Fig 5. Percentage fBOLD signal intensity changes under different CO₂ concentrations. The prehypercapnic and posthypercapnic fBOLD remains similar at the level of 3%–4%. During hypercapnia, changes in the fBOLD signals tend to increase in the transient state but step down in the steady state as the CO₂ concentration becomes higher. TS indicates transient state; SS, steady state.