This study examines the reliability and agreement of autonomic nervous system measurements obtained from different anatomical sites during transcutaneous auricular vagus nerve stimulation (taVNS). taVNS is a non-invasive electrical stimulation delivered to the ear using a small stimulator. Healthy volunteers aged 18-40 years will participate in one laboratory session. Heart rate and heart rate variability will be recorded from the chest (reference), finger, and arm. Blood pressure and pulse will also be measured. Data will be collected in three standardized periods: T0 (5-minute resting baseline), T1 (10 minutes during taVNS), and T2 (5-minute recovery). The main goal is to determine how closely finger- and arm-based measurements match the chest reference and how consistent these measurements are across the study periods. Participation is voluntary, and participants may withdraw at any time. Expected risks are minimal and may include temporary tingling or mild discomfort at the ear and, rarely, lightheadedness. No direct medical benefit is expected, but the findings may help improve how autonomic responses are monitored during taVNS in future research.

Heart rate variability (HRV) is widely used to quantify autonomic regulation, and non-invasive transcutaneous auricular vagus nerve stimulation (taVNS) may acutely influence cardiac autonomic activity. In both clinical and research settings, autonomic measures can be recorded from different sites using different sensor technologies. However, it is not always clear whether HRV-related indices derived from peripheral recordings (e.g., finger or arm photoplethysmography-based signals) provide comparable results to chest-based reference recordings. This study is designed as a methodological reliability and agreement investigation to determine the consistency of autonomic measures across anatomical sites during a standardized taVNS session.

Participants will attend a single laboratory visit with standardized pre-test instructions (e.g., avoidance of heavy exercise and stimulants before measurement, as applicable). Autonomic measures will be recorded from multiple sites in parallel, using the chest recording as the reference for beat-to-beat intervals. Finger and arm recordings will be obtained simultaneously to allow direct, time-aligned comparisons across sites within the same physiological state. Blood pressure and pulse will be measured during the session to provide complementary hemodynamic context.

The session consists of consecutive measurement periods representing baseline, stimulation, and recovery. Data will be processed using standard HRV pipelines (including artifact inspection and correction when needed) to derive time-domain and frequency-domain indices. The primary analyses will focus on (1) inter-site agreement (bias and limits of agreement) and (2) reliability/consistency metrics across the measurement periods. Agreement will be evaluated using Bland-Altman approaches and related indices, while reliability will be quantified using intraclass correlation coefficients (ICCs) with appropriate model specification. Secondary analyses may explore the association between HRV indices and hemodynamic parameters across periods.

This study involves minimal risk. Expected adverse effects, if any, are transient (e.g., mild ear discomfort or tingling, and rarely lightheadedness). Participant safety will be monitored by the study team throughout the session, and the procedure will be stopped if a participant requests discontinuation or if concerning symptoms occur. Data will be recorded and stored in a de-identified format, and results will be reported at the group level.

Inclusion Criteria:

• Age 18 to 40 years.
• Healthy volunteers without known chronic disease or acute illness at the time of participation.
• Able and willing to comply with study procedures and instructions.
• Provided written informed consent.

Exclusion Criteria:

• Any cardiovascular disease, arrhythmia, hypertension, or similar condition. Any neurological disorder (e.g., diabetes mellitus, peripheral neuropathy, epilepsy).
• Any diagnosed psychiatric disorder.
• Pregnancy or suspected pregnancy.
• Ear conditions that prevent stimulation (infection, open wound, pain/tenderness) or the presence of a piercing at/near the stimulation site.
• Vigorous exercise within 24 hours prior to the measurement.
• Caffeine intake, smoking, or alcohol consumption within 4-6 hours prior to the measurement.
• Marked intolerance or hypersensitivity to the devices or the procedure.
• Inability to follow instructions during measurements or refusal to complete the session.

Individual participant data (IPD) will not be shared at this time. Only aggregated, de-identified results will be reported in publications. The dataset includes sensitive physiological measurements, and access will be limited to the research team to protect participant privacy. IPD sharing may be reconsidered in the future upon reasonable request and with appropriate ethical and institutional approvals.

No masking was implemented; this is an open-label, single-session methodological study without sham stimulation.