To familiarize the subjects with the character identification task, they performed five training trials for each treatment, and each treatment took about 4 min. There was a 1-2 min break between treatments to avoid successive contrast effects and visual fatigue. The overall experiment lasted about 2.5 hr for each subject, including regular breaks to reduce fatigue. For each subject, the three within-subject factor treatments were administered randomly. Before the experiment, the treatment sequence for each subject was determined by drawing lots. To maintain work motivation, subjects were paid NT$ 100 per hr, plus an extra NT$ 100 if their overall average percentage of correctly identified letters exceeded 60%.