TORONTO - What is it about a baby's face that draws us like bits of metal to a magnet? Well, scientists think they may have the answer -- and it's all in the brain.

In experiments using a neural scan, researchers at Oxford University found that a specific region of the brain associated with emotion lights up with activity within milliseconds of subjects seeing pictures of infants' faces.

But images of adult faces didn't elicit the same response, said co-principal investigator Morgen Kringelbach, noting that the reaction was the same for both men and women, and for people with and without children.

"We can see what happens immediately in the brain," the neuroscientist said in a phone interview from England on Tuesday. "And what you see is activity starting from the back of the brain, just like you would with any visual stimuli."

Quite to the scientists' surprise, they found that when subjects looked at babies' faces - and only babies' faces - the region of activity shot ahead to an area of the brain above the eyes, known as the medial orbitofrontal cortex.

Kringelbach said the response in the brain to a baby's face - which occurs within one-seventh of a second, or a little more than in the blink of an eye -- is "almost like a signature."

"So that's what makes us think that what happened is really almost like a parental instinct, certainly something that tells us that babies are special, that they need caring for."

"And we know that people who have lesions to that region (of the brain) basically lack emotions."

The researchers, whose study is published in Wednesday's online edition of a Public Library of Science journal, believe their finding could have important applications for identifying postpartum depression.

They suggest brain scans could provide a potentially predictive test for people prone to postpartum depression, a disease marked by an inability to bond with a newborn, which occurs in about 13 per cent of mothers and three per cent of fathers.

"One thing that's well-established in postnatal depression is there's something wrong or something different about the way the mothers and the fathers approach the kids, the way they respond to them," he said. "It's almost as if some of them don't look at their babies."

While there may be many reasons why postpartum depression occurs, Kringelbach said it may be that parents with the disorder don't have the "neural signature" that allows them to respond to their infant.

"If you don't get that signal, maybe that's why you get into this state: 'I should be feeling something for my baby, yet I don't."'

"If that's true, and that's what we're assessing at the moment, then we might be able to use that to pick up much earlier who is more likely to become postnatally depressed."

Dr. Ariel Dalfen, a psychiatrist at Toronto's Mount Sinai Hospital, said the study lends credibility to the belief that depression and postpartum depression are biological in origin.

"I see a lot of women with postpartum depression, so I spend a lot of time helping them realize that this is really an illness and it's not just they're being weak or lazy people," Dalfen said.

"They don't feel love for their children, and they can't figure it out. They don't want to be with their kids, they don't get excited by their kids, they don't enjoy their kids," she explained.

"And it becomes a sort of feedback loop, because the mother sees the child, doesn't respond and then the infant doesn't respond when it doesn't get the kind of response it needs to feel safe and loved."

Kringelbach said it's important to identify and treat postnatal depression in a parent, not just for that individual but also for their child's long-term well-being.

"Because we know that children of people with postpartum depression ... don't do as well on emotions. They are much more likely to be depressed themselves and much more likely to have anxiety disorders."