By Sharon Begley

NEW YORK (Reuters) - Having a baby alters new mothers' brain activity, researchers have found, and a new study adds the first evidence of such changes in the brains of gay men raising children they adopted through surrogacy.

The men's pattern of brain activity resembles that of both new mothers and new fathers in the study.

The research, reported on Monday, could feed into the debate over whether gay men should be allowed to adopt children. Many U.S. adoption agencies will not work with same-sex couples, and some states prohibit them from adopting.

The current study, published in Proceedings of the National Academy of Sciences, was conducted in Israel, and builds on work by neuropsychologist Ruth Feldman of Bar-Ilan University and others, who showed that the brains of new mothers become hyper-reactive to their child's cries and other emotional cues.

It was not clear if that pattern is a result of the hormonal and other changes that accompany pregnancy or a response to the experience of motherhood.

To find out, Feldman and her colleagues videotaped 89 new mothers and fathers interacting with their infants at home. They then measured the parents' brain activity while watching the videos in an MRI tube, and again (to establish a baseline) while watching videos that their kids did not star in.

In the 20 mothers in the study, all primary caregivers, watching their babies triggered heightened activity in the brain's emotion-processing regions, particularly in a structure called the amygdala, which was five times more active than at baseline.

"These are regions that respond unconsciously to signs of an infants' needs, and that derive deep emotional reward from seeing the baby," Feldman said.

For the 21 heterosexual fathers - who were very involved in raising their baby but whose wives took the parenting lead - watching their infant increased activation of cognitive circuits, particularly a structure that interprets a baby's cries and non-verbal cues. It is the region that knows which squirm means "I'm about to scream" and which means "change me."

The 48 gay fathers raising children with their husbands seemed to be both mom and dad, brain-wise. Their emotional circuits were as active as those of mothers and the interpretive circuits showed the same extra activity as that of heterosexual fathers'.

Ideally, scientists would perform neuroimaging on men and women before and then after they became parents, to show definitely that any heightened activity followed junior's arrival and was not present before. Until they can do that, Feldman said, she is confident that the telltale brain activity results from parenting.

One clue: in gay fathers, but not heterosexual ones, the brain also had extra communication lines between emotional and cognitive structures. The more time a man spent as primary caregiver, the greater the connectivity. It was as if playing both parental roles caused the brain to integrate the structures required for each.

"Fathers' brains are very plastic," Feldman said. "When there are two fathers, their brains must recruit both networks, the emotional and cognitive, for optimal parenting."

(Reporting by Sharon Begley; Editing by Steve Orlofsky)