Development and habituation of the heart rate orienting response to auditory and visual stimuli in the rat.

Development and habituation of responsitivity to sound and visual stimuli were studied in 8-19 day-old pups by using a change in heart rate (HR) as the indicant response. A change in HR to sound (white noise) first occurred around 14-15 days of age and was acceleratory in nature. Within 3 days the direction of the HR response shifted to bradycardia. The HR response to light was also characterized by tachycardia in young pups (14 days) which shifted to bradycardia as the pups matured (16 days and older). Experiment 2, the HR response to 15- and 19-day-old rats was studied at three auditory stimulus intensities (60, 70, and 90 dB, B scale). Stimulus intensity did not alter the direction of the HR response, but it did influence the magnitude of the cardiac response in the 19-day-old pups. These data suggest that even weak sensory stimuli elicit a defensive reaction (HR acceleration) during the first few days that follow onset of function in each sensory system. Later in development the same stimulus elicit an orienting response (cardiac deceleration).