Transcutaneous Vagus Nerve Stimulation in Humans Induces Pupil Dilation and Attenuates Alpha Oscillations

Abstract

Vagus nerve stimulation (VNS) is widely used to treat drug-resistant epilepsy and depression. While the precise mechanisms mediating its long-term therapeutic effects are not fully resolved, they likely involve locus coeruleus (LC) stimulation via the nucleus of the solitary tract, which receives afferent vagal inputs. In rats, VNS elevates LC firing and forebrain noradrenaline levels, whereas LC lesions suppress VNS therapeutic efficacy. Noninvasive transcutaneous VNS (tVNS) uses electrical stimulation that targets the auricular branch of the vagus nerve at the cymba conchae of the ear. However, the extent to which tVNS mimics VNS remains unclear. Here, we investigated the short-term effects of tVNS in healthy human male volunteers (n = 24), using high-density EEG and pupillometry during visual fixation at rest. We compared short (3.4 s) trials of tVNS to sham electrical stimulation at the earlobe (far from the vagus nerve branch) to control for somatosensory stimulation. Although tVNS and sham stimulation did not differ in subjective intensity ratings, tVNS led to robust pupil dilation (peaking 4-5 s after trial onset) that was significantly higher than following sham stimulation. We further quantified, using parallel factor analysis, how tVNS modulates idle occipital alpha (8-13Hz) activity identified in each participant. We found greater attenuation of alpha oscillations by tVNS than by sham stimulation. This demonstrates that tVNS reliably induces pupillary and EEG markers of arousal beyond the effects of somatosensory stimulation, thus supporting the hypothesis that tVNS elevates noradrenaline and other arousal-promoting neuromodulatory signaling, and mimics invasive VNS.SIGNIFICANCE STATEMENT Current noninvasive brain stimulation techniques are mostly confined to modulating cortical activity, as is typical with transcranial magnetic or transcranial direct/alternating current electrical stimulation. Transcutaneous vagus nerve stimulation (tVNS) has been proposed to stimulate subcortical arousal-promoting nuclei, though previous studies yielded inconsistent results. Here we show that short (3.4 s) tVNS pulses in naive healthy male volunteers induced transient pupil dilation and attenuation of occipital alpha oscillations. These markers of brain arousal are in line with the established effects of invasive VNS on locus coeruleus-noradrenaline signaling, and support that tVNS mimics VNS. Therefore, tVNS can be used as a tool for studying how endogenous subcortical neuromodulatory signaling affects human cognition, including perception, attention, memory, and decision-making; and also for developing novel clinical applications.

Keywords: EEG; alpha oscillations; noradrenaline; pupil; tVNS; transcutaneous vagus nerve stimulation.

Figure 1.

Experimental design. A, Schematic illustration of the rationale of tVNS. B, Stimulation electrode placement: (1) location of the tVNS on the cymba conchae of the left ear; (2) location of the sham stimulation on the left earlobe; and (3) photograph of the commercial stimulation electrode. C, Experimental design. Each experiment started with a method of limits procedure to adjust the stimulation current according to the individual subjective pain report (Rating), and then increased incrementally by 0.2 mA until a current matched to a rating of 8 was selected. Eight blocks were then conducted, each of 5 min and including 11 stimulation trials of 3.4 s and stimulation intervals of 25–27 s.

Figure 2.

tVNS leads to greater pupil dilation than sham stimulation. A, Grand average pupil dilation in response to tVNS (red trace) and sham stimulation (black trace). Shaded areas around the trace indicate the SEM. The gray transparent rectangle indicates that the active current is on. The top red line indicates FDR-corrected statistical significance using the Wilcoxon signed-rank test. The dashed black bar indicates the time interval used to compute individual subject dilation values in B. B, Single-participant values in both tVNS and sham conditions between the two points of half-maximum (FDHM, 3.2–10.4 s; dashed black bar in A). The solid black lines denote tVNS > sham stimulation, while the dashed gray lines denote sham stimulation > tVNS. C, I, II, Single-participant traces. C, Two representative single-subject pupil time courses as indicated in B, with identical graphic representation as in A.

Figure 3.

PARAFAC to identify individual alpha activity. Graphic illustration of the PARAFAC method we used to decompose the stimulus-free (break) data and create subject-specific topographical and frequency bands of interest. A, Illustration of the PARAFAC model with two components, in which f1 and f2 refer to the frequency features, t1 and t2 indicate temporal features, and c1 and c2 represent the spatial features of the components in the channel space. B, Spectrogram of five single 3 s “trials” derived from the break, the same subject as in the top left in C. C, D, Representative examples of the decomposition result for two participants. Each panel includes two components: 1 (pink), and 2 (blue), together with their associated frequency (f) and trial (t) profiles. The spatial (channel) dimension is presented as scalp topographies on the right side.

Figure 4.

tVNS leads to greater attenuation of EEG alpha activity than does the sham stimulation. A, Median alpha component topography. The median weights across participants are colored pink. The blue points mark electrodes with the highest alpha activity (selected using a threshold applied to the median weights) to facilitate visualization in subsequent panel E, but these electrodes are not used in any statistical analyses. B, Alpha attenuation relative to baseline in individual subject data between 0 and 4 s, using the weighted topography in A and using the spectral profile in C. Black solid lines mark participants with higher alpha decreases in the tVNS condition (19 of 21), whereas dashed gray lines mark participants with higher alpha decreases in the sham condition (2 of 21). C, Alpha component spectral profile (median across participants). D, The mean alpha component time course (using the spectral profile depicted in C, and the topographical profile depicted in A). E, The difference in induced power between the tVNS and sham conditions (shown separately in G and H). White contours mark statistically significant time–frequency clusters (after correction for multiple comparisons). Note that tVNS causes alpha attenuation lasting several seconds. F, Topographical dynamics following stimulation (at a resolution of 1 s) reveal occipital alpha attenuation following on tVNS (top) but not in the sham condition (bottom). The yellow points mark electrodes comprising the statistically significant time–space cluster that exhibits tVNS attenuation > sham attenuation (after correction for multiple comparisons). G, Mean induced spectrogram following on tVNS; the white contour is identical to that shown in E. H, The mean induced spectrogram following on sham stimulation; the white contour is identical to that shown in E.