Discussion
We examined putative neural representations of imminent monetary rewards and losses using an established probe, the MID task (Our findings stand in contrast to our a priori hypotheses. Peripheral ghrelin has been proposed to promote appetite and increase the reward value of food in part by potentiating mesolimbic DA signaling (Neural activations in the MID task as assessed with fMRI are highly correlated with, and likely reflect, striatal DA release (Effects of ghrelin on delay discounting were equally unexpected. Our simplistic a priori hypothesis was that increased motivation for food would generalize across reward categories and make people prioritize immediate rewards over those that are distant in time. Findings in rodents, obtained using food rewards, supported this notion (Meta-analyses have linked angular gyrus activity, primarily in the left hemisphere, with number processing and conflict resolution, in particular in go/no-go–type tasks (Finally, it is unclear why effects of ghrelin in the delay discounting task appear to be restricted to women, while attenuated loss aversion affected men and women alike. This apparent discrepancy is unlikely to be related to pharmacokinetic differences, as ghrelin levels achieved through the infusion were somewhat higher in men than in women, and the biomarker of ghrelin activity, GH release, was virtually identical between male and female participants. A methodological issue may account for the observation that women drove the effect of ghrelin in the delay discounting task. In agreement with published findings (Among the strengths of our study is its randomized, placebo-controlled, double-blind design and biomarker confirmation of GHSR activation. This allowed us to establish a causal role of ghrelin in humans for cognitive processes outside appetite regulation and food- or drug-related cues. Our sample size was based on an a priori power analysis and allowed us to robustly replicate several previously published findings in the MID task (In summary, our results support the general notion that ghrelin plays a role in processes subserving value-based decision making beyond its established role as an appetite-stimulating hormone. They do not, however, support predictions based on animal studies that ghrelin generally enhances the value of prospective rewards through actions on mesolimbic DA signaling.