2.3. Tasks for the training and the fMRI scans

The working memory training task was a visual-spatial span task (see Supplement Fig. S2A). Stimuli were green-colored squares presented sequentially in a 5 × 5 empty grid on a computer screen. After an intermission, subjects were required to recall the sequence of locations by clicking on the empty grid. Each stimulus was presented for 500 ms, the interstimulus interval was also 500 ms, and the intermission was 1000 ms. Difficulty level was determined by the number of the stimuli. The task difficulty was adaptive to the participants’ performance for the adaptive training group. In the first training session, the training started with 3 stimuli and the number of stimuli automatically increased by 1 if the subjects made 5 continuous correct responses at the current level. In subsequent training session, the training started with 2 stimuli less than the maximum level in the previous session. For the non-adaptive training control group, the task difficulty was kept at a low level with 3 stimuli in all the training sessions. The training consisted of 20 sessions and was conducted in a neuropsychological laboratory over the course of 4 weeks (5 sessions per week, no>1 session per day). Each training session consisted of 80 trials (lasting 30 ~ 40 min). Then we revised the training task and created an assessment version, which included two trials for each span. Testing ended when subjects failed both trials. The length of the longest correct span was used to reflect the memory span.

The fMRI task was similar to the training task, which included the working memory condition (72 trials) and the baseline condition (36 trials) (see Supplement Fig. S2B). The stimuli were presented in cue-probe pairs. For the working memory condition, the green-colored cue stimuli were presented in the same way as in the training task and the number of the stimuli was set at 5. The probe stimulus was an empty grid. Subjects were asked to judge if the Arabic number indicated the correct order of the cue stimuli by pressing the button (left button for the correct number and right button for the incorrect number). For the baseline condition, the 5 red-colored cue stimuli were presented in a fixed order (top left corner → top right corner → bottom right corner → bottom left corner → center), and the probe stimuli were the same in all trials. Each trial started with a centrally placed fixation cross for 500 ms, followed by 5 cue stimuli. Each cue stimulus was presented for 500 ms, followed by a 500 ms interstimulus interval. After a 1000 ms intermission screen, probe stimuli were presented for 2000 ms.