One important way that children, including infants, learn how to respond to their environment—for instance, what is “safe” and what is “dangerous”—is by taking cues from their parents or caregivers. But as 2010 NARSAD Young Investigator Grantee Jacek Debiec, M.D., Ph.D., and a colleague point out in new research, that cue-based learning is a two-edged sword.
It’s a bad idea to get too close to the kitchen stove when the burners are on. Cues leading children to learn such things are crucial to their development and adaptation to different environments—everything does not have to be learned by “trial and error.” But what if a parent who has just come back from a tour in Afghanistan, suffers from post-traumatic stress disorder (PTSD) and takes cover under the kitchen table or hides in a closet every time he hears a loud sound? Learning to imitate that behavior or register the fear that drives it would not be at all helpful to a child.
Dr. Debiec, of the University of Michigan, and Dr. Regina Marie Sullivan, Ph.D., of New York University, confirm in a paper published online in the Proceedings of the National Academy of Sciences today, that emotional trauma and fear are indeed transmitted across generations, even when the response is pathological, as in PTSD or with various phobias. The team has gained new insight into how this fear learning develops in the brain and can have long-lasting impact.
Working with animal models, the researchers demonstrated that a mother rat trained to fear a peppermint-like odor transmits that fear to her infant pups when she expresses her fear in their presence. This is the experimental equivalent of a non-normal, pathological fear—for example, being undone by loud sounds because one has been exposed to trauma in a war zone.
Drs. Debiec and Sullivan examined brain activity in the rat pups, and traced the fear response to neural activity in two parts of the amygdala, the brain area known to coordinate the response to fear. They also noted an associated increase in the pups’ stress response, evidenced by elevated levels of the stress hormone corticosterone; this stress response occurred even when the mother wasn’t present if the pups had already picked up the cued fear response from her. The researchers found this fear-associated learning that causes an elevated stress response was hard-wired early in pups’ development and had an enduring effect.
If the researchers used a medication to “inactivate” the amygdala, the pups were prevented from learning their mother’s fear of peppermint odor. Similarly, if they inactivated the alarm response system in the brain that induces elevated levels of corticosterone, the maternal fear was also not passed on to the pups. While this is not a practical possibility—or wish—it does help identify the structures in the brain that are impacted by cue-associated fear learning that gets passed from parent to child.
This study has also been featured in the following new sources: