The key to the young rat pups’ behavior is an almond-shaped structure known as the amygdala, a part of the brain involved in emotionally charged memories—especially fearful ones. The presence of the mother during a painful event is enough to suppress activity in the amygdala of a rat pup. The mechanism proved to be surprisingly simple: pups still living in the nest, or older pups in the mother’s presence, did not release the stress hormone corticosterone. The infant amygdala is uniquely dependent upon increases in corticosterone to learn and express fear. In a later study, we showed that the neurotransmitter dopamine is also released when the rat is learning to avoid the odor. Though dopamine is often considered a “reward” chemical, when increased in the amygdala it helps to form fear-related memories. In this study, young rats still in the mother’s nest showed a large decrease in dopamine in the amygdala, indicating a mechanism to further block pups from learning to fear.4

The benefits of this maternal off switch seem obvious. Nearness to the mother offers comfort and courage in what might otherwise be a frightening situation, serving to strengthen the bond between mother and infant, and to remind an older child where its haven is in times of danger. This phenomenon is known as “social buffering,” and it continues throughout life. When a child gets an injection, the presence of a parent can help the child cope with the pain. Later on, when someone we care about is there to comfort us in a stressful situation, our stress hormones and fear are greatly reduced.

The switch is flipped, however, when the intrepid older rat pup is out exploring. Then, the threatening event sparks the fear response, propelling the youngster away from danger and, preferably, back into the nest while searing an indelible memory into the brain.

But when the parent and the nest are themselves sources of danger, the suppression of fear circuits in the amygdala unfortunately still works. The fear, avoidance, and even memories associated with pain are extinguished—explaining why an abused child, even while trying to escape pain, will later seek contact with the abuser.

Neurons Build Walls Around Memories

Under normal circumstances, the ability to remember danger is so vital to survival that memories based on fear cannot be erased. They can be overwritten through “extinction,” an active-learning process in which a new memory supplants the older one (like teaching a rat that a shock will no longer follow the sound that used to precede it). But extinction is shaky and impermanent, and the underlying fear can re-emerge at any time.

Key neurons in the brain build structures that reinforce fear-based memories. These “perineuronal nets” are made of cartilage-like tissue known as proteoglycans. Like miniature chain-link fences, they are thought to protect important memories by blocking the re-modeling, or plasticity, that might otherwise dismantle the memory in favor of new information.

Once again, the formation of these barriers is age-dependent. Andreas Luthi of the Friedrich Miescher Institute for Biomedical Research in Switzerland and Cyril Herry of INSERM in France showed that in adult mice whose fear of a shock has been reprogrammed through extinction, treatment with an enzyme that degrades the “nets” will bring the memory back in full force; the mice showed the same freezing behavior in response to a warning odor that they did before extinction training took place.5 But younger pups, less than three weeks old, can lose a fearful memory completely. Like our work with norepinephrine circuits, this research also illustrates a concrete mechanism that prevents young children from forming memories based on fear—even when it might be in their best interest to do so.