CARDIAC ACTIVITY IN INFANCY - RELIABILITY AND STABILITY OF INDIVIDUAL-DIFFERENCES

The goals of this study were to examine: (a) normative developmental c hanges 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 st udy were 5, 7, 10, and 13 months old, a 5-min sample of cardiac activi ty was collected while the infants sat on their mothers' laps in a qui et, attentive state. A second 5-min sample of cardiac activity was als o collected at 7, 10, and 13 months following a 20-min long battery of emotion-eliciting stimuli. Significant developmental increases were o bserved 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. Individu al differences in measures of heart period and cardiac vagal tone were stable over 2- and 3-month periods.