Conditioning-specific reflex modification of heart rate as indexed by a Pretest to Post Test increase in interbeat interval (IBI, top panel) and PQ interval (bottom panel) at the highest intensities of the unconditioned stimulus (US). Data are shown as the mean change (± SEM) in inter-beat interval (IBI) from baseline during the first 20-trial block of US presentations of the Pretest and first Post Test (Post1) at the five different US intensities (0.1 mA, 0.25 mA, 0.5 mA, 1.0 mA, and 2.0 mA), collapsed across duration and averaged across subjects (n=58). Single and double asterisks indicate significant differences between groups at p < 0.05 and p < 0.01, respectively.