Cardiac activity in infancy: Reliability and stability of individual differences
The goals of this study were to examine: (a) normative developmental changes in heart period and cardiac vagal tone: and (b) the reliability and stability over time of individual differences in the two measures. When the 73 infants participating in this short-term longitudinal study were 5, 7, 10, and 13 months old, a 5-min sample of cardiac activity was collected while the infants sat on thier mothers' laps in a quiet, attentive state. A second 5-min sample of cardiac activity was also collected at 7, 10, and 13 months followin a 20-min long battery of emotion-eliciting stimuli. Significant developmental increases were observed in heart period but not in the measure of cardiac vagal tone. Stress-related decreases in cardiac vagal tone and increases in heart rate were observed after the emotion-eliciting stimuli ended. Individual differences in measures of heart period and cardiac vagal tone were stable over 2- and 3-month periods.