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For the O2-breathing experiment, the special gas mixture used hyperoxic gas (98% O2 + 2% CO2). For the sham control experiments, the special gas mixture used medical air (21% O2 + 79% N2). One asterisk = P<0.05 for the comparison between red and blue symbols. Two asterisks = P<0.01 for the comparison between red and blue symbols. Error bar = standard error.
