Parents might say a baby lights up their life, but a new study shows that an image of a smiling baby also "lights up" the reward centres of the mother's brain.

Researchers wanted to find out more about the effects of different factors in child development, and made use of a functional magnetic resonance imaging machine to scan the moms' brains as they looked at photos of their own baby as well as unknown babies.

"One of the most critical factors is the relationship an infant develops with the parent," said Dr. Lane Strathearn, assistant professor of pediatrics at Baylor College of Medicine and Texas Children's Hospital.

"So I wanted to look at those factors more closely," he said in a phone interview from Houston.

The researchers recruited 28 pregnant women in their final trimester who remained in the study for a year and a half.

Strathearn said that several months after birth, the research team videotaped the babies, and extracted still images of their faces in all different stages of emotion — smiling, crying, neutral and everything in between.

"And then we were able to use these images to present to the mothers while they were being scanned in the MRI scanner, to look at how their brains responded when they saw pictures of their own baby compared to a matched unknown baby that they'd never seen before," he said.

During the study, published Monday in the journal Pediatrics, researchers were particularly interested in parts of the mother's brain involved in reward processing, and associated with the neurotransmitter dopamine.

"When we did the analysis, we looked at the mothers' responses to her own baby's smiling compared to an unknown baby smiling, and also neutral and crying comparisons," Strathearn said.

"And it was really just the smiling faces where we saw difference in activation in these dopamine-reward processing regions of the brain. So we saw that when the mothers saw their own baby smiling these areas were very strongly activated in comparison to the unknown baby face."

He said it reinforced to researchers that the emotional response a mother has when her baby smiles activates reward pathways in the brain.

And this, in turn, reinforces and establishes caregiving behaviours in the mother, he surmised.

"We think that these cues that mothers receive from their babies actually process and stimulate a mother's response to her baby with regard to responsive caregiving."

But the researchers didn't notice any significant difference in the mothers' brains when they were looking at images of their own baby crying and an unknown baby crying. And likewise, when the babies had neutral expressions on their faces.

The researchers are further analyzing their data to see if they can detect differences in the responses of women based on their different styles of attachment to their babies.

"And we think this may have important implications in cases where that relationship doesn't become established as we would normally hope, and that may predispose to problems with child neglect or even child abuse," Strathearn said.

His study is not the first to involve brain scans and baby faces.

Researchers at Oxford University published a study in the online edition of a Public Library of Science journal in February that showed images of baby faces elicited a response in the medial orbitofrontal cortex, while images of adult faces did not.

Strathearn said the women in his study spent about 20 minutes in the MRI machine.

"It was interesting, some of the mothers when they did come out of the scanner told me that they felt like reaching out to their baby when they saw their baby on the screen — for some of these mothers at least, it was really a very strong stimulus for them, even being in the noisy scanner, lying completely still.

"The most important thing is that this response that mothers have to their babies is biologically driven, that there are particular brain systems in place to help forge this important relationship between a mother and baby. I think where it leads now is to look at where those systems aren't working normally, aren't functioning as we'd hope they would, and how that may be associated with difficulties in the relationship between mothers and their babies."

Dr. Jean Wittenberg, head of the Infant Psychiatry Program at the Toronto Hospital for Sick Children, said it's an important and interesting study.

"It's part of a general development in psychiatry and neuropsychology looking at finding the areas of the brain related to specific behaviours," he said. "It's a way of helping us understand more about the psychosomatic continuum."

Maternal responses are both psychological and physical and "each one has influence and impact on the other," he said.

Wittenberg noted that what happens in the first couple of years sets down patterns that can be lifelong. Caring for an infant can be tedious and frustrating, he said, but if the mother is rewarded, it becomes worthwhile.

"Mothers are responding very specifically to their own babies in a physiological way," he said.