Fig. 1. Study design showing the timing and order of the individual positron emission tomography measurements at baseline and during target-controlled S -ketamine infusion aiming at subanesthetic (150 ng/ml) and anesthetic (1,500–2,000 ng/ml) concentrations. The baseline measurement for cerebral glucose metabolic rate was performed approximately 3 weeks apart from the other assessments (for further details, see Materials and Methods). The mean durations of the subanesthetic and anesthetic levels were 50 min and 2 h, respectively. The S -ketamine serum concentration measurements are indicated with Roman numerals ( I – IV ) and corresponding arrows .