Figure 1.

Behavioral tasks. Successive screenshots displayed in one trial, with duration in milliseconds. A, The incentive force task. After a fixation cross, subjects were shown the monetary incentive as a coin image (€0.1, €0.2, €0.5, €1, €2, €5). This was the trigger to exert a force on a hand grip. Online feedback on the force produced was provided as a cursor moving up and down within a scale graduated from 0 to the maximal force of the subject. The height reached by the cursor determined the fraction of the monetary incentive earned in the current trial. B, The binary choice task. After a fixation cross, subjects were shown two options side by side, each corresponding to a potential monetary reward (coin image) associated with a required force level (orange bar). Subjects selected their preferred option on a keyboard and then had to produce the associated force (i.e., raise the cursor up to the orange bar). In both tasks, cumulative total of monetary earnings was indicated at the end of the trial.