Fig. 2

a BOLD response to the CO₂ ramp with the ETO₂ tension clamped at ~ 100 mmHg. The expected response with increased BOLD signal to the ramp stimulus is depicted by orange voxels. A diffuse response is seen. The scale is t scores based on fit to the GLM from SPM first-level analysis with “hotter” colors indicating a better fit to the GLM. The blue voxels depict the inverse response with “colder” colors indicating a better fit to the inverse response to the CO₂ ramp. The t score had to exceed 3.11 (p = 0.001) to be colorized. b The response of one voxel to the CO₂ ramp. The incremental and rapid increase with the step change in CO₂ (gray scale) is seen by the red dots representing the BOLD scan signal intensity at that moment in time. c The inverse response of one voxel to the CO₂ ramp. The incremental and rapid decrease with the step change in CO₂ (gray scale) is seen by the red dots representing the BOLD scan signal intensity at that moment in time. These data are from subject 3