Fig. 4. The effects of chronic hypercapnia (CHC; open squares), chronic hyperoxia (CHO; filled circles) and chronic hyperoxic hypercapnia (CHH; filled triangles) on (A) fictive breathing frequency (fictive breaths min^-1), (B) fictive episodes min^-1, (C) fictive breaths per episode, (D) integrated fictive breath area (Vxs), (E) total fictive ventilation index (Vxs min^-1) and (F) fictive breath duration (s) recorded from in vitro brainstem-spinal cord preparations. The data are plotted as the mean ± 1 s.e.m. Letters (a-c) represent significant differences at the different artificial cerebral spinal fluid (aCSF) pH levels within any group. A plus sign (+) represents a difference from the CHC value. A number sign (#) represents a difference from the CHH value.