Transcutaneous Auricular Vagus Nerve Stimulation Enhances Emotional Processing and Long-Term Recognition Memory: Electrophysiological Evidence Across Two Studies

Abstract

Recently, we found that continuous transcutaneous auricular vagus nerve stimulation (taVNS) facilitates the encoding and later recollection of emotionally relevant information, as indicated by differences in the late positive potential (LPP), memory performance, and late ERP Old/New effect. Here, we aimed to conceptually replicate and extend these findings by investigating the effects of different time-dependent taVNS stimulation protocols. In Study 1, an identical paradigm to our previous study was employed with interval stimulation (30-s on/off). Participants viewed unpleasant and neutral scenes on two consecutive days while receiving taVNS or sham stimulation and completed a recognition test 1 week later. Replicating previous results, unpleasant images encoded under taVNS, compared to sham stimulation, elicited larger amplitudes in an earlier window of the LPP during encoding, as well as more pronounced late Old/New differences. However, no effects of taVNS on memory performance were found. In Study 2, we followed up on these findings by synchronizing the stimulation cycle with image presentation to determine the taVNS effects for images encoded during the on and off cycles. We could replicate the enhancing effects of taVNS on brain potentials (early LPP and late Old/New differences) and found that taVNS improved recollection-based memory performance for both unpleasant and neutral images, independently of the stimulation cycle. Overall, our results suggest that taVNS increases electrophysiological correlates of emotional encoding and retrieval in a time-independent manner, substantiating the vagus nerve's role in emotional processing and memory formation, opening new venues for improving mnemonic processes in both clinical and non-clinical populations.

FIGURE 1

Schematic representation of the task design for Study 1 (A) and Study 2 (B).

FIGURE 2

Memory performance for recollection‐related (highest confidence) judgments. Depiction of behavioral performance for unpleasant and neutral images in Study 1. Error bars represent standard error. Bar plots represent mean values. Dots indicate individual scores.

FIGURE 3

ERP results during encoding in Study 1. ERP‐averaged waveforms across the significant sensor clusters. The blue box indicates the significant time window. Upper right inset: Mean averaged ERPs during the significant time window and sensor clusters, showing a larger emotion discrimination during taVNS than sham condition. Upper left inset: Scalp topographies showing emotional differences during the taVNS and sham conditions, separately.

FIGURE 4

ERP results during retrieval in Study 1. ERP‐averaged waveforms across the significant sensor cluster (left for unpleasant images and right for neutral images). The blue box indicates the significant time window. Upper inset: Mean averaged ERPs during the significant time window (672–776 ms) and sensor cluster, showing a larger activity for correctly retrieved unpleasant images encoded during taVNS, compared to sham. Lower inset: Topographical Old/New differences for taVNS and sham conditions, separately.

FIGURE 5

Memory performance for recollection‐related (highest confidence) judgments. Depiction of behavioral performance for unpleasant and neutral images of Study 2. Error bars represent standard error. Bar plots represent mean values. Dots indicate individual scores.

FIGURE 6

ERP results during encoding in Study 2. ERP‐averaged waveforms across the significant sensor clusters. The blue box indicates the significant time window. Upper right inset: Mean averaged ERPs during the significant time window and sensor clusters, showing a larger emotion discrimination during taVNS than sham condition. Upper left inset: Scalp topographies showing emotional differences during for taVNS and sham groups, separately.

FIGURE 7

ERP results during retrieval in Study 2. ERP‐averaged waveforms across the significant sensor cluster. Left for unpleasant images and right for neutral images. The blue box indicates the significant time window. Left inset: Mean averaged ERPs during the significant time window (500–636 ms) and sensor cluster, showing larger ERP Old/New effects for unpleasant images in the taVNS group compared to the sham group. Right inset: Topographical Old/New differences for the taVNS and sham groups, separately.