In the new study, moms from 11 countries consistently picked up, held and talked to their infants when they heard their infants sob. MRI scans were also taken of mothers' brains, which revealed heightened activity in regions tied to caregiving, movement and speech.
Finding such connections between the brain and behavior is in part what neuroscience is all about, said the study's lead author Marc Bornstein, chief of the National Institute of Child Health and Human Development's section in child and family research.
"As for the 'practical' side, infant cry is one of the most talked about and asked about issues for new parents. Cry also signals the health status of a child," Bornstein said.
"Infant cry excites some adults, mothers included, to respond with empathy and care but others with neglect or even abuse. Infant cry is a trigger to maltreatment. So understanding how mothers normally respond to cry at the behavioral and nervous systems levels is potentially telling," he said. "We hope this research will spur others to study brain responses associated with non-normal variations in parenting, such as mothers who maltreat."
The study involved 684 first-time healthy mothers from Argentina, Belgium, Brazil, Cameroon, France, Kenya, Israel, Italy, Japan, South Korea and the United States.
The researchers observed and recorded one hour of interactions between each mom and her baby, around 5½ months old, while at home.
The researchers found that the mothers had surprisingly consistent responses to their crying babies, "and in a very short amount of time from the start of the cry, five seconds, they preferred to pick up and hold or to talk to their infant," Bornstein said.
Using MRI technology, the researchers also scanned the brains of a separate group of 43 healthy first-time mothers in the United States. The mothers were scanned while they heard their own infant cry or make other noises. MRI scans were also taken of yet another group of 44 healthy moms in China, who were more experienced with infants, while they heard infant cries and other sounds that came from a database.
The MRI scans showed that in both groups, hearing infant cries generally activated regions in the brain tied to the intention to move, grasp and speak, the processing of auditory stimuli and caregiving.
Those brain areas that were activated in the study could be described as "readiness" or "planning" areas, said Robert Froemke, a neuroscientist at New York University who was not involved in the study.
"There's also widespread activation of the hearing part of the brain," Froemke said. "It also makes sense that there would be widespread activation because these (infant cries) are alarm cries."
'New mother's brains undergo dynamic changes'
Froemke has studied oxytocin
, a hormone that plays an important role in mother-infant bonding, in mice, and he has examined how it helps shape a mother's brain to respond to her offspring's needs.
In human mothers, such as the women in the new study, oxytocin and other brain chemicals could be at play in reinforcing the urgency of responding to a crying baby, Froemke said.
On separate occasions, previous studies unrelated to the new research have found associations between giving birth vaginally
to a mother having stronger brain responses to
her baby's cries.
Now, "the current study contributes to the existing literature on the human mother's brain by identifying the common brain regions that are sensitive to baby cry sounds across cultures," said Pilyoung Kim, associate professor of psychology at the University of Denver, who was not involved in the new study but led separate previous research on the neuroscience of motherhood.
"This is an important step toward future studies to better understand common, as well as unique responses that mothers in different cultures show to their own babies, in their brains and behaviors," she said. "New mothers' brains undergo dynamic changes to help the mothers to cope with stress and support their transition to motherhood."
Froemke praised the new paper for involving a cross-cultural sample of mothers, yet the study had some limitations.
All of the first-time mothers from 11 countries were not necessarily representative of their entire nations, Bornstein said, so more research is needed to determine whether similar findings would emerge in a larger sample.
"Also, we did not measure the brains of the same mothers for whom we measured behavior or vice versa, and so we are assuming that these brain-behavior associations hold," Bornstein said.
"After all, the robustness of the behavioral results across 11 countries and the fMRI results across three countries tells us at least that chance is unlikely to be operating," he said. "Scientists like to be cautious about assigning causality. This was not an experiment but the coordination of two sets of observations, about behavior and brain."
More research is also needed to determine whether similar responses to infant cries would appear in adults who are not mothers, Bornstein said.
On the other hand, by just 2 years old, many children have developed the social intelligence to understand adults' emotional reactions and their expressions of emotional sounds, like "awww" and "mmm," according to a separate study published in the Proceedings of the National Academy of Sciences
"Our study suggests that if the parent suddenly exclaims with delight or makes an affectionate coo at something in a scene, babies might be able to make a good guess at whether the parent is looking at something exciting or something adorable in the room," said Yang Wu, a doctoral student in the department of brain and cognitive sciences at the Massachusetts Institute of Technology and lead author of the study.
Toddlers could know what makes you go 'awww'
The study included a series of five experiments involving 230 children, 1 to 4 years old, and 16 adults. In the experiments, the children were asked to complete tasks, some of which involved linking the positive sounds or expressions made by the adults, such as "mmm," "awww" or "whoa," to images or objects that could be the probable triggers of those expressions, like a toy or food.
The researchers found that by around age 2, the children could make nuanced distinctions about the adults' positive emotions and connect the adults' emotional reactions to possible causes.
"The results were surprising in the sense that we found infants were able to make fine-grained distinctions among positive emotions while most previous research on early emotion understanding has focused on a few basic emotions," Wu said.
"So far, we've just looked at a handful of distinctions among positive emotions, chosen fairly arbitrarily, so we don't know the full space of infants' emotional understanding," she said. "There are a lot of open questions about infants' emotion representations in the first year of life."