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To investigate the effect of resting blood oxygen concentration on the hemodynamic response to functional brain activation, we compared activation-induced changes in hemoglobin oxygenation during normoxia with systemic hyperoxia or mild hypoxia. Hemoglobin oxygenation changes were measured by microfiber optical spectroscopy (500–590 nm) in response to physiological whisker barrel cortex activation by whole whisker pad deflection (4 s, 4 Hz) in α-chloralose/urethane anesthetised male Wistar rats. During systemic hyperoxia (n=6), the stimulation-induced hyperoxygenation response was decreased and prolonged, whereas during mild systemic hypoxia (n=7) the peak response was significantly increased followed by a faster return to baseline. During normoxia, a poststimulation under- (oxy-hemoglobin) and overshoot (deoxy-hemoglobin) was observed, which disappeared during systemic hyperoxia …