Fig. 1

Task procedure. a Set up. The monkey sat in the primate chair equipped with three bars. b Decision-making part. By touching the center bar, two different choice targets were presented simultaneously one on each side of a fixation spot. The monkey could choose one of the alternatives by touching a bar on the side corresponding to the choice. After choosing one of them, the chosen reward schedule began. c Choice target set. The brightness and the length of choice targets were proportional to the reward amount and schedule length, respectively. d Color discrimination trial. When the monkey touched a center bar, a fixation spot appeared. The monkey must release the bar within 1 s after the red square changed to green. If the monkey successfully released the bar, the color of the square changed to blue and liquid reward was given. e Reward schedule part (an example of 4 trial schedule with 4 drops of reward). The monkey was required to perform 1, 2, 3, or 4 repeats of the color discrimination trial successfully to earn 1, 2, 3, or 4 drops of liquid reward. Throughout the trials, a white rectangle visual cue was presented at the top of the monitor, and its brightness and length indicated the reward amount and number of progressed trials, respectively. Schedule states were abbreviated as ‘trial number/schedule length’ (for example, the second trial in a four-trial schedule was labeled ‘2/4’). Trials in which the monkey failed to release the bar were scored as an error, and the same trial was repeated