Fig. 4.

The raw BOLD signal for 2 voxels from the same patient is shown. One voxel demonstrates positive reactivity to CO₂ (black line) while the other demonstrates negative reactivity (grey line). The P_ETCO₂ (dashed line) varies between approximately 40 and 50 mm Hg. The grey signal shows an initial drop as soon as PCO₂ is elevated. However, this is followed by a signal increase during the high PCO₂ plateau, which we interpret as a combination of some residual component of true reactivity (vasodilation of vascular beds within this voxel) coupled with steal phenomenon as described in the text.