Fig 2. Signal intensity variations during a single experiment for the illustration of data analysis, with the subject breathing a gas mixture containing 5% CO₂. A, Signal intensity of the activated visual cortex consists of both fBOLD and hBOLD components. For the prehypercapnic and posthypercapnic phases, the signals corresponding to the 2 visual stimulation blocks (2 OFF and 2 ON, as indicated in the figure) are used for fBOLD signal intensity analysis. For the hypercapnic phase, the signals of the first OFF-ON cycle and 2 OFF-ON cycles at the end are used to reflect the transient-state response and the steady-state response, respectively. The fBOLD signal intensity change in each phase is calculated by the difference between the average of 2 ON-state signals and the average of 2 OFF-state signals. Only 1 OFF-ON cycle is chosen for transient-state hypercapnia analysis to minimize the influence from hBOLD-induced baseline changes. B, Signal intensity of the brain area near the visual cortex but without showing fBOLD activations consists of the hBOLD component only. The hBOLD signal intensity change is computed as the difference between the average of the last 25 scans in the hypercapnic phase and the average of the last 50 scans.