Fig. 1.

Illustration of free-operant VI-10 task. A fractal image remained present throughout the block. The filled-in yellow square indicated which button to press. Responses were self-paced. After non-rewarded responses, a dark gray circle was presented for 50 ms. A reward became available with a probability of 0.1 per second. After the subsequent button press, a picture of an M&M or Frito was shown for 1 s, indicating a food reward of the corresponding type. The two stimulus-response-outcome pairings used for each subject remained consistent throughout the experiment. A third fractal, shown with empty yellow squares, indicated a rest block.