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Mothers soothe their infants with physiological signals

It is well known that a mother can soothe her distressed baby better than anyone else can. With little more than a calm embrace and some soothing words, she can usually stop her baby from crying when nobody else succeeds. What is the source of this magic? 

The calming effect of a mother has been researched in the fields of pediatrics and behavioral neuroscience, but is not yet well understood. What is known, however, is that mothers who experience post-partum depression (PPD), which affects up to 20 percent of new mothers, don’t seem to have this magic touch.

“Myriad studies have shown that mothers with post-partum depression struggle calming their distressed babies,” said John Krzeczkowski, postdoctoral fellow in the Department of Psychology and with the LaMarsh Centre for Child and Youth Research. “However, it is not known how mothers’ soothing signals are transmitted to their baby, how postpartum depression disrupts this process, or if treating depressed mothers can alter these signals.”

In order to investigate the links between mother and infant physiology when infants were distressed, Krzeczkowski and his team worked with two sets of mother-infant pairs. In the control group, the 32 pairs of mothers and babies were healthy, but in the other group of 26 pairs, the mothers had received a PPD diagnosis within a year of giving birth.

The mother-infant pairs all went through three stages of interaction at the start of the investigation. In the play stage, mothers interacted with their infants in a normal way, for example by playing, singing, talking and touching. In the still-face stage, mothers had to maintain eye contact with their infants but show no expression and refrain from talking to or touching them. This “poker” face behavior was expected to make the infant distressed and agitated. The final reunion stage involved the mother re-engaging with her now-distressed infant and initiating normal interaction and play again. This last stage was the focus of the study. 

During each of the stages, the mothers and infants were monitored for heart rate variability. In healthy people, the heart does not beat at a perfectly regular rate but exhibits slight beat-to-beat variations. This is especially noticeable when comparing the heart rate while a person is breathing in with the heart rate as the person exhales. Usually, the heart rate speeds up during inspiration and slows down during expiration. The phenomenon is termed respiratory sinus arrhythmia (RSA) and it is an important component of overall heart rate variability. It is also a known indicator of a person’s emotional state.

In the reunion phase in particular, the researchers were watching the RSA readings for evidence of a “feedback loop” of reciprocal heart-rate signals passing between mother and baby. They also examined whether the influence of maternal RSA on subsequent infant RSA (measured at the next second) strengthened during the time of the reunion phase. 

The groups were tested in this way twice: once to establish a baseline and then a second time, nine weeks later. In the intervening time, the mothers with PPD received, and were observed to benefit from, a course of cognitive behavioral therapy (CBT) that was aimed at improving their affective state.   

The findings of the study show that heart rate changes in healthy mothers preceded those of their infants. According to Krzeczkowski, this suggests that the mothers were leading a “soothing dance” that calmed their infants.  In the case of the PPD-afflicted mothers, however, it was the infants whose physiological signals led the dance, initially at least. After these mothers had received CBT treatment, their physiological signaling improved such that they then led the back-and-forth, just as the healthy mothers did.    

Krzeczkowski said the results represent a breakthrough in what has long been a blind spot in behavioral neuroscience.

“This study demonstrates empirically, for the first time, that synchronized physiology between mothers and babies plays a role in soothing distressed infants, and that treating PPD with CBT can improve the synchronicity patterns and thereby augment mothers’ ability to soothe their distressed babies,” explained Krzeczkowski. 

“So we now know a bit more than we ever did before about how soothing signals are transmitted in real time on a moment-to-moment scale between mothers and infants.”

The authors call for further studies to test whether an improvement in the RSA feedback loop’s soothing effects is directly attributable to the CBT treatment received by the mothers with post-partum depression. 

“Because of our study’s observational design, we can’t say categorically that positive changes were specifically due to CBT treatment,” said Krzeczkowski. “The two are correlated but there could be confounding factors – for example, the specific methods that individual mothers use to calm their babies, like singing, talking and touch. But we want more people to get treatment for PPD. We hope that by showing causality and efficacy, it can increase the idea that these programs can benefit them.”

The research is published in the Journal of Psychopathology and Clinical Science.

By Alison Bosman, Staff Writer

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