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This article is from the In-Depth Report The Mother-Baby Bond

Baby's Little Smiles: Building a Relationship with Mom

How smiles—and pouts—are helping researchers probe the essence of the complex mother-infant bond.
baby smiling activates mom's reward centers in brain



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It’s probably not surprising that mothers excel at recognizing and interpreting the moods and emotions of their infants. Although infants can’t speak, mothers seem to know what their babies are thinking: they smile when their baby smiles and they frown when their baby is upset. Research suggests that the mother’s ability to understand the needs of her infant is very important for establishing a secure mother-infant relationship. However, the neural mechanisms that underlie these behaviors are poorly understood. Such knowledge is crucial for understanding normal as well as abusive and neglectful mothering. 

Maternal Rewards
In recent years, several studies have been carried out using functional magnetic resonance imaging (fMRI) to better understand how a mother’s brain responds to her own child’s cues. The most recent, led by neuroscientist Lane Strathearn and colleagues at Baylor College of Medicine, investigated what happens inside the brain of a mother when she looks at the facial expressions of her own infant. In the study, 28 first-time mothers were shown pictures of their seven-month old child that they had never seen before. (The pictures were taken when the mother was not present.) The pictures spanned a wide range of human emotion and included images of the child making happy, sad or neutral faces. These pictures were then matched with images of an unknown infant. The central finding was that seeing the happy face of the mother’s own infant activated all of the key areas in the brain associated with reward processing. These regions include the ventral tegmental area, substantia nigra and the striatum. This finding suggests that for mothers the sight of their smiling baby is a potent reward and represents a uniquely pleasurable experience. Furthermore, this neural response was graded, so that happy faces led to more activation than neutral faces. Sad faces generated the least activation. In other words, the response of mothers in their reward areas seemed to directly mirror the emotions the infant displayed.

The argument put forth by Strathearn and colleagues is that maternal behavior is fundamentally rooted in these reward areas. Positive sensory cues from infants, such as a smiling facial expression, stimulate dopamine release and thus promote responsive maternal care. However, many questions remain. In their paper, Strathearn et al. do not discuss the significance of the orbitofrontal cortex, although its activity was clearly influenced in the experiment by seeing one’s own infant’s compared with an unknown infant. The importance of the orbitofrontal cortex shouldn’t be too surprising, as this brain area is believed to receive ascending dopamine projections from reward areas and is critical in representing the “value” of a reward. Other studies have also demonstrated that the orbitofrontal cortex is correlated with the positive feelings of the mother, suggesting that it plays a key role in modulating maternal behavior.

Is the Smile Enough?
This study also raises a more fundamental question: is the infant smile the most important element for motivating maternal behavior? It goes without saying that the smiling face of one’s own baby is highly rewarding and encourages maternal care. On the other hand, babies aren’t always smiling and mothers must also learn to respond to infants in distress. In fact, a human mother’s response to an infant in distress is a good indicator of how responsive she is to other infant cues. Studies also show that abusive and neglectful mothers show less empathy and more aversive feelings towards a crying infant when compared with nurturing mothers, suggesting that how a mother reacts to a baby when it’s upset and not smiling is a crucial test of maternal behavior.

According to our own recent work, the orbitofrontal cortex and striatum were more activated in the brain of a mother when she looked at her own infant compared with other infants regardless of the infant’s situation or mood. These brain areas also showed a greater activation when mothers were viewing their infant when he or she was crying (the distressed condition) as opposed to when he or she was happy (the play situation). This discovery makes sense, as a distressed baby might require more effort and thought as the mother must quickly identify the source of the distress and respond appropriately. Another reason to highlight the importance of the orbitofrontal cortex in guiding maternal behavior is that, in our experiment, the activity of this brain area showed a positive correlation not only with pleasurable feelings but also the anxious feelings experienced by the mother. Of course, these anxious feelings are important for maternal care, as anxiety and worry can be powerful motivators. Taken together, these findings suggest that maternal behavior is guided by elaborate and complex neural mechanisms. Although reward processing is clearly an important part of this mental process—it seems to mediate maternal love and feelings of joy—other mechanisms are required in order to explain the full range of the mother-infant relationship.

In conclusion, a smiling face of a mother’s own infant is certainly rewarding and it motivates maternal care, but this is not the only motivator. We hope that in the future other aspects of maternal behavior—such as the maternal desire to protect her infant, which is a biologically essential mechanism for preservation of the species—will get investigated and explored. 

 

Are you a scientist? Have you recently read a peer-reviewed paper that you want to write about? Then contact Mind Matters editor Jonah Lehrer, the science writer behind the blog The Frontal Cortex and the book Proust Was a Neuroscientist.

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