Persistent and emergent cardiac responses elicited by simple stimuli

Heart rate and respiration were recorded as rabbits were exposed to visual and auditory events: light-on, light-off, light-change, noise-on, noise-off and noise-change. Each event lasted one second. One kind of stimulus event was presented ten times each session. Each kind of event was used for five sessions. On the following sessions (forty) two kinds of stimulus events were continued and used during the same session in order to introduce novelty. A beat-to-beat analysis of heart rate with corresponding respiration analysis showed (1) that rise and decay time of the reactions tended to decrease although magnitude might increase with repetition of the stimulus, (2) that some reactions elicited by some stimuli persisted beyond 400 trials, (3) that some reactions, not seen at first, appeared after 20–30 trials, (4) that respiration could be ruled out as a necessar causal factor for the heart rate changes, (5) that rate changes are specific to the modality or quality of the stimulus (e.g. noise-on vs. noiseoff or light-on), (6) that some reactions may be attenuated by stimulus novelty.