Vagus nerve stimulation boosts the drive to work for rewards

Abstract

Interoceptive feedback transmitted via the vagus nerve plays a vital role in motivation by tuning actions according to physiological needs. Whereas vagus nerve stimulation (VNS) reinforces actions in animals, motivational effects elicited by VNS in humans are still largely elusive. Here, we applied non-invasive transcutaneous auricular VNS (taVNS) on the left or right ear while participants exerted effort to earn rewards using a randomized cross-over design (vs. sham). In line with preclinical studies, acute taVNS enhances invigoration of effort, and stimulation on the left side primarily facilitates invigoration for food rewards. In contrast, we do not find conclusive evidence that acute taVNS affects effort maintenance or wanting ratings. Collectively, our results suggest that taVNS enhances reward-seeking by boosting invigoration, not effort maintenance and that the stimulation side affects generalization beyond food reward. Thus, taVNS may enhance the pursuit of prospective rewards which may pave avenues to treat motivational deficiencies.

Fig. 1. Schematic summary of the effort allocation task.

First, a fixation cross is shown. The trial starts in sync with the stimulation and the reward cue is shown for 1 s. During the effort phase, participants have to keep a ball above the red line by vigorously pressing a button with their right index finger to earn rewards. As task conditions, we manipulated reward type (food vs. money), reward magnitude (low vs. high), and difficulty (easy vs. hard). The inset shows a representative time series in one high-difficulty trial depicting effort output as button press rate, BPR, in % relative to the maximum frequency of the participant. Invigoration slopes were estimated to capture how quickly participants ramp up their effort during a trial to obtain the reward at stake. Effort maintenance was estimated by taking the average relative frequency on the trial.

Fig. 2. Invigoration is associated with reward magnitude and wanting, but not with exertion.

a Participants were quicker to invigorate if more reward was at stake, p < 0.001, and slower if difficulty was high, p = 0.002. b Participants exerted more effort when more reward was at stake, p < 0.001, and less when it became more difficult to obtain it, p < 0.001. Moreover, they worked more for difficult reward when the magnitude was high, p < 0.001. c Food and monetary rewards elicited comparable invigoration, p = 0.45. d Food and monetary rewards elicited similar investment of effort, p = 0.45. e Effort maintenance was related to both ratings of exertion and wanting, ps < 0.001, but invigoration was only related to wanting, p < 0.001, not exertion, p = 0.88. n = 7776 trials of 81 participants; dots depict condition means per participant, bars depict data as sample mean values; error bars depict 95% confidence intervals at the trial level (a–d) or of fitted coefficients at the participant level (e). %/s = button press rate in % of maximum per s. Statistics refer to two-sided t-contrasts of the mixed-effects models reported in Supplementary Tables 1, 2, approximate degrees of freedom = 78, no adjustments for multiple comparisons. Source data are provided as a Source Data file.

Fig. 3. Transcutaneous auricular vagus nerve stimulation (taVNS) increases invigoration.

a During taVNS, participants were faster to invigorate instrumental behavior (stimulation main effect, p = 0.004, BF₁₀ = 7.34). The invigorating effect of taVNS was significantly more pronounced for food vs. monetary rewards (stimulation × reward type, p = 0.049) which was primarily driven by stimulation side (cross-level interaction, p = 0.037). b In contrast to invigoration, taVNS did not enhance effort maintenance compared to sham, p = 0.09, BF₁₀ = 0.51. n = 7776 trials of 81 participants; dots depict condition means per participant; bars depict data as sample mean values; error bars depict 95% confidence intervals at the trial level. %/s = button press rate in % per s. Note that apparent differences between sham conditions were not significant due to considerable inter-individual differences (ps > 0.072). Statistics refer to two-sided t-contrasts of the mixed-effects models reported in Supplementary Tables 1, 2, approximate degrees of freedom = 78, no adjustments for multiple comparisons. Bayes factors were calculated using Bayesian two-sided t-tests based on order-corrected ordinary least squares effects of individual estimates. Source data are provided as a Source Data file.

Fig. 4. Individual estimates and group densities of the stimulation effects induced by transcutaneous auricular vagus nerve stimulation (taVNS).

a During taVNS, participants showed an increase in invigoration across conditions (main effect of stimulation, S; b = 2.93, 95% CI [0.98, 4.88], p = 0.004). The S × Food interaction was significantly higher during taVNS, (b = 1.33, 95% CI [0.03, 2.63], p = 0.049) which was primarily driven by stimulation on the left side. b No significant changes in effort maintenance were induced by taVNS. Diff = difficulty, RewM = reward magnitude. The plot depicts empirical Bayes estimates (after taking the distribution across the group into account). Statistics refer to two-sided t-contrasts of the mixed-effects models reported in Supplementary Tables 1, 2, approximate degrees of freedom = 78, no adjustments for multiple comparisons. Source data are provided as a Source Data file.

Fig. 5. Vagus nerve stimulation boosts the drive to work for less wanted rewards.

a Overall, participants are slower to invigorate behavior if they want the reward at stake less as depicted in the 2d-density polygon (brighter colors indicate higher density of data). In line with univariate analyses, robust regression lines show that the slope reflecting the association between invigoration and wanting is decreased by transcutaneous auricular vagus nerve stimulation (taVNS: red line; sham is depicted as blue line). Again, no change was observed for monetary rewards after taVNS on the left side. b Compared to permuted data, taVNS induces significant changes in the association between invigoration and wanting. By fitting robust regression coefficients, b, after permuting the labels for taVNS vs. sham stimulation, we compared the observed difference in slopes for taVNS—sham (in red) to a null distribution (violin plot in gray). This two-sided permutation test showed a significant main effect across both stimulation sides (l + r = left + right) and for taVNS on the right, but not the left side. On the left side, we observed an interaction with reward type instead, p = 0.029. VAS = visual analog scale. p-values are unadjusted for multiple comparisons. Source data are provided as a Source Data file.

Fig. 6. Vagus nerve stimulation does not affect cost-evidence accumulation.

a Schematic summary of the computational model of effort allocation as proposed by Meyniel et al.. b Posterior densities of transcutaneous auricular vagus nerve stimulation (taVNS)-induced changes in the model parameters. The credible intervals of the posteriors all include 0 suggesting that there is no significant effect on cost-evidence accumulation. c Simulations show that the length of segments can be recovered very well by the fitted model for both stimulation conditions. Source data for b, c are provided as a Source Data file.