Classical functional analysis
The statistical analysis was performed on SPM12. In the first level analysis, we used 3- and 0-back as regressors of interests and six estimated parameters of motion as regressors of no interests. Signal drifts were adjusted by a high-pass filter of 128 s and serial correlations using an AR(1) model. We contrasted the activation by workloads, i.e., 3-back > 0-back, 3-back < 0-back, and load-independent responses. In the group-level analysis, we used a flexible factorial model to estimate condition effects as the fixed effect, subject effects as the random effect, and age as a covariate, in each workload contrast. In order to control for the caffeine- and withdrawal-induced changes in perfusion, we used individual whole-brain cerebral blood flow, measured by arterial spin labelling (sequence detail see the 