The relationships between brain functions and the respiratory system are complex. Disentangling brain activity related to CO 2 changes from nonspecific vasoreactivity is a challenge when studying brain activity involved in the control of breathing with fMRI. In this work, we analyzed a dose dependent relationship between arterial CO 2 levels and brain response. To accomplish this goal, we developed a gas administration protocol, together with multi-subject ICA and specific nonlinear post-processing analysis. Our results highlighted a supra-linear response to CO 2 challenges in brainstem, thalamus and putamen. Results were discussed in the light of current knowledge about the central respiratory network.