Fig. 2. Reaction time for digit–symbol substitution increased at hemoglobin concentrations of 6 and 5 g/dl but not at 7 g/dl, compared with 14 g/dl in nine healthy subjects. Data are mean ± 95% confidence interval. *P < 0.05 for comparison of the difference between experimental and control days for the difference between the value at the indicated hemoglobin concentration and the value at the baseline hemoglobin concentration of 14 g/dl.