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.