Figure 1.

Experimental design. The experiment included 2 sessions. During the experiment, subjects performed 2 types of mental tasks involving advanced trail-making tests (ATMT). During the first session, subjects performed Task 1 of ATMT while during the second session they performed Task 2. At the end of each positron emission tomography (PET) scan, subjects were asked to rate their subjective fatigue sensation on a visual analogue scale (VAS) from 0 (no fatigue) to 100 (total exhaustion). Each session included 10-min fixation and 35-min fatigue-inducing task trails. The session, which included 1 fixation scan and 4 task scans, was performed with 10-min interval. The rest period between the sessions was 8 minutes.