(A) Scatter plot of the effect of treatment on ventilation of study 1 at baseline (B), clamped ventilation (C) and time points P1 (carbon dioxide-clamp plus alfentanil (ALF)-low), P2 (carbon dioxide-clamp plus ALF-low plus GAL021-low), P3 (carbon dioxide-clamp plus ALF-low plus GAL021-high), and P4 (carbon dioxide-clamp plus ALF-high plus GAL021-high). The subjects that participated in studies 1 and 2 are indicated in orange. (B) Scatter plot of the effect of treatment on ventilation of study 2 at baseline (B) and time points P1 (ALF-low), P2 (ALF-low plus GAL021-low), P3 (ALF-low plus GAL021-high), and P4 (ALF-high plus GAL021-high). The subjects that participated in studies 1 and 2 are indicated in orange.