Previous studies have reported that spontaneous startles occur most frequently in the quiet sleep states, and have posited an energy release model in which spontaneous startles occur to release energy which would otherwise wake the infant. An alternative suggestion is that startles serve a homeostatic function by increasing the activity of the infant during periods of low arousal. The purpose of the current study was to examine the function of spontaneous startles using the ongoing cardiac and respiratory activity as indices of arousal.

Twenty-six newborn infants were assigned to two groups. The first group was exposed to auditory stimulation which previously had been shown to decrease heart rate and respiratory rate for the first half of the one hour observation period. To the extent that spontaneous startles are related to periods of low arousal, decreasing the ongoing activity via the auditory intervention was expected to increase the rate of startles. The second group received no auditory stimulation.

The occurrence of spontaneous startles was preceded by periods of lower than average heart rate and decreasing respiratory rate. Startles were followed by periods of increasing heart rate and further decreasing respiratory rate. In addition, the auditory intervention group reliably showed both a lower heart rate and an increased number of startles while exposed to the auditory stimulation, while the nonintervention group showed comparable rates of startles and heart rates in both halves of the observation period. Further, regardless of group status, most infants had their higher rate of startles in the period in which they had their lower heart rate. Finding lower heart rate and decreasing respiratory rate preceding startles, and lower heart rate and increased number of startles when exposed to the auditory stimulation, suggests that spontaneous startles modulate periods of low arousal in newborn infants.