Stress and the Architecture of the Brain

By Dorian Friedman

At one end of the spectrum, the latest research demonstrates that young children exposed to stressful conditions in a setting as common as a child-care center often do respond physiologically. National Scientific Council member Megan Gunnar, a developmental psychologist at the University of Minnesota’s Institute of Child Development, has shown that youngsters who must manage being with large groups of children for many hours each day experience rising levels of cortisol (the stress hormone) as the day progresses.

By afternoon, toddlers and preschoolers, especially those in large centers and those receiving poorer-quality child care, have stress hormone levels often double or triple what they show at home on non-child-care days. Researchers do not yet know how these rising stress-hormone levels in poorer-quality child-care arrangements affect brain development. However, because they are observed in the same settings that are associated with poorer behavioral outcomes for children, there is reason for concern, says Gunnar.

At the other end of the spectrum, we are slowly coming to understand the link between more severe forms of stress—which can be referred to as “toxic stress”—and the healthy growth of the brain’s architecture. At least one study of youngsters who suffered serious child abuse drew conclusions that were most disturbing: Compared with their peers, these children had measurably smaller brain volume, with more ventricles (the fluid-filled, squiggly cavities) and weaker connections between the organ’s left and right sides. Sadly, the longer they endured the abuse, the more severe were the effects on their developing brains.

Dance of Genes and Environment

Importantly, the relationship between stress and the brain is a function of both “nature” and “nurture.” “All of this is a dance between genes and experience,” says Gunnar, who offers the following analogy: The genes we are born with can be thought of as our “genetic library.” The experiences we have influence which books in that library we take out and read at different points in brain development. And this intertwining of genes and experience shapes our brain architecture. Scientists are just beginning to understand how particular genes in our library affect how we react to stress early in life and what effect this has on how our brains develop and respond to stress later.

We know that some children are much more adversely affected by stress than others, and we strongly suspect this is partly due to the genes they have in their genetic libraries, Gunnar says. For example, a study done in New Zealand (Caspi et al, 2003, Science, 301: 386-389.) demonstrated that abused children with a faulty version of the gene that regulates the brain chemical serotonin were more likely to develop depression, while those who had a more efficient serotonin gene were not likely to get depressed. What we don’t yet know is how this particular gene influences brain development in abused children and whether the faulty version participates in producing the smaller brain volumes and weaker connections seen in many abused children. What we do know is that in the absence of abuse, even the children with the faulty serotonin gene were not likely to become depressed. So, it’s not the genes in the child’s library or that child’s experiences that determine how the brain develops—it’s very much both.

What science also knows is that stress hormones connect with regions on many genes that open or shut the books’ covers, allowing them to be read (turned on) or putting them back on the shelf (turned off). Increases in stress hormones mean that gene books are opened and shut all over the brain, allowing the experience of stress to affect the brain’s development. We also know that nature has provided children with a powerful way of keeping stress from affecting brain development by blocking the rise in stress hormones, thus keeping the librarian sitting at her desk. Loving, supportive relationships are the “stress hormone blocker.”

The power of this protective stress blocker can be seen in something as common as a visit to the pediatrician for immunizations. Gunnar’s research shows that the way children react to this frightening situation varies greatly based on their relationships with their caregivers. Children who feel loved and secure seem to have a biological buffer against the threat. “That secure attachment literally blocks the baby’s hormonal response to stress,” she says, explaining that chemicals in the brain react differently in response to the perceived danger. Conversely, children who don’t feel a sense of security from outside sources often overreact to fear of the shot; their bodies produce elevated levels of stress hormones when they are even mildly afraid.

Everything in Moderation

While toxic stress is clearly harmful to the developing brain, it turns out that exposure to mild stress is important, physiologically speaking. Animal studies illustrate this point. When rat pups are handled by researchers only occasionally or for short periods of time, they seem to acquire the ability to cope better with stressful situations as adults. In fact, they mature into more skillful “stress responders” than rat babies exposed to extreme stress—and, interestingly, they handle stress better than those rat pups that endured no stress at all in the same lab tests. The simple hypothesis: Early exposure to manageable amounts of stress helps the body’s chemical stress-response system evolve in an effective way.

Pat Levitt likens this finding to a familiar problem for any parent: How to introduce a child to candy without causing one of two undesirable outcomes: addiction to sugar because of too much early in life, or craving it because of deprivation. Experience and common sense—if not hard science—suggest the wisdom of moderate exposure to cultivate a healthy response in later years.

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