To the lay person, it's one of life's mysteries - operating almost as if by magic - and not even in the research fields of pediatrics and behavioural neuroscience is the process well understood.
The story is also different for new mothers who experience postpartum depression (PPD), which affects up to 20 per cent of people who give birth.
John Krzeczkowski
"Many studies have shown that mothers with postpartum depression struggle to calm their distressed babies," says John Krzeczkowski, postdoctoral Fellow in the Department of Psychology at York University's Faculty of Health and a trainee at 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.
"To investigate this, our team examined links between mother and infant physiology when babies were distressed."
Krzeczkowski is the lead author of "Follow the leader: Maternal transmission of physiological regulatory support to distressed infants in real-time," published in the Journal of Psychopathology and Clinical Science.
Krzeczkowski's team - which included Professor of behavioural neuroscience Louis A. Schmidt (University of McMaster), epidemiologist Mark A. Ferro (University of Waterloo), as well as Dr. Ryan J. Van Lieshout, a psychiatrist specializing in perinatal mental health at McMaster - worked with two sets of mother-infant pairs: a control group of healthy mothers and babies; in the other group, mothers who had received a PPD diagnosis within a year of giving birth.
In the study, mothers and babies were observed during three phases of interaction. In a play phase, mothers played with their infants as they normally would (e.g. singing, talking, touching). In a still-face phase, mothers were instructed not to touch or speak to their babies but to maintain eye contact and adopt an expressionless "poker" face, which can distress infants. The final reunion phase was the focus of the study: here, mothers were permitted to re-engage with their now-distressed infants as they did in the play phase.
Throughout these phases, the team monitored both mother and baby heart-rate variability, a known indicator of emotional state. In the reunion phase, the researchers were watching these readings for evidence of synchrony of heart-rate signals between mother and baby.
The groups were tested in this way twice: once to establish a baseline and then a second time, several weeks later, after the mothers with PPD had received a course of cognitive behavioural therapy (CBT), improving their mood.
In the healthy control group, the team found that mothers' heart rate variability changes influenced those of infants, suggesting that they were leading what Krzeczkowski calls the "soothing dance."
Conversely, before treatment in the PPD-affected group, it was the infants whose physiological signals led the dance. But after CBT treatment, the PPD mothers' physiological signalling improved such that they led the interaction, just as the healthy mothers had.
Krzeczkowski says these results provide novel evidence in what has long been a blind spot in behavioural 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 synchrony patterns and thereby augment mothers' ability to soothe their distressed babies.
"This study provided important clues into how soothing signals may be transmitted in real time on a moment-to-moment scale between mothers and infants."
The paper calls for future studies to test whether improvement in the mother-infant respiratory sinus arrhythmia (RSA) synchrony pattern soothing effects can be casually linked to CBT treatment of PPD-suffering mothers.
"Because of our study's observational design," Krzeczkowski says, "we can't say conclusively that positive changes were specifically due to CBT treatment. However, it may be contributing, and we now need to replicate this finding and understand how it works. Our goal is to ensure that more people can get treatment for PPD when they need it. We hope that by conducting future studies that can show causality, it can increase the idea that these programs can benefit them."
Funding for the study came from the Brain and Behaviour Research Foundation.
More about John Krzeczkowski's research
Krzeczkowski describes his research as concerned with investigating the foundations of social and emotional development and examining how early interventions can optimize the developing brain. He says he was inspired to pursue these aims by his grandmother, who, as a public health nurse, established the Niagara Region's first maternal postpartum-depression support group in the late 1970s. In her honour, he says he has dedicated his career to determining whether programs that support the health of mothers/birthing parents not only improve their health but also benefit their children and families.
"The overall goal of my research is to determine how we can best harness the immense plasticity of the developing brain to prevent mental disorders and optimize the health of families in Canada and beyond," he says.
More information is available online.
This story was originally featured in YFile, York University's community newsletter.
