Ventilatory response, expressed as % baseline, to focal acidification of: (A) the retrotrapezoid nucleus (RTN) alone (with simultaneous acidification of sites lying outside the raphe obscurus (ROb)), (B) the ROb alone (with simultaneous acidification of the sites lying outside the RTN), and (A + B) both the RTN and the ROb simultaneously. Asterisk at far right of A and of A + B indicates a significant (P < 0.01) increase as determined by repeated-measures ANOVA on the absolute values for ventilation. Asterisks at individual time periods indicate a significant (P < 0.05) difference from baseline as determined by post hoc comparison with Dunnett’s test. Modified from Figs. 3, 4, and 5 in Dias et al., J. Appl. Physiol. 2008, (45) and used with permission.