Figure 1.

(Top) Stroop Match-to-Sample paradigm, illustrating four conditions: (1) congruent-match, (2) incongruent-match, (3) congruent-nonmatch, and (4) incongruent-nonmatch. A color cue (XXXX) presented for 450 ms was followed by an incongruent or congruent Stroop target stimulus that appeared for 1100 ms after an inter-stimulus interval (ISI) of 300 ms. The inter-trial interval (ITI) was 1450 ms. Subjects matched the color (red, green or blue) of the cue to the ink color of the Stroop stimulus. (Bottom) fMRI block design illustrated for 8 blocks: Stroop stimuli in each block were either congruent (word BLUE written in blue font) or incongruent (word BLUE written in red font). Four blocks contained mixed YES- and NO responses: 2 incongruent with match and non-match trials (incongruent, INC), 2 congruent with match and nonmatch trials (congruent, CON). In addition, four same-response blocks were presented: congruent-match, congruent-nonmatch, incongruent-match, incongruent-nonmatch. Trials presented in same- and mixed-response blocks were the same, only the order of trials differed, i.e., in same response blocks, either six match trials or six non-match trials were presented in a row, whereas in mixed-response blocks, three match and three nonmatch trials were presented in random order. Each block consisted of 6 trials, and lasted for of 19.8 s. In response-repetition blocks, cue-target color either matched or did not match, in response-switching blocks, match and nonmatch trials were mixed. In total 32 blocks were presented in pseudo-random order ensuring that each condition was equally often represented (24).

Thus, Debriefing after fMRI scanning revealed that none of our subjects was aware about the blocked nature of the task.