Everyone knows nothing melts a mother's heart like a baby's smile.

But Houston researchers have found that it also activates a region of the brain known as the reward center, a middle area associated with feelings of euphoria. Previous research has shown the reward center also flips on when addicts take drugs.

"This could be described as a natural high," said Dr. Lane Strathearn, a pediatrics professor at Baylor College of Medicine and Texas Children's Hospital and the research's lead investigator. "Mothers just had to look at their baby's smiling face, and blood flow in the region picked up."

Strathearn said the research is part of an effort to use new brain-imaging technology to better understand the mother-infant bond, a key to healthy child development. When something goes wrong with that bond and the relationship doesn't develop normally, he said, it typically leads to abuse or neglect, which are devastating to a child's development.

Currently, little is known about what exactly occurs when the bond goes wrong, or even when it's right.

The finding, reported in today's edition of the journal Pediatrics, one day could lead to the development of a drug treatment for mothers emotionally detached from their children, Strathearn said. Right now, he said, researchers are just trying to understand the process in the brain.

In conducting the study, Strathearn's team captured still images of each child smiling, crying and displaying neutral expressions.

They then showed 28 mothers the photos of their own babies and infants they had never seen before while the women were in a functional imaging scanner, a machine that measures blood flow in the brain.

Though blood flow picked up when the mothers were shown the photos of other women's infants, the greatest activity occurred when each mother looked at her own baby's pictures, researchers found. The reaction was strongest when the baby was smiling.

The activity occurred in reward center pathways associated with dopamine, a pleasure-related chemical that ferries signals between brain cells.

Unexpectedly, the researchers found little difference in blood flow when mothers were shown photos of their own crying babies compared with pictures of the other children.

The finding about the "natural high" caused by a smiling baby contributes to a growing body of research about what triggers the brain's reward center. Besides cocaine and other drugs, some triggers include humor, favorite foods, photographs of attractive faces, praise, sex, gambling and even charitable giving, according to recent studies.

The study was conducted at Baylor's Human Neuroimaging Laboratory.

todd.ackerman@chron.com