Ever wonder why adults tend to go ga-ga when they see a baby?

Scientists report that sophisticated imaging showed that seeing a baby’s face lit up a specific region of the adult brain associated with reward circuitry. This “Christmas tree” effect didn’t occur when adults looked at another adult face, suggesting there’s a neural basis for protective, nurturing feelings triggered by babies.

And the findings could also shed some light on postnatal depression, which affects some 13 percent of new mothers, the study authors said.

Other experts, however, were divided on the clinical implications of the findings.

“It’s interesting that clearly a brain area may be related to an instinctive behavior such as looking at a baby’s face,” said Paul Sanberg, director of the University of South Florida Center for Aging and Brain Repair. “I think [the study authors] are right that this could [be related to] an underlying brain mechanism of postnatal depression, where this part of the brain may be altered and could effect the ability of mothers to respond to infants’ cues.”

But Dr. Jonathan Friedman, assistant professor of surgery and neuroscience and experimental therapeutics at Texas A&M Health Science Center College of Medicine, was a bit more cautious when discussing the findings’ significance.

“It’s an interesting finding without question — it’s never been recorded before,” said Friedman, who is also director of the Texas Brain and Spine Institute in College Station. “It’s not known if it [the new study] will have clinical significance.”

In the 19th century, famed naturalist Charles Darwin noticed that something about infants caused adults to respond instinctively and care for them. In his paradigm, this increased the chances that one’s own genes would endure.

And Nobel Prize-winning 20th century zoologist Konrad Lorenz suggested that it was facial structure that prompted these nurturing responses in adults.

But the biological or neural basis for this phenomenon has yet to be explained — until, perhaps, now.

Using an imaging technique called magnetoencephalography, scientists led by researchers at the University of Oxford in England scanned the faces of 12 adults as they looked at images of 13 infant and 13 adult faces. The study participants had never seen the images before. The faces were matched for emotional content, attractiveness and other features.

Brain activity started in the medial orbitofrontal cortex region of the brain within one-seventh of a second after seeing infant faces, but not adult faces. The responses were considered too rapid to be consciously controlled.

This region of the brain has been implicated in reward behavior; it also appears to be involved in visual object recognition. And depression has been linked to another region of the brain — the subgenual cingulate cortex — that is connected to the medial orbitofrontal cortex.

The next step, Sanberg said, would be to see how the adult brain reacts to one’s own child. “It could be worth taking this further to see if this is involved in imprinting,” he said. “Are there different connections when it’s your own child? It’s of interest from an evolutionary point of view.”

(The study results were published Feb. 27 in the journal PLoS ONE.)