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Abstract:
Introduction: Anhedonia is a core symptom in many mental disorders such as major depression. Generally defined as the 'inability to experience pleasure', recent debates push towards a conceptual revision emphasizing an understanding of anhedonia as a motivational deficit to work for rewards. Emerging evidence suggests that the vagus nerve affects dopaminergic circuits and metabolic signaling. Therefore, we expected transcutaneous vagus nerve stimulation (tVNS) to enhance the motivation to exert effort for rewards by modulating the effect of costs and benefits on instrumental behavior. Methods: In this study we applied tVNS/sham stimulation in a randomized crossover design to healthy subjects. They completed two sessions of a modified effort allocation task (EAT) and had to maintain a varying proportion of their individual maximum button press frequency to gain rewards. Results: Preliminary results in 24 participants indicate that the EAT tracks important aspects of costs and benefits of action as reflected in significant effects of reward magnitude, t(23) = 3.310, p = .003, and task difficulty, t(23) = -2.721, p = .012, on average button press frequency. Moreover, we found that higher reward magnitude increased effort invigoration by 8%, t(23) = 4.131, p < .001. Critically, the stimulation increased the invigoration by an additional 6%, t(22) = 3.042, p = .006. However, tVNS-effects on effort maintenance were modulated by session order and not significant at the group level, t(22) = 0.274, p = .787. Discussion: These findings confirm the suitability of the EAT to study effort-based decision making over time. We found that effort invigoration, but not maintenance was facilitated by tVNS. Nevertheless, more data is needed to assess whether the absence of tVNS effects on effort maintenance is due to a carry-over effect. To conclude, anti-depressant effects of the stimulation could be explained by an enhanced incentive value conferred by rewards.
