Mary C. Larson’s research while affiliated with University of Minnesota Duluth and other places

To describe the behavioral and physiological responses associated with colic, the responses of 20 two-month-old infants with and 20 without colic were studied during a physical examination. Parents kept a diary of infant behaviors (including crying and fussing) for 3 days following the visit. Using Wessel, Cobb, Jackson, Harris, & Detwiler criteria, colic was defined as fussing/crying for 3 hr or more on each of the 3 days. Behavioral data coded by ‘blind’ observers showed that during the physical exam, colic infants cried twice as much, cried more intensely, and were more inconsolable than were control infants. Despite these behavioral differences, heart rate, vagal tone, and cortisol measures indicated no appreciable difference in physiological responsivity for the two groups. At home, parents collected saliva cortisol samples at wakeup, midmorning, midafternoon, and evening for 2 days. In a finding similar to that shown by the laboratory data, the colic and control infants did not have different levels of daily average cortisol. These laboratory and home data provide no evidence of greater responsivity in the physiological substrate of difficult temperament for colic infants and are consistent with evidence of similarity in temperament once colic is resolved. At home, compared with control infants, colic infants did display a blunted rhythm in cortisol production. By diary, they also slept about 2 hr less per day than did control infants. Nighttime sleep was still significantly different when fussing/crying was statistically controlled. These data suggest that colic might be associated with a disruption or delay in the establishment of the circadian rhythm in activity of the hypothalamic-pituitary-adrenocortical axis and associated sleep – wake activity.

The decrease in responsiveness of the hypothalamic-pituitary-adrenocortical (HPA) system is marked over the first months of life. Seventy-eight healthy infants (44 girls), 7 to 15 weeks old, were given a laboratory mock physical examination. Salivary cortisol samples were collected pre- and postexamination and at home. Behavioral state during the examination and home sleep/wake activity were measured. Subjects younger than 11 weeks showed an increase in pre- to postexamination cortisol, while older subjects did not. Further, there was no decrease in behavioral distress to the examination with age. Infants who showed an early- morning peak (EMP) in home cortisol levels were significantly older and were likely to be those who slept through the night. However, the presence of an EMP was not associated with a lack of cortisol response to the examination. The decrease in cortisol responsiveness witnessed around the age of 3 months is presumably due to other processes associated with age, and not with the expression of the day-night rhythm in basal cortisol.

The decrease in responsiveness of the hypothalamic–pituitary–adrenocortical (HPA) system is marked over the first months of life. Seventy-eight healthy infants (44 girls), 7 to 15 weeks old, were given a laboratory mock physical examination. Salivary cortisol samples were collected pre- and postexamination and at home. Behavioral state during the examination and home sleep/wake activity were measured. Subjects younger than 11 weeks showed an increase in pre- to postexamination cortisol, while older subjects did not. Further, there was no decrease in behavioral distress to the examination with age. Infants who showed an early-morning peak (EMP) in home cortisol levels were significantly older and were likely to be those who slept through the night. However, the presence of an EMP was not associated with a lack of cortisol response to the examination. The decrease in cortisol responsiveness witnessed around the age of 3 months is presumably due to other processes associated with age, and not with the expression of the day–night rhythm in basal cortisol. © 1998 John Wiley & Sons, Inc. Dev Psychobiol 33: 327–337, 1998

To investigate the relations among popular measures of neonatal stress and their link to subsequent temperament, 50 full-term newborns from a normal care nursery were examined responding to a heelstick blood draw. Baseline and heelstick measures of behavioral state, heart period, vagal tone, and salivary cortisol were obtained. Recovery measures of behavioral and cardiac activity were also analyzed. Mothers completed Rothbart's Infant Behavior Questionnaire when their infants reached 6 months of age. Baseline vagal tone predicted cortisol in response to the heelstick, suggesting that baseline vagal tone reflects the infants' ability to react to stressors. Greater reactivity to the heelstick (more crying, shorter heart periods, lower vagal tone, and higher cortisol) was associated with lower scores on "Distress-to-Limitations" temperament at 6 months. This finding was consistent with the expectation that the capacity to react strongly to an aversive stimulus would reflect better neurobehavioral organization in the newborn. Recovery measures of cardiac activity approximated and were correlated with baseline measures indicating the strong self-righting properties of the healthy newborn. Finally, vagal tone and salivary cortisol measures were not significantly related, suggesting the importance of assessing both systems in studies of the ontogeny of stress-temperament relations.

The effect of a positive first-time experience on adrenocortical activity was examined in 48 6- to 13-month-old infants who took part in two sessions of a YMCA-like mother-infant swim class. Experience was manipulated by comparing Novice swimmers with infants who previously had taken a swim class, and by examining responses to first versus second class session. In addition, the effects of temperamental fear of novelty were examined by maternal report prior to the first session. Preswim and Postswim cortisol levels were compared to measures obtained at the same time of day in two comparison groups, one tested at home and one tested in a playroom at the University. The results yielded no evidence of an adrenocortical stress response (elevated cortisol) to the swim sessions for either the Novice or Experienced swimmers. Infant temperamental fearfulness predicted behavioral but not adrenocortical responses during swimming. Postswim cortisol levels were significantly lower than posttest levels in the two baseline comparison groups, and lower postswim cortisol levels were associated with less negative and more positive emotional behavior and engagement of the swimming experience. These results are consistent with the hypothesis that emotions serve as a major pathway influencing adrenocortical activity, and provide no support for the uncertainty hypothesis of adrenocortical activation.

This manuscript reports on the results of 2 experiments dealing with behavioral and adrenocortical responses to separation among 9-month-old human infants. In both experiments the social context of separation was manipulated. The results of Experiment 1 yielded evidence of a statistically significant adrenocortical response to 30 min of separation under conditions in which the substitute caregiver responded sensitivity to infant distress, but was busy and relatively noninteractive when babies were not distressed during the separation period. Altering the behavior of the substitute caregiver such that she was warm, responsive, and interactive throughout the separation produced a significant reduction in adrenocortical activity and in negative affect. In fact, these measures were not significantly different than those obtained when the mother and infant remained together in the playroom (No Separation). In Experiment 2, the effects of group versus singleton care were examined using the less stressful mode of substitute caregiving as described above. No significant condition differences in behavioral distress or cortisol were found. Furthermore, neither condition elicited a significant increase in cortisol over basal levels. Finally, these data provide evidence that maternal reports of infant Distress to Limits temperament, using Rothbart's Infant Behavior Questionnaire, predict adrenocortical responses to separation, while reports of Fear of Novelty do not.

This manuscript reports on the results of 2 experiments dealing with behavioral and adrenocortical responses to separation among 9-month-old human infants. In both experiments the social context of separation was manipulated. The results of Experiment 1 yielded evidence of a statistically significant adrenocortical response to 30 min of separation under conditions in which the substitute caregiver responded sensitively to infant distress, but was busy and relatively noninteractive when babies were not distressed during the separation period. Altering the behavior of the substitute caregiver such that she was warm, responsive, and interactive throughout the separation produced a significant reduction in adrenocortical activity and in negative affect. In fact, these measures were not significantly different than those obtained when the mother and infant remained together in the playroom (No Separation). In Experiment 2, the effects of group versus singleton care were examined using the less stressful mode of substitute caregiving as described above. No significant condition differences in behavioral distress or cortisol were found. Furthermore, neither condition elicited a significant increase in cortisol over basal levels. Finally, these data provide evidence that maternal reports of infant Distress to Limits temperament, using Rothbart's Infant Behavior Questionnaire, predict adrenocortical responses to separation, while reports of Fear of Novelty do not.

This manuscript reports on the effects of stressors repeated at a 24-hr interval on three samples of human newborns. In Study 1, newborns meeting criteria for obstetric and perinatal optimality experienced either 2 mock Discharge Exams, viewed as a type of handling stressor, or 2 Heelstick Blood Draws, viewed as a type of nocioceptive stressor. As in a previous study, newborns meeting optimality criteria showed habituation of the adrenocortical response to the repeated Discharge Exam. The adrenocortical response to the repeated Heelstick tended to increase or sensitize. In Studies 2 and 3, samples of newborns with more nonoptimal obstetric complication scale scores were examined. These newborns failed to show habituation of the adrenocortical response to the Discharge Exam, but responded like "Optimal" newborns to the repeated Heelstick. The applicability to these data of the Groves and Thompson (1970) Dual-Process Model of Habituation is discussed. The results also provided evidence of stability in behavioral distress independent of stability in adrenocortical activity. The implications of these data for studies using cortisol as an index of the physiological basis of early temperament are considered.