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Original article: Revealing Differential Gamma Responses to Transcutaneous Auricular Vagus Nerve Stimulation Through Human Intracranial Recording.pdf 

You may have heard of "vagus nerve stimulation" — a neuromodulation technique widely studied for epilepsy, depression, and stroke rehabilitation. Transcutaneous auricular vagus nerve stimulation (taVNS), as its non-invasive version, has become a hot direction in both clinical and research settings due to its surgery-free nature and ease of operation. But did you know? With taVNS, simply adjusting the stimulation intensity can lead to vastly different brain responses: some intensities activate key brain regions, while others inhibit their activity.

A research team from Washington University School of Medicine, through direct human intracranial recording, has for the first time clearly revealed the differential gamma-band EEG responses elicited by different intensities of taVNS, providing direct human brain experimental evidence for precisely optimizing taVNS treatment parameters and achieving personalized neuromodulation. The related study was published in Neuromodulation.

Research Dilemma: The "Parameter Puzzle" of Non-Invasive taVNS, Awaiting Human Brain Evidence

The clinical value of vagus nerve stimulation (VNS) has been well established, but traditional invasive VNS requires surgical electrode implantation, whereas taVNS can achieve non-invasive stimulation through ear electrodes, making it easier to disseminate. However, a key question has long persisted in taVNS research: how stimulation parameters affect neural activity in the human brain has always lacked direct intracranial recording evidence.

Previous studies mostly employed a single intensity of taVNS, and were largely based on animal models or non-invasive brain imaging (such as fMRI). Meanwhile, the parameter patterns of invasive VNS cannot be directly applied to taVNS due to differences in nerve fiber distribution and stimulation methods. At the same time, the effects of taVNS varied across different studies, and the academic community speculates that the core reason is inconsistency in stimulation parameter selection.

To solve this puzzle, the research team focused on the intensity parameter of taVNS, utilizing stereoelectroencephalography (sEEG) electrodes implanted intracranially in patients with refractory epilepsy to directly record neural activity in the human brain under different intensities of taVNS — the most precise method currently available for capturing local neural responses in the brain, filling the research gap in "non-invasive taVNS human brain neural responses."

Study Design: Precise Parameter Control + Intracranial Recording, Rigorously Validating the Effect of Intensity

01 Study Subjects

10 patients with refractory epilepsy (8 ultimately included in analysis, 4 male and 4 female, mean age 39 years), all of whom had sEEG electrodes implanted for clinical monitoring purposes, with electrodes covering key brain regions of the vagus nerve pathway including the limbic-frontal-insular areas, providing natural conditions for intracranial recording.

02 Stimulation Protocol

A standardized taVNS device was used, stimulating only the left ear, with a unified setting of 30 Hz frequency, 250 μs pulse width, and 1-minute stimulation duration (30 seconds on + 1 second interval), varying only the stimulation intensity:

Active stimulation: Three intensity levels — 0.5 mA, 1.0 mA, 1.5 mA

Sham stimulation: 1.5 mA (electrode placed on the earlobe, not targeting the auricular branch of the vagus nerve)

03 Core Measurement Indicators

High-gamma band power (95–170 Hz) was used as the core indicator — neural activity in this band is the "gold standard marker" of local cortical processing, precisely reflecting the neural activation/inhibition status of specific brain regions.

04 Analysis Approach

Comparing high-gamma power changes across the whole brain, left hemisphere (stimulation side), right hemisphere, and key brain regions (orbitofrontal cortex, insula, anterior cingulate cortex, inferior frontal gyrus, etc.) under different intensities, while also analyzing the temporal dynamics of neural responses (whether transient activation or sustained response), and distinguishing differences between active and sham stimulation.

Core Findings: Intensity Determines Brain Response, Activation/Inhibition Shows Clear Gradient Characteristics

Through direct intracranial recording, the study found that the neural responses to taVNS exhibit significant intensity dependence. Different intensities not only affect the amplitude of activation but can even alter the direction of the response. Furthermore, responses in the left hemisphere (stimulation side) were far greater than in the right hemisphere, with key brain regions showing specificity:

1.0 mA and 1.5 mA: Significant Brain Activation, 1.5 mA Achieves Sustained Response

Both intensities elicited significant increases in whole-brain high-gamma power, with activation primarily concentrated in key vagus nerve pathway regions of the left hemisphere including the orbitofrontal cortex, insula, inferior frontal gyrus, and middle frontal gyrus. Among these, 1.5 mA was the optimal intensity, sustaining high-gamma activation in the orbitofrontal cortex and insula throughout the entire stimulation period, a sustained response that sham stimulation could not achieve.

Figure 2:PSD curves for selected electrode contacts in the sample subject. a. Positions of four contacts in radiological view. b. This panel shows all four contacts in both the 1.5 mA active condition and the 1.5 mA sham condition. Three of the contacts — orbitofrontal cortex (OFC), middle frontal gyrus (MFG), and insula — were selected from the ROI, while the remaining contact — precentral gyrus — was selected as the control location. The stimulation frequency (30 Hz) and its subsequent harmonics have been removed. Mean curves are shown with SD as shaded regions, and smoothed using a Gaussian-weighted average within a five-element sliding window for visualization. Colored frequency bars below each plot indicate the frequency bands where PSD during stimulation (active or sham) significantly differed from baseline (two-tailed Wilcoxon rank-sum test, uncorrected p < 0.05).

Figure 3:Group-level high-gamma responses across three active conditions, excluding sham responses. a. Distribution of high-gamma responses, covering whole-brain and hemispheric ranges. Kruskal-Wallis tests performed on the overall distributions (regardless of magnitude) showed significant differences across all analyzed groups. Post-hoc pairwise comparisons (Bonferroni-corrected) revealed significant differences between conditions. b. A threshold of 0.5 was set, showing only regions with absolute z-score ≥ 0.5 (z-score median relative to baseline) for visualization. Increased responses relative to baseline are shown in red tones, while decreased responses relative to baseline are shown in blue tones. Slices are displayed in radiological view. c. Differences between active and sham group-level high-gamma responses. A threshold of 0.5 was set, as in panel b, showing only regions where the absolute difference in z-score from sham was ≥ 0.5, also for visualization. Red indicates higher responses during active compared to sham, blue indicates lower responses during active compared to sham. Slices are also displayed in radiological view. ∗, Bonferroni-corrected p<0.05; ∗∗, Bonferroni-corrected p<0.01.

0.5 mA: Mild Inhibition of Specific Brain Regions, More Pronounced in the Right Hemisphere

The low intensity of 0.5 mA did not activate the brain, but instead produced mild inhibition in some brain regions, particularly a decrease in high-gamma power in the right hemisphere frontal regions and the right insula, an effect completely opposite to that of sham stimulation and high-intensity stimulation.

Figure 4:Temporal dynamics of high-gamma responses within the insula during stimulation in the sample subject. a. Each score represents the median z-score of a single contact within a 2-second window during stimulation. This figure includes all classified contacts within the insula. Solid lines represent the median z-score across all insular contacts for each time segment. b. Each bar shows the percentage of total stimulation time with z-score >1 and ≤1, across all four conditions. Response duration percentage was calculated by taking the number of time bins in panel a where z-score was >1 or ≤1, divided by the total number of time bins during stimulation. Bar plots show the median response duration across all contacts within the insula for the subject, with error bars representing median absolute deviation. Two-tailed Wilcoxon signed-rank test: ∗, Bonferroni-corrected p<0.05; ∗∗, Bonferroni-corrected p<0.01.    

Figure 5:Temporal dynamics of ROI group-level high-gamma responses during stimulation. ROIs include OFC, ACC, insula, IFG, and MFG. Each bar shows the percentage of total stimulation time with z-score >1, across all four conditions. Response duration percentage was calculated by dividing the number of time bins with z-score >1 by the total number of time bins during stimulation. Bar plots show the median response duration across all contacts in each ROI for each subject, with error bars representing median absolute deviation. Two-tailed Wilcoxon rank-sum test: ∗, Bonferroni-corrected p<0.05; ∗∗, Bonferroni-corrected p<0.01.

Sham Stimulation Elicits Weak Responses, but Significantly Differs from Active Stimulation

Although 1.5 mA sham stimulation could elicit slight high-gamma activation, the magnitude was far lower than that of active stimulation at the same intensity, and lacked sustained response characteristics, confirming that the specific neural response of taVNS originates from targeted stimulation of the auricular branch of the vagus nerve, rather than mere electrical stimulation perception.

Additionally, the study also found that the anterior cingulate cortex showed the shortest activation duration across all stimulation intensities, suggesting that this brain region has far lower sensitivity to taVNS compared to the orbitofrontal cortex and insula.

Mechanism Interpretation: Understanding the Logic Through the "Neuromodulation Switch" Model

The core of taVNS is to transmit signals to the central nervous system by stimulating the auricular branch of the vagus nerve in the ear, thereby modulating brain activity. The stimulation intensity determines the strength of the vagal afferent signals, ultimately eliciting different brain region responses, which can be simplified as an "intensity-signal-brain response" regulatory model:

High intensity (1.5 mA): Sufficient to activate effective afferent fibers of the auricular branch of the vagus nerve, with signals continuously transmitted to the basal forebrain (the core relay station of the vagus nerve pathway), thereby sustaining activation of its projected key brain regions such as the orbitofrontal cortex and insula, manifested as sustained elevation of high-gamma power — this is an "effective activation-type" response, with the core being the achievement of sustained neural conduction through the vagus nerve pathway.

Medium intensity (1.0 mA): Can activate some afferent fibers, producing transient central signals and eliciting transient brain region activation, but the signal strength is insufficient to maintain a sustained response, belonging to a "partial activation-type" response.

Low intensity (0.5 mA): Can only activate a small number of fibers, and may even produce mild inhibition of basal vagal nerve conduction. The signals transmitted to the central nervous system are weak and counter-directional, leading to inhibition of neural activity in some brain regions, manifested as decreased high-gamma power — this is a "mild inhibition-type" response.

The reason why the insula and orbitofrontal cortex become core responsive brain regions is also closely related to anatomical structure: the insula is an important hub in the vagus nerve pathway, participating in autonomic regulation and immune perception; the orbitofrontal cortex has direct anatomical connections with the basal forebrain, which is the key structure mediating VNS/taVNS neural responses. Therefore, these two brain regions become the "core targets" of taVNS.

Research Significance: From Basic Research to Clinical Application

The value of this study goes beyond understanding the "relationship between current intensity and brain response" — it moves taVNS from "empirical stimulation" toward "precision modulation," pointing out a clear direction for clinical translation:

01 Clinical Application Stratification

✦ Activation needs (epilepsy control, cognitive improvement, stroke rehabilitation): select medium-to-high intensity around 1.5 mA to achieve sustained activation of core brain regions; inhibition needs (inflammation regulation, subarachnoid hemorrhage intervention): 0.5 mA low intensity can be explored, modulating the brain-immune pathway through inhibition of brain regions such as the insula;

02 Breaking the One-Size-Fits-All Parameter Approach

✦ Clearly establishing current intensity as the core variable of taVNS efficacy, providing direct intracranial electrophysiological evidence for the design of personalized treatment plans;

03 Strengthening the Research Foundation

✦ First to reveal the intensity-dependent mechanism of taVNS through human intracranial recording, providing important reference for subsequent exploration of higher intensity and bilateral stimulation effects.

Conclusion

Of course, the study also has certain limitations, such as the subjects being patients with refractory epilepsy, only left ear stimulation being used, and limited sample size. Future research needs to expand the sample, conduct bilateral stimulation studies, and validate results in healthy populations and different disease populations. But undeniably, this study opens a new door for the development of non-invasive taVNS, making "precise, personalized neuromodulation" more within reach.

In the future, as further exploration and optimization of taVNS parameters continues, this non-invasive technology is expected to play a role in the treatment of more diseases, bringing safer and more effective treatment options to patients.

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