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Abstract:
Anhedonia is a core symptom in many mental disorders such as major depression. It is defined as the 'inability to experience pleasure', but recent debates argue for a conceptual revision emphasizing a motivational deficit to work for reward instead. Recently, stimulation of the vagus nerve has been shown to improve symptoms in treatment-resistant depression. While the therapeutic mechanism remains poorly understood to date, motivation could be facilitated via improved dopaminergic transmission and metabolic signaling. Here, we expected transcutaneous vagus nerve stimulation (tVNS) to enhance the motivation to exert effort for reward by modulating perceived costs and benefits of instrumental behavior. Therefore, as part of an ongoing study, we applied tVNS or sham stimulation in a randomized crossover design to 24 healthy participants who completed a modified Effort Allocation Task (EAT). As expected, higher reward magnitude increased the invigoration of work, t = 4.13, p < .001. Critically, the stimulation further increased the invigoration of work, t = 3.04, p = .006. In contrast, effects of tVNS on effort maintenance (i.e. average button-press frequency) were strongly dependent on session order and not significantly different at the group level (p = .78). To summarize, we found that invigoration, but not maintenance of work was facilitated by tVNS. We conclude that anti-depressant effects of the stimulation could be partly explained by an enhanced incentive salience conferred by rewards.
