MID task.
SENSITIVITY
First-level analyses included regressors modelling the conditions of anticipation and feedback, resulting in a total number of 11 regressors. Three anticipation regressors were defined as the onset of the cues associated with either reward (high/low), loss (high/low), or neutral (no loss/no reward) until the onset of the target. Feedback was modelled using onset-to-offset of the outcome presented on each trial. Loss and reward feedback regressors were collapsed over high and low amounts to increase the number of stimuli per regressor. Neutral feedback was represented by the outcome of the neutral no loss/no reward cue (± 0 SEK). Regressors representing losses as punishments and missed rewards as nonrewards were used to model unsuccessful trials with respect to behavioral performance. A motor regressor was also included to model button presses related to the task. Correction for multiple comparisons of group-level data was performed with 3dClustSim (Group-level analyses were run using AFNI 3dMVM (The objective of the whole-brain MVM analysis was to empirically delineate the anticipation-responsive striatal cluster in our data, rather than relying on a literature-based region-of-interest (ROI). To properly account for the within-subject design of our study, we also included intervention (ghrelin versus placebo) as a factor in the whole-brain analysis. The main effect of this factor was not of interest for identifying the anticipation-responsive cluster, while the power required to detect an anticipation × intervention interaction in the whole brain analysis vastly exceeds that for detecting main effects. To test for effects of ghrelin on striatal activations during anticipation, the whole brain analysis was therefore followed up by extracting β coefficients from the striatal cluster identified by the anticipation factor, and analyzing these β coefficients using factorial ANOVAs. These analyses were performed in SPSS version 28 (IBM). Several sensitivity analyses supporting the robustness of our approach are presented in Biological sex (male/female) was included as a covariate in all analyses. Sensitivity analyses were also carried out to examine potential contributions from age and BMI as covariates, but their inclusion did not change the results, and they were therefore dropped in the final analysis. The order of active ghrelin challenge and placebo was balanced across subjects in the randomization and was therefore not included as a factor.