N-back
We employed N-back tasks to assess visual working memory during the laboratory (10th) day of each condition. Four sessions were scheduled during the course of the day and allowed us to examine performances through and after caffeine intake. The timings of the four sessions relative to the treatments were: 15 min. after the first treatment, 1 h after the second, as well as 1 h and 5 h after the last treatment. Every session consisted of 9 trials of 3-back and 5 trials of 0-back, each trial consisted of 30 stimuli with 1.5 s interval in a quasi-random order. Each of the consecutively presented stimuli was presented for 1 s. Participants keyed down “1” when the presented stimulus (a letter) matched the one three stimuli earlier (3-back, “high workload”) or when it was a “K” (0-back, “low workload”), otherwise pressing “2”. Participants performed one practice session in the evening of the ninth day.Performances were indexed by (a) error rate, calculated by the ratio of “incorrect rate” (missed + false alarms) to “correct response” (hits + correct rejections), and (b) reaction time (RT) during correct answers. We employed generalized linear mixed models to analyze the condition and time effects using R packages 