Fig. 1.

Schematic of the 2-back paradigm. (a). The stimulus train: a 1 s blank screen preceded a 1 s letter presentation to make a single trial. Whenever the current letter matched that of the one two trials earlier, the subject pressed a button. 15 trials made up a 30 s task block. (b). Each task block was followed by a 30 s rest block. 12 task blocks, each followed by a rest block, made up a single experiment, lasting 12 min.