Fig. 1. Behavioral task and performance.
(a) To begin a trial, subjects fixated a central point for 450 ms. On choice trials, two pictures at ± 5° visual angle predicted different reward amounts. Subjects freely viewed both images and chose one by fixating it for 450 ms. After a choice, another cue appeared instructing a right or left joystick response, which, if executed correctly, resulted in the reward associated with the picture chosen at the beginning of the trial. Single picture trials were identical to choice trials, except only one randomly selected picture was shown. Subjects had to fixate the picture for 450 ms and make the subsequently instructed joystick response to obtain reward. (b) Both subjects learned eight reward-predicting pictures well, choosing more valuable pictures on choice trials (left; regression of percent chosen per session on picture value. Subject M: n = 96 (24 sessions × 4 values) r2 = 0.96 p = 2×10−67, Subject N: n = 80 r2 = 0.94 p = 3×10−50), and making faster joystick responses for higher value pictures on single picture trials (right; RT = Reaction Time; Regression of log(RT) on picture value: Subject M r2 = 0.34 p = 6×10−10, Subject N r2 = 0.25 p = 3×10−6). Error bars are ± SEM.