Why Does Stimulation Location Matter in taVNS?

How can a small patch of skin on the ear serve as a site for vagus nerve stimulation? A branch of the vagus nerve reaches the ear and carries sensory information from it to the brainstem. Transcutaneous auricular vagus nerve stimulation (taVNS) uses electrodes on the ear to deliver electrical pulses intended to activate this auricular branch of the vagus nerve (ABVN). Where the electrode sits determines which area of nerve supply lies beneath it.

The cymba conchae is often a preferred site because its vagal nerve supply is relatively well documented, and stimulation there produces responses in related brainstem regions. Human studies comparing it with sites such as the tragus and ear canal have also found advantages worth considering.

In Figure 1, the arrow labeled “Cymba conchae” points to the small hollow above the crus of the helix, the ridge that crosses the concha. The larger hollow below it is the cavum conchae, labeled “Cavity of conchae.” The tragus is the small projection in front of the ear canal opening. These landmarks are close together, but their nerve supply differs. They help us locate the electrode on the surface; to understand what lies beneath it, we need to look under the skin.

Figure 1. The crus of the helix divides the concha into upper and lower parts. The cymba conchae lies above it, and the cavum conchae, labeled “Cavity of conchae,” lies below it. The tragus sits in front of the ear canal opening.

What Makes the Nerve Supply of the Cymba Conchae Distinctive?

The cymba conchae contains the auricular vagal branch that stimulation aims to reach. In a classic anatomical study, this branch supplied all the sensory innervation identified in that area, while the neighboring cavum conchae often shared its supply with another nerve.

Several nerves provide sensation to the outer ear. Alongside the auricular vagal branch are the great auricular nerve from the cervical plexus and the auriculotemporal nerve, a branch associated with the trigeminal nerve. Moving an electrode can therefore change the mix of nerves exposed to stimulation. Figure 2 shows the approximate territories.

Figure 2. Green marks the territory associated with the auricular vagal branch, and blue marks the auriculotemporal nerve territory. Much of the remaining area is supplied mainly by the great auricular nerve. The colored boundaries summarize classic anatomical descriptions and are approximate.

In 2002, Peuker and Filler dissected 14 ears from seven cadavers and traced the origins of the nerves supplying the outer ear. In these specimens, the cymba conchae was supplied exclusively by the auricular vagal branch. The cavum conchae differed: 45% had an exclusively vagal supply, while 55% also received branches from the great auricular nerve. A single ridge separates the two hollows, yet their nerve supply was different.

Identifying these tiny branches required magnification. The researchers exposed the nerves, traced their origins, then colored and photographed them before compiling a map of the ear. The colored territories reflected nerves they had followed through the tissue.

The boundaries on a nerve map do not confine electrical current to the same colored areas. Even where the sensory supply comes mainly from one nerve, electrode placement and current still affect which fibers are stimulated. Nor can observations from 14 ears tell us that everyone has exactly the same anatomy.

Finding the nerve in the cymba conchae raises a further question. What happens in the brainstem when electrical pulses are applied to the skin above it?

How Does the Brainstem Respond to Cymba Conchae Stimulation?

Stimulation at the cymba conchae can produce a response well beyond the ear. In a human study by Frangos and colleagues, stimulation of the left cymba conchae produced greater activity than earlobe stimulation in vagus-related regions, including the nucleus of the solitary tract and the locus coeruleus.

The ear’s connections with the rest of the body sometimes show up in unexpected ways. Some people cough when their external ear canal is stimulated. This is Arnold’s ear-cough reflex, which involves sensory input through the auricular vagal branch. A stimulus in the ear can trigger a cough elsewhere in the body. During electrical stimulation of the cymba conchae, however, a person’s sensations cannot tell us exactly what is happening in the brain. Imaging allows researchers to observe those changes.

The nucleus of the solitary tract (NTS) lies in the brainstem, receives information carried by the vagus nerve, and has connections with the locus coeruleus (LC). The LC is involved in noradrenaline regulation and functions such as arousal and attention. Changes in these regions during ear stimulation can help reveal how stimulation affects related pathways in the brain.

In 2015, Frangos and colleagues studied 12 healthy adults who received stimulation at the left cymba conchae and the earlobe. The team recorded brain responses using functional magnetic resonance imaging (fMRI). The earlobe was a useful comparison because it produces a sensation when stimulated but lacks an auricular vagal supply in classic anatomical descriptions. Comparing the two sites helps distinguish the response to cymba conchae stimulation from the effects of feeling stimulation on the ear more generally. Earlobe stimulation can itself affect brain activity.

fMRI measures nerve activity indirectly through changes in blood oxygenation. When an area “lights up” in an image, that reflects a corresponding change in the blood oxygen signal; it does not identify the individual nerve fibers activated in the ear. This study observed responses in the NTS, LC, and other regions, but did not measure whether symptoms of a particular disease improved.

The cymba conchae showed these responses compared with the earlobe. How would the tragus and ear canal perform? Both have also attracted interest as stimulation sites.

How Does the Cymba Conchae Compare with Other Stimulation Sites?

In a study of 37 people, the cymba conchae was the only site that met the statistical significance threshold against the earlobe control in both the NTS and the LC. The inner tragus met that threshold only in the NTS. The lower rear wall of the ear canal met it in neither region.

Published in 2017 by Yakunina and colleagues, the study tested four locations on the left ear in the same healthy participants: the inner tragus, the lower rear wall of the ear canal, the cymba conchae, and the earlobe. The team then examined the same brain regions for each location. Having each person receive stimulation at different sites reduced the influence of differences between separate groups of people.

Table 1. Comparisons of mean brainstem t-values between each stimulation site and the earlobe control within the same study. [4]

Note. Adapted from Figure 7 and the brain-region analysis in the original paper, with correction for multiple comparisons. A mean t-value is a statistical measure, not the percentage of nerves activated. A result that is not statistically significant does not mean there was no response.

“Significant” in the table means that the statistical difference in brain-region activity between that site and the earlobe met the study’s threshold. The mean t-value does not tell us what percentage of nerves were activated. The cymba conchae met the threshold in both target regions, making it a site worth prioritizing when the aim is to influence these pathways.

The tragus falling short of that threshold in the LC does not mean it produced no response there. Brain-imaging data vary. Observing a change and establishing a statistical difference from the earlobe are separate things.

Likewise, a significant difference between the cymba conchae and the earlobe does not automatically establish one between the cymba conchae and the tragus. Deciding which of those two sites performs better requires a direct comparison and a clear outcome of interest. For the two brainstem regions examined in this experiment, the cymba conchae’s results make it an appealing choice.

The strongest practical case for the cymba conchae comes from studies that place it beside other ear targets and measure what changes. García de Gurtubay and colleagues did this in 26 healthy volunteers. They stimulated the cymba conchae alone, the cymba and cavum conchae together, and the earlobe, then recorded vagus somatosensory evoked potentials (VSEPs). Clear responses appeared whenever the cymba conchae was included. Neither earlobe condition produced a measurable VSEP. For a reader trying to choose one reliable place to start, that result points back to the cymba conchae.

Covering the neighboring cavum conchae increased the VSEP amplitude, and the larger electrode was more comfortable at higher current levels. That finding matters for device design. A wider contact can recruit more fibers and spread charge over a larger surface. Yet the cymba conchae remains the anchor of the setup. It has the clearest vagal anatomical rationale, it generated a reproducible response on its own, and adding the cavum broadened coverage rather than replacing the cymba target. VSEP amplitude measures an immediate electrical response in healthy volunteers, so it cannot predict clinical benefit by itself. Taken together with the anatomical and fMRI findings already discussed, however, this study makes the choice clearer. The cymba conchae is currently the best-supported single auricular target for taVNS, while combined cymba-cavum stimulation is a promising way to extend that target.

Figure 4. Stimulation topographies and electrode sizes tested by García de Gurtubay et al. CC indicates simultaneous cymba and cavum conchae stimulation, C indicates cymba stimulation, and L indicates earlobe stimulation; X and S indicate extra-large and small electrodes. Reproduced from García de Gurtubay et al. (2021), Figure 1, under the Creative Commons Attribution License.

How Was Electrical Stimulation Delivered?

Once the cymba conchae has been selected, stimulation still has an intensity and a rhythm. In this comparison study, pulse frequency and duration were the same across sites, while current was adjusted according to what each participant felt at each location.

The electrodes delivered pulses at 25 Hz, or 25 pulses per second. Each pulse lasted 500 μs, equivalent to 0.5 milliseconds. Stimulation continued for 30 seconds, followed by a 60-second rest. Four cycles made up one six-minute scan. Each site was tested in two scans, totaling 12 minutes, of which four minutes involved active stimulation.

Different parts of the same person’s ear may feel the same current differently. The researchers first measured the level at which a participant could just feel the stimulus, then the level at which it became painful. These are the sensory and pain thresholds. For the experiment, current was set 0.1 mA below the pain threshold, allowing the intensity at each site to reflect the participant’s tolerance.

With this approach, the average current was 0.91 mA at the cymba conchae and 0.77 mA at the tragus, a statistically significant difference. Brain responses were therefore recorded under conditions in which both location and actual current differed. The differences in response cannot all be attributed to location alone.

Stimulation also requires two electrodes. When the team stimulated the tragus, ear canal, or cymba conchae, the other electrode sat on the outer tragus. For earlobe stimulation, it sat on the back of the earlobe. Both positions influence how current passes through tissue, so “cymba conchae stimulation” also involves a choice about where to place the second electrode.

From the outside, the electrode sits on a small hollow. The nerves it aims to influence lie beneath the skin, alongside blood vessels and other tissue. A closer view shows what is under the electrode.

What Lies Beneath the Skin of the Cymba Conchae?

Nerves in the cymba conchae run between the epidermis and cartilage, with blood vessels nearby. Their paths and depths are difficult to show on a surface map of the ear. Three-dimensional imaging offers a view inside this small hollow.

In 2020, Dabiri and colleagues used high-resolution episcopic microscopy (HREM) to reconstruct a series of tissue sections as a three-dimensional model. Figure 3 comes from one cymba conchae specimen. Start with the epidermis at the top and the cartilage beneath it, then look between them for the green nerves, red arteries, and blue veins.

Figure 3. A three-dimensional reconstruction of cymba conchae tissue shows the spatial relationships between the epidermis, nerves, blood vessels, and cartilage. Colors distinguish the structures; the green nerves lie below the skin surface. Reproduced in full from Dabiri et al., 2020, original Figure 3, under CC BY.

Following the green structures reveals nerves running below the epidermis, near blood vessels. The electrode rests on the ear’s surface, while the tissue it aims to influence has depth. Even within the cymba conchae, the precise contact point and the position of the second electrode remain relevant to the stimulation setup.

This image shows the internal structure of one specimen; the arrangement may differ between people. Green identifies nerves, but their appearance alone cannot establish whether each branch comes from the vagus nerve.

Why Is the Cymba Conchae a Preferred Stimulation Site?

The cymba conchae deserves priority because this small hollow contains the auricular vagal branch that stimulation aims to reach, and human studies have observed responses in related brainstem regions when it is stimulated. Small shifts in position on the ear can place an electrode over a different nerve supply.

Auricular vagus nerve stimulation aims to use sensory nerves in the ear to send signals toward the brainstem. The exclusively vagal supply reported in the classic anatomical study made the cymba conchae a suitable candidate. Later human experiments found responses in regions including the NTS and LC compared with earlobe stimulation. These regions are associated with incoming vagal signals, giving us a reason to keep this site in focus.

When tested alongside the tragus and ear canal in the same experiment, the cymba conchae was also the only site to meet the statistical significance threshold against the earlobe control in both the NTS and the LC. For approaches intended to influence these brainstem pathways through ear stimulation, it is a site worth considering first.

Its nerve supply and the responses observed in people help explain why the cymba conchae is so often chosen. Site selection is one part of a stimulation protocol; electrode placement and intensity also affect the result. Whether stimulation improves symptoms of a particular disease depends on the clinical outcomes measured for that condition.

This is why ZenoWell’s electrode design targets the cymba conchae and also extends to the neighboring cavum conchae. By bringing both regions into a single earpiece, the design provides broader coverage of the concha while keeping the cymba conchae as its primary target. It puts the anatomy discussed here into practice, with stimulation contacts positioned across both the upper and lower conchal hollows.

FAQ

Where is the cymba conchae located?

The cymba conchae is the small hollow above the crus of the helix in the outer ear. The cavum conchae is the larger hollow below it, while the tragus is located in front of the ear canal opening.

Why is the cymba conchae used in taVNS?

The cymba conchae is commonly studied in taVNS because of its anatomical relationship with the auricular branch of the vagus nerve (ABVN). Research has also observed responses in vagus-related brain regions when this area is stimulated.

Is the cymba conchae better than the tragus for taVNS?

Current evidence does not establish that one stimulation site is universally better. The cymba conchae has strong anatomical support and is one of the most studied taVNS targets, but results depend on factors such as stimulation parameters, electrode placement, and the outcome being measured.

References

[1] Peuker, E. T.; Filler, T. J. — The nerve supply of the human auricle — Clinical Anatomy, 15(1), 35–37 — 2002. https://doi.org/10.1002/ca.1089

[2] Butt, M. F.; Albusoda, A.; Farmer, A. D.; Aziz, Q. — The anatomical basis for transcutaneous auricular vagus nerve stimulation — Journal of Anatomy, 236(4), 588–611 — 2020. https://doi.org/10.1111/joa.13122

[3] Frangos, E.; Ellrich, J.; Komisaruk, B. R. — 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 — 2015. https://doi.org/10.1016/j.brs.2014.11.018

[4] Yakunina, N.; Kim, S. S.; Nam, E.-C. — Optimization of Transcutaneous Vagus Nerve Stimulation Using Functional MRI — Neuromodulation, 20(3), 290–300 — 2017. https://doi.org/10.1111/ner.12541

[5] Mercante, B.; Deriu, F.; Rangon, C.-M. — Auricular Neuromodulation: The Emerging Concept beyond the Stimulation of Vagus and Trigeminal Nerves — Medicines, 5(1), 10 — 2018. https://doi.org/10.3390/medicines5010010

[6] Dabiri, B.; et al. — High-Resolution Episcopic Imaging for Visualization of Dermal Arteries and Nerves of the Auricular Cymba Conchae in Humans — Frontiers in Neuroanatomy, 14, 22 — 2020. https://doi.org/10.3389/fnana.2020.00022

[7] García de Gurtubay, I.; Bermejo, P.; Lopez, M.; Larraya, I.; Librero, J. — Evaluation of different vagus nerve stimulation anatomical targets in the ear by vagus evoked potential responses — Brain and Behavior, 11, e2343 — 2021. https://doi.org/10.1002/brb3.2343