tVNS, Motivation, and Insulin Sensitivity

To assess where individual reward deficits manifest, participants will undergo an intake session that includes clinical interviews, a fasting blood draw, and a battery of reward tasks (Reward Rating, Effort Allocation, Taste Test, Go-Nogo-Learning). Changes in symptoms and glucose levels will be evaluated using a wearable glucose sensor and ecological momentary assessments (EMA) over 2 weeks. To assess insulin sensitivity, the investigators will perform an oral glucose tolerance test, with concurrent stimulation (tVNS vs. sham; i.e., two sessions, randomized). Finally, in two neuroimaging sessions, the investigators will assess the effect of acute tVNS (vs. sham; randomized) on motivation and stomach-brain coupling using concurrent functional magnetic resonance imaging (fMRI) and electrogastrography (EGG). Washout between tVNS/sham days will be a minimum of 2 days. Condition order will be randomized, and the design is single-blind (participant). To characterize our sample, the investigators will also collect information using standardized questionnaires assessing personality traits, eating behavior, psychiatric symptoms, and physical activity. To further characterize our sample metabolically, the investigators will also collect blood samples to determine metabolic parameters (e.g., acyl-ghrelin, des-acyl ghrelin, insulin, glucose, triglycerides, HDL, LDL), and participants can opt in to collect data for genetic analyses as part of a Biobank. These measures will be used to describe the sample and will be explored as predictors to explain inter-individual intervention effects. * Personality traits related to reward and motivation: Behavioral Inhibition/Activation System * Eating behavior: Three Factor Eating Questionnaire * Psychiatric symptoms: depressive symptoms and anhedonia (BDI-II; Snaith-Hamilton Pleasure Scale, German version, SHAPS-D; Temporal Experience of Pleasure Scale, TEPS; Dimensional Anhedonia Rating Scale, DARS). * Physical activity: International Physical Activity Questionnaire (IPAQ). During the EMA period, participants will measure changes in glucose using a continuous glucose monitor (CGM) using the Freestyle Libre 3 sensor. Participants will answer questions with respect to: * state ratings (e.g. hunger, mood) * anticipated rewarding activities (wanting, time planned) * consummated rewarding activities (liking, time spent) * as well as complete a food choice task.