Fig. 1.

fMRI experimental procedure. (A) During Experiment 1, subjects alternated between periods of task (30 s) and periods of rest (30 s). During task periods, subjects made odd/even judgments on a random set of numbers ranging from 1 to 1,000. During the passive rest periods, subjects were instructed to simply maintain fixation on a crosshair that was presented at a central location on the screen. (B) During Experiment 2, subjects alternated between periods of task (1.25 s) and variable periods of rest (0–10 s). During task periods, subjects responded at the onset and offset of a flickering checkerboard stimulus. During the passive rest periods, subjects were instructed to simply maintain fixation on a crosshair that was presented at a central location on the screen.