A baby's smile does more than warm a mother's heart - it also lights up the reward centers of her brain, according to the results of a brain imaging study.

The finding, investigators say, could go a long way in helping researchers dissect the unique bond between mother and infant and how it sometimes goes wrong.

"The relationship between mothers and infants is critical for child development," Dr Lane Strathearn, of the Human Neuroimaging Laboratory at Baylor College of Medicine, Houston noted in a statement.

"For whatever reason, in some cases, that relationship doesn't develop normally. Neglect and abuse can result, with devastating effects on a child's development," Strathearn explained.

Strathearn and colleagues used magnetic resonance imaging (MRI) to scan the brains of 28 first-time mothers of 5- to 10-month-old infants while they looked at photos of their own babies and other infants.

In some of the photos babies were smiling or happy. In others, the infants were sad and in some they had neutral expressions.

The investigators found that when the mothers saw their own infants' faces, key areas of the brain associated with reward lit up during the scans, suggesting increased blood flow to that area.

The areas stimulated by the sight of their own babies were those involved in thinking, movement, behavior and emotion. "These are areas that have been activated in other experiments associated with drug addiction," said Strathearn.

"It may be that seeing your own baby's smiling face is like a 'natural high,'" the investigator added.

The strength of mom's reaction depended on her baby's facial expression. "The strongest activation was with smiling faces," Strathearn said. There was less effect from pictures of their babies with sad or neutral expressions.

"We were expecting a different reaction with sad faces," the researcher explained. In fact, the team found little difference in the reaction of the mothers' brains to their own babies' crying face compared to that of an unknown child.

Overall, the mothers responded much more strongly to their own infants' faces than to those of an unknown baby.

"Understanding how a mother responds uniquely to her own infant, when smiling or crying," Strathearn said, may be the first step in understanding the neural basis of mother-infant attachment.

(China Daily/Agencies July 16, 2008)