Figure 1.

Procedure for Study 1 and Study 2. (A) In Study 1, the participant in the scanner was randomly paired with an anonymous partner on each trial. The task for the participant and the partner was to quickly estimate the number of dots presented briefly on the screen. The outcome of their performance was presented under the photo of the participant and under a blurred picture of face representing the partner. If at least one of them estimated incorrectly, the partner would receive a number of mildly painful electric shocks. The participant then indicated the level of pain he/she would be willing to take for the partner as a compensation. Finally, the pain stimulation of the participant’s choice was delivered to him/her (see Yu et al. 2014 for details). (B) In Study 2, two participants took turns in either performing or observing the otherʼs performance in a dot-estimation task. The dot-estimation task required the player to indicate which side of the screen contained a larger number of dots. The participant outside the scanning room would receive either painful or nonpainful (i.e., warm) thermal stimulation after each trial, depending on the performance of the current player. The full 2 × 3 factorial design resulting from the different feedback type in the two task conditions (playing or observing) is displayed in the table (see Koban et al. 2013 for details).