Frequency Effects of Transcutaneous Trigeminal Nerve Stimulation (eTNS) on Brain and Autonomic Function

Xidian University25 enrolled

Overview

This study aims to investigate how different frequencies of transcutaneous Trigeminal Nerve Stimulation (eTNS) affect brain activity and the autonomic nervous system in healthy individuals. Participants will undergo a 3T functional magnetic resonance imaging (fMRI) scan while receiving three different types of nerve stimulation: 2Hz eTNS, 120Hz eTNS, and a sham (inactive) stimulation. The order of these stimulations will be randomly assigned. During the brain scan, researchers will simultaneously monitor the participants' breathing and heart rhythms (using a finger sensor). The main goal is to understand how changing the frequency of eTNS influences the connection between the brain's networks and the body's unconscious physiological responses.

This is an exploratory, randomized, sham-controlled, within-subject crossover fMRI study designed to evaluate the frequency-dependent effects of transcutaneous Trigeminal Nerve Stimulation (eTNS) on central-autonomic coupling in healthy volunteers. Participants will undergo functional neuroimaging in a 3T MRI scanner. The imaging protocol consists of a structural T1-weighted scan and three functional Blood Oxygenation Level-Dependent (BOLD) sequences corresponding to three intervention arms: 2Hz-eTNS, 120Hz-eTNS, and Sham stimulation. The order of the three stimulation conditions will be randomized for each participant. To minimize potential carryover effects, the structural T1 scan will be acquired between the two active (real) stimulation sequences. Each functional BOLD sequence will last for 7 minutes and 30 seconds and will utilize a block design. This design consists of alternating 30-second "ON" blocks (where the stimulation is active) and 30-second "OFF" blocks (where the stimulation is inactive). Concurrently with the fMRI acquisition, continuous physiological monitoring will be conducted. This includes recording respiratory effort and photoplethysmography (PPG) via a finger pulse oximeter to capture peripheral autonomic metrics. The primary and secondary analyses will focus on quantifying BOLD signal responses, evaluating dynamic changes in brain functional connectivity, assessing variations in autonomic nervous system metrics (such as heart rate variability derived from the PPG), and exploring the mechanisms of central-autonomic coupling across the different stimulation frequencies.

Study Type

INTERVENTIONAL

Allocation

RANDOMIZED

Interventions

2Hz Transcutaneous Trigeminal Nerve StimulationDEVICE

Active electrical stimulation applied over the skin of the forehead targeting the branches of the trigeminal nerve. The stimulation frequency is set at 2Hz. It is synchronized with the fMRI sequence and continuous physiological monitoring (respiration and photoplethysmography).

120Hz Transcutaneous Trigeminal Nerve StimulationDEVICE

Active electrical stimulation applied over the skin of the forehead targeting the branches of the trigeminal nerve. The stimulation frequency is set at 120Hz. It is synchronized with the fMRI sequence and continuous physiological monitoring (respiration and photoplethysmography).

Sham Transcutaneous Trigeminal Nerve StimulationDEVICE

An inactive or sensory-matched sham electrical stimulation designed to mimic the physical sensation of the device being active without delivering the therapeutic frequencies (2Hz or 120Hz). It is administered synchronously with the fMRI BOLD sequence and physiological monitoring to serve as a baseline comparator.

Eligibility

Sex: ALLMin age: 18 YearsMax age: 35 YearsHealthy volunteers:

Medical Language ↔ Plain English

Inclusion Criteria: 1. Healthy volunteers, aged 18 to 40 years old. 2. Right-handed. 3. Generally healthy with no history of neurological, psychiatric, or severe cardiovascular diseases. 4. Normal physical and neurological examinations. 5. Capable of understanding the study procedures and voluntarily signing the written informed consent form. Exclusion Criteria: 1. Contraindications to MRI scanning (e.g., claustrophobia, cardiac pacemakers, artificial cochlea, metallic braces, or any other ferromagnetic implants). 2. Female participants who are pregnant, lactating, or suspect they might be pregnant. 3. Contraindications to transcutaneous electrical stimulation (e.g., personal or family history of epilepsy/seizures, trigeminal neuralgia, facial skin lesions, or active skin diseases on the forehead). 4. Current or recent (within the past month) use of any medications known to affect the central nervous system or the autonomic nervous system (e.g., antidepressants, beta-blockers, sedatives, or sympathomimetics). 5. History of substance abuse, heavy smoking, or excessive daily consumption of alcohol or caffeine. 6. Irregular sleep patterns, shift work, or severe sleep deprivation within 24 hours prior to the scanning session.

Outcomes

Primary Outcomes

Difference in Blood Oxygenation Level-Dependent (BOLD) Signal Amplitude in the Brainstem

The metric is the percent signal change in BOLD amplitude within predefined Regions of Interest (ROIs) of the brainstem (e.g., trigeminal nuclei, locus coeruleus, nucleus tractus solitarii). It is calculated as the BOLD signal difference between the 30-second stimulation "ON" blocks and the 30-second "OFF" blocks. Data will be aggregated as the mean percentage of BOLD signal change for each stimulation condition.

Time frame: During the 7-minute and 30-second fMRI scan for each stimulation condition.

Secondary Outcomes

Functional Connectivity (FC) within and outside the Central Autonomic Network

The strength of dynamic and static functional connectivity between key nodes of the CAN and other whole-brain regions. Connectivity matrices will be computed and compared across the 2Hz-eTNS, 120Hz-eTNS, and Sham conditions to evaluate how different stimulation frequencies modulate brain network communications.

Time frame: During the 7-minute and 30-second fMRI scan for each stimulation condition.

Change in Heart Rate Variability Assessed by Low Frequency to High Frequency (LF/HF) Ratio

The LF/HF ratio is a frequency-domain measure of heart rate variability derived from PPG data, reflecting the sympathovagal balance. Data will be aggregated as the mean LF/HF ratio during each stimulation condition.

Time frame: Continuously recorded during the 7-minute and 30-second fMRI scan for each stimulation condition.

Change in Respiratory Rate

Respiratory rate is extracted from concurrent respiratory belt signals to assess respiratory patterns. Data will be aggregated as the mean respiratory rate (measured in breaths per minute) during each stimulation condition.

Time frame: Continuously recorded during the 7-minute and 30-second fMRI scan for each stimulation condition.

Central-Autonomic Coupling Index

The degree of synchronization between central neural activity and peripheral autonomic output. This will be quantified by calculating the correlation coefficients or mutual information between the low-frequency fluctuations of the BOLD signals in CAN regions and the concurrent peripheral autonomic time-series data (HRV and respiration). The coupling strength will be compared between the 2Hz and 120Hz conditions.

Time frame: Computed from data acquired during the 7-minute and 30-second fMRI scan for each stimulation condition.