Introduction
Caffeine is the most commonly consumed psychostimulant worldwideAmong the studies controlling for low-order task performance, some of them also measured working memory-related blood-oxygen-level-dependent (BOLD) activity and consistently reported an acute increase in brain activity by caffeine, but no concomitant change in behavioral performanceTo date, it is unclear if such cerebral effects persist during daily exposure to caffeine, especially in a healthy cognitive state. Chronic administration of caffeine can lead to adaptions in the adenosine system, such as upregulated extracellular endogenous adenosine concentrationHence, we aimed to use a double-blind randomized placebo-controlled crossover study to answer whether daily caffeine intake increased working memory-related brain activity and improved behavioral performance. We measured working memory by N-Back tasks repeatedly across 13 h after waking up in a caffeine, a withdrawal, and a placebo condition. We defined a “