Figure 2.

The stimulus sequence for a representative control subject (ID 26, F, 38). The PETCO₂ stimulus (black line) and whole brain average BOLD voxel response (red line) are shown. The first part of the stimulus, between points (a) and (b), consisted of a step change used to measure time delay reported previously (12). The subject’s resting PETCO₂ of 41 mmHg (a) was increased by 10 mmHg for 2 min then returned to resting PETCO₂ (b). The ramp portion shown between points (c) and (e) was used for assessing the resistance response. The subject’s resting PETCO₂ was decreased by 10 mmHg (c) and then slowly increased to resting PETCO₂ (d), further increased to ~15 mmHg above resting (e), and returned to resting PETCO₂.