40 Hz Transcutaneous Auricular Vagus Nerve Stimulation Enhances Verbal Free Recall in Healthy Adults by Modulating Frontal Theta Oscillations

Background Transcutaneous auricular vagus nerve stimulation (taVNS) is a non-invasive neuromodulation approach that may influence memory-related brain activity. Theta oscillations are closely related to episodic memory processing, but it remains unclear whether taVNS modulates memory-related theta activity in a frequency-specific manner and how these effects change after stimulation.Objective This study investigated the sustained effects of 20 Hz and 40 Hz taVNS on free recall and resting-state EEG theta oscillations in healthy young adults.Methods Forty-four participants were enrolled in this randomized, single-blind, sham-controlled, three-arm parallel-group study. They were randomly assigned to the sham stimulation group, the 20 Hz taVNS group, or the 40 Hz taVNS group. Free recall performance was assessed before and after stimulation, and resting-state EEG was recorded at baseline and multiple time points after stimulation.Results The results showed that participants receiving 40 Hz taVNS exhibited better free recall performance than those receiving sham stimulation or 20 Hz stimulation. 40 Hz taVNS increased the relative power of theta waves, particularly in the frontal region. This enhancement was an immediate increase followed by a gradual decrease over time. Frontal theta activity and stimulation-induced theta changes were positively correlated with free recall performance. Furthermore, exploratory Bayesian mediation analysis supported a theta-dependent model.Conclusion This study suggests that 40 Hz taVNS may promote verbal free recall performance by inducing transient enhancement of frontal theta wave activity, providing preliminary evidence for the modulation of memory-related neural oscillations by frequency-specific taVNS.