Fig. 4. Combined data for midbrain reticular formation (MRF) single-unit activity (peristimulus-time histogram, bin width 1 s; mean ± SD). (A ) When torso isoflurane was 1.2 ± 0.2% (cranial isoflurane = 1.2 ± 0.2%), noxious stimulation evoked a minimal response. (B ) When torso isoflurane was 0.3 ± 0.1%, the noxious stimulus evoked a marked response. *P < 0.05 compared with spontaneous activity when torso isoflurane was 0.3% and spontaneous and evoked activity when torso isoflurane was 1.2%.