Doctors agree that when it comes to feeding your baby, breast is best. Most research has focused on health advantages to the infant and, more recently, on physiological and psychological benefits for the mother. Now research highlights a mechanism by which nursing may influence the mother-infant bond: it seems the brain of a breast-feeding mother is especially receptive to signals from her baby.

Graduate student Pilyoung Kim and her colleagues at Yale University’s Child Study Center used functional MRI to scan the brains of 20 women while exposing them to their baby’s cry or image. Preliminary results suggest that three weeks after giving birth, breast-feeding mothers showed greater responses to indicators of their own infant (as compared with those of another baby) than formula-feeding mothers did, especially in limbic, hypothalamic and midbrain areas—brain regions involved in emotion and motivation.

Kim’s team believes this difference stems mostly from oxytocin, a hormone that has received much attention for its role in social bonding. Nursing stimulates the production of oxytocin, which is thought to facilitate a mother’s attentiveness to her baby.

Three to four months after they gave birth, the difference in the overall amount of brain activity between breast- and formula-feeding moms was smaller, suggesting that over time a mother’s reaction to her infant may start to depend more on experience than on hormone levels. The areas of the brain more strongly activated in formula-feeding mothers, however, were different from those activated in breast-feeding mothers. They included the prefrontal cortex and other regions typically linked to social and cognitive behaviors.

Because all the subjects in this study were healthy women from similar backgrounds, Kim warns that the specific patterns of brain activation found in this study may not generalize to a more diverse population. The results may be valuable, however, for mothers who have trouble with their newborns because of depression or environmental factors such as poverty. Breast-feeding could be one way for these mothers to tap into the positive cycle involving oxytocin and the early mother-infant relationship, which has long-lasting effects on a child’s development.