right-ear taVNS conditions. Adapted from Ng et al. (2024).
Neuser and colleagues studied effort for food and monetary rewards. Left-ear stimulation produced a stronger food-specific effect, while the right ear did not show the same selectivity. The overall motivation effect, however, did not favor the left side. The left-sided advantage appeared in one specific reward-related effect, while the overall motivation effect did not favor either side.
Monti and colleagues studied 50 healthy adults with left, right, and bilateral stimulation during fMRI. Left and right stimulation changed different functional connections. That pattern suggests partially distinct brain-network effects, but it does not provide a single “stronger side.”
Figure 3. Brainstem-related functional connectivity changes during left-, right-, and bilateral-ear taVNS. Adapted from Monti et al. (2025).
So why does the left ear remain the default?
Because it is familiar, extensively used, and easy to compare with earlier studies. Reusing the left-sided protocol reduces one source of variation between experiments. That is a reasonable research convenience. It is not evidence that the left ear always produces better treatment outcomes.
Figure 4. Evidence-tiered working model of the ear-to-brain pathways engaged by taVNS. Adapted from the 2026 disorders-of-consciousness review.
Figure 5. Auricular stimulation targets used in taVNS research. Adapted from Barbetti et al. (2025).
For now, left-ear taVNS is a sensible default when a protocol needs a well-established starting point. If the goal is to treat a specific symptom, the decision should follow direct comparisons using the same ear region, stimulation parameters, and outcome measures.The relevant choice depends on the side, the ear region, the stimulation parameters, and the clinical problem being treated.
参考文献
Ardell, J. L., & Randall, W. C. (1986). Selective vagal innervation of sinoatrial and atrioventricular nodes in canine heart. American Journal of Physiology, 251(4), H764–H773. https://doi.org/10.1152/ajpheart.1986.251.4.H764
Badran, B. W., et al. (2018). Short trains of transcutaneous auricular vagus nerve stimulation have parameter-specific effects on heart rate. Brain Stimulation, 11(4), 699–708. https://doi.org/10.1016/j.brs.2018.04.004
Barbetti, L., et al. (2025). Effects of stimulation site and protocol on autonomic responses to auricular vagus nerve stimulation. Frontiers in Neuroscience, 19, 1692530. https://doi.org/10.3389/fnins.2025.1692530
Butt, M. F., et al. (2020). The anatomical basis for transcutaneous auricular vagus nerve stimulation. Journal of Anatomy, 236(4), 588–611. https://doi.org/10.1111/joa.13122
De Couck, M., Cserjesi, R., Caers, R., Zijlstra, W. P., Widjaja, D., Wolf, N., Luminet, O., Ellrich, J., & Gidron, Y. (2017). Effects of short and prolonged transcutaneous vagus nerve stimulation on heart rate variability in healthy subjects. Autonomic Neuroscience, 203, 88–96. https://doi.org/10.1016/j.autneu.2016.11.003
Frangos, E., Ellrich, J., & Komisaruk, B. R. (2015). Non-invasive access to the vagus nerve central projections via electrical stimulation of the external ear: fMRI evidence in humans. Brain Stimulation, 8(3), 624–636. https://doi.org/10.1016/j.brs.2014.11.018
Gerges, A. N. H., et al. (2024). Clinical application of transcutaneous auricular vagus nerve stimulation: A scoping review. Disability and Rehabilitation, 46(24), 5730–5760. https://doi.org/10.1080/09638288.2024.2313123
Kim, A. Y., et al. (2022). Safety of transcutaneous auricular vagus nerve stimulation: A systematic review and meta-analysis. Scientific Reports, 12, 22055. https://doi.org/10.1038/s41598-022-25864-1
Mei, S., Fu, G., Gao, R., & Li, W. (2026). Transcutaneous auricular vagus nerve stimulation for disorders of consciousness: A narrative review of neurophysiological mechanisms, clinical evidence, and future directions. Frontiers in Human Neuroscience, 20, 1837913. https://doi.org/10.3389/fnhum.2026.1837913
Monti, D. A., Wintering, N., Vedaei, F., Steinmetz, B., Mohamed, S., & Newberg, A. (2025). Changes in brain functional connectivity associated with transcutaneous auricular vagus nerve stimulation in healthy controls. Frontiers in Human Neuroscience, 19, 1531123. https://doi.org/10.3389/fnhum.2025.1531123
Neuser, M. P., et al. (2020). Vagus nerve stimulation boosts the drive to work for rewards. Nature Communications, 11, 3555. https://doi.org/10.1038/s41467-020-17344-9
Ng, K. B., Guiu Hernandez, E., Haszard, J., Macrae, P., Huckabee, M.-L., & Cakmak, Y. O. (2024). Transcutaneous auricular vagus nerve stimulation alters cough sensitivity depending on stimulation parameters: Potential implications for aspiration risk. Frontiers in Neuroscience, 18, 1265894. https://doi.org/10.3389/fnins.2024.1265894
Stavrakis, S., et al. (2020). TREAT-AF: Transcutaneous electrical vagus nerve stimulation to suppress atrial fibrillation: A randomized clinical trial. JACC: Clinical Electrophysiology, 6(3), 282–291. https://doi.org/10.1016/j.jacep.2019.11.008
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Zhang, et al. (2026). Effectiveness of transcutaneous auricular vagus nerve stimulation in stroke rehabilitation: A systematic review and meta-analysis of randomized clinical trials. Frontiers in Neurology, 17, 1786103. https://doi.org/10.3389/fneur.2026.1786103
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