Transcutaneous Auricular Vagus Nerve Stimulation on the Human Ear

Begin with a human participant seated in a comfortable position.

On the left ear, identify the tragus, a small, rounded structure located at the entrance of the outer ear canal.

Behind the tragus lies the anterior wall of the outer ear canal, which is innervated by the auricular branch of the vagus nerve or ABVN.

The wall is a target site for transcutaneous auricular vagus nerve stimulation, or taVNS, a noninvasive approach for modulating brain function.

Clean the ear and apply conductive gel to the electrodes.

Position the positive electrode on the target site and the negative electrode over the tragus.

Deliver the lowest stimulation intensity that elicits a sensation at the target site, known as the perceptual threshold.

Then, apply an intensity higher than the threshold to stimulate the ABVN.

The resulting nerve impulse travels to the brainstem and is relayed to various brain regions to influence their activity.

Begin by seating the participant in a relaxed position, with the legs elevated, and the head supported. Inspect the left ear of the participant, confirming that no jewelry is attached, that all makeup and lotion are removed, and that there are no skin-related contraindications at the site of stimulation.

Find the anterior wall of the outer ear canal, landmarked by the tragus, to locate the stimulation target. Alternative sites, like the cymba concha, may be explored as alternative active targets. Then, use an alcohol prep pad to gently scrub the target site, both internally and externally, to decrease the skin resistance and to increase the conductance.

Before placing the electrodes, visually inspect the materials to confirm that a clean, corrosion-free surface is exposed before disinfecting the electrodes to prevent the spread of bacteria between participants. Next, use a narrow wooden applicator to spread a less than 1 millimeter layer of conductive paste evenly across the surface of each electrode.

And connect the electrode cables to the stimulation device in the off mode. After verifying the polarity of the electrodes, clip the spring electrode onto the tragus, with the anode contacting the anterior wall of the outer ear canal and the cathode contacting the anterior region of the tragus.

Then, confirm that the participant is comfortable, and that the regional blood flow has not been disrupted by asking participant if they are OK. To determine the perceptual threshold, turn on the stimulator and set the output to 3 milliamps. Deliver a 1 second train of transcutaneous auricular vagus nerve stimulation, or TAVNS, at the appropriate experimental pulse width and frequency.

Ask the participant whether they felt the stimulation. The sensation is typically reported as a tickle or pricking sensation. If the answer is yes, turn down stimulation intensity by 50% and repeat the stimulation. Repeat stimulation until a minimum of four yes responses are achieved, for which the fourth yes response comes after a no.

The perceptual threshold intensity is the value at which the participant gives the fourth yes. Once the perceptual threshold has been determined, use a computer running a pulse-generating graphic user interface connected to a data acquisition unit to drive the stimulation system. The software should output transistor-transistor logic or TTL pulses as programmable settings.

Set the stimulation to be delivered at 200% of the perceptual threshold, and set the guidelines for the duty cycles, as appropriate for conducting a long stimulation session. The length of the total stimulation session time should be varied between sessions.