Fig. 1.

Reward-giving task and behavioral results. (A) Experimental setup. Two monkeys sat opposite each other at a horizontal computer touch screen, each holding a resting key. On each trial, light gray and black backgrounds indicated actor and conspecific roles to the respective animals. (B) Task sequence: shape of conditioned cue predicted absence (square) or presence (circle) of reward for each animal (yellow for left, purple for right animal). Appearance of a subsequent blue go signal was followed by key release, stimulus touch and reward for actor, and 1 s later for conspecific. (C) The four reward conditions used: reward for neither, own reward only, conspecific's reward only, and reward for both. (D) Mean reaction times for the four reward conditions (from go signal to stimulus touch) between the two animals. Error bars show SEM. (E) Eye fixation density between onset of conditioned stimuli and go signal. (F) Eye fixation density on conspecific's face and spout after reward delivery to conspecific.