Effects of photic stimulation on heart rate of infant rats.

We present the results of three experiments which characterize the heart rate (HR) response of infant rats at a number of different ages to the onset, or the offset, of a bright light. Experiment 1 demonstrated that at ages 5, 8, 11, 14, 17 and 21 days, a sudden increase in ambient illumination caused a HR deceleration, whereas a sudden decrease in ambient illumination caused a HR acceleration. At each age, the HR acceleration did not habituate over 60 stimulus presentations, whereas at all ages beginning at 8 days, the HR deceleration did habituate. By varying the intensity of either the visual or thermal component of the stimulus at ages 8 and 17 days, Experiment 2 revealed that the directional effects were a result of the visual and not the thermal component of the stimulus, thereby ruling out any account of the findings in terms of local adaptational reflexes. Finally, Experiment 3 varied the rate of stimulation, tested for spontaneous recovery, and demonstrated that the decrement in response magnitude observed across trials conformed to several of the accepted parametric characteristics of habituation, and could not be accounted for by changes in baseline HR across trials. These results are discussed in terms of traditional interpretations of HR changes as orienting and defensive reactions.