The n-back task (Callicott et al., 1999) required subjects to continually update their mental set while responding to previously seen stimuli. Subjects viewed stimuli consisting of numbers (1–4) shown in random sequence displayed on a diamond-shaped box (see Figure 1). On each trial, subjects press one of four buttons to indicate the appropriate numeral. In the 0-back condition, the correct response is the numeral currently displayed on the screen; for 2-back, the correct response is the numeral seen 2 screens back. The actual task included five conditions (0-, 1-, 2-, 3-back and rest); each of 5 runs consisted of 30-second blocks of each load, pseudorandomly ordered, with 15 stimuli per block. Total task time was approximately 15 minutes with all responses recorded.
