Figure 2.

Experimental paradigm. (A) Subjects had to complete an experimental session of 12 blocks of 2 minutes each. At the end of each block, subjects had to judge to what extent pressing the space bar caused the occurrence of the reward, on a scale from −100 (pressing the space bar always prevented reward) to 100 (pressing the space bar always caused reward). During the experimental session, the participant was presented with a white triangle and could decide whether to press the space bar or not. Rewards were delivered contingently on pressing of the space bar or noncontingently in the absence of a response. In addition, a running total of the amount of money earned within a block was continuously displayed in the upper corner of the screen (not shown in figure). In cases where the participant was not pressing the space bar for multiple (hidden) 1-second bins in a row, the white triangle was continuously displayed on the screen, unless a nonresponse contingent reward occurred. In those cases, a reward was displayed on the screen noncontingently. (B) Each block was divided into 120 unsignaled time periods (bins) of 1 second. When a response occurred within each bin, the triangle turned yellow until the bin ended. If a response was recorded during the bin, a contingent reward was delivered at the end of that bin according to the applicable probability of outcome delivery given a response [P(O|A)]. If no response occurred during the bin, a noncontingent reward was delivered according to the applicable probability of outcome delivery given the absence of a response [P(O|∼A)]. (C) By varying P(O|A) and P(O|∼A), different levels of contingencies were achieved so that each experimental session included positive, degraded, and negative contingency blocks. O, outcome; P(O|A), probability of outcome given an action; P(O|∼A), probability of outcome given the absence of an action.