Figure 1.

The behavioral task and performance. A, The experiment consisted of two tasks presented in alternating blocks. In the AO task, the subject moved the lever (indicated by red arrows) in two directions to receive two different liquid rewards. He then repeated one of the movements to receive more of that corresponding reward. For instance, the subject could move left to receive orange juice, then right to receive apple juice. If the subject prefers orange juice, he will repeat the leftward movement in the choice period to gain more orange juice. In the SO block, the subject viewed two pictures that were paired with different rewards. Then he selected the picture associated with his preferred reward via a lever movement. Pictures were equally likely to appear on either side of the screen at the choice phase. B, Mean (±SEM) proportion of trials in which the subject selected his more preferred juice in each juice pairing.