Figure 1.

Experiment setup and trial structure. (A) Schematic of experimental setup. Participants lay in the MRI scanner while a confederate partner sat at the experiment computer outside (shown as hand with buttons, not to scale; confederate button presses were obscured from view). Participants viewed the entire experiment via a live video camera feed aimed at the outside monitor. Responses from inside the scanner were connected to the experiment computer, and liquid outcomes were delivered by pumps controlled by the experiment computer. (B) Trial structure and schematic of participant view. Two trial types are shown, as viewed on participant screen. Participant screen showed experiment computer and confederate hand (with button presses obscured). In Experienced trials, participants saw a slot machine cue on their half of the screen (e.g., top) and made their response. After a variable delay, the cue was followed by a liquid reward or neutral outcome, as well as a colored indicator (reward shown). In Observed trials, participants saw the slot machine cue on the confederate’s half of the screen and observed her response; the liquid outcome was then nominally delivered to the confederate instead of the participant, whereas the visual indicator was identical to Experienced trials (neutral outcome shown). On instrumental trials, the participant or confederate selected between two arms; on noninstrumental trials, the computer chose one side or the other (left indicator shown) and the participant or confederate responded to the chosen side with a button press. (C) Representative reward probability curves for two conditions. Each line indicates the probability of reward for one arm or side of a given condition’s slot machine over the experiment for a single participant.