Through a new rat study that could have implications for humans, researchers found the fears of rat mothers are transferred to their offspring through odors emitted when they encounter a stimulus that makes them fearful.
The team, from the University of Michigan (U-M) Medical School and New York University, published their results in the journal Proceedings of the National Academy of Sciences.
They say their findings helped them identify the specific brain area where fear transmission settles during the early days of life, which could lead to a better insight as to why not all children of distressed mothers experience the same effects.
For their study, the researchers, led by neuroscientist and psychiatrist Dr. Jacek Debiec of U-M Medical School, taught female rats to fear the smell of peppermint by subjecting them to mild electric shocks, pre-pregnancy, while smelling the aroma.
Then, once they gave birth, the researchers introduced the peppermint smell - without shocks - to the mothers to elicit fear. Additionally, they used a control group of mothers that did not have a fear of peppermint.
The team introduced the peppermint smell to the pups of both groups, both with and without their mothers nearby.
"During the early days of an infant rat's life," says Dr. Debiec, "they are immune to learning information about environmental dangers. But if their mother is the source of threat information, we have shown they can learn from her and produce lasting memories."
Dr. Debiec adds that their research on rats allows them to observe what happens inside the brain during fear broadcasting, which they could never do in humans.
Offspring 'acquire their mothers' experiences'
The team focused on a brain structure called the lateral amygdala by using special brain imaging and studies of genetic activity in single brain cells, as well as studying cortisol in blood.
This brain area is they key location where threats are detected and dealt with, so the researchers say it makes sense that this area would also be the center for learning new fears.
Not only did the baby rats learn the fears of the mother through the odors she gave off when experiencing fear, but the introduction of the scent of their absent mother reacting to the peppermint smell also elicited the same fear reaction in the pups.
However, the researchers also found that when they gave the baby rats a substance that blocked amygdala activity, the pups failed to learn the fear of peppermint from their mothers.
Dr. Debiec says this suggests there could be a way to prevent children from learning harmful fear responses from their mothers.
"Our research demonstrates that infants can learn from maternal expression of fear, very early in life. Before they can even make their own experiences, they basically acquire their mothers' experiences. Most importantly, these maternally transmitted memories are long-lived, whereas other types of infant learning, if not repeated, rapidly perish."
What are the implications for humans?
The researchers note that mental health experts have been observing, for generations, that emotional trauma is transmitted across generations. Dr. Debiec himself has worked with children of Holocaust survivors who had nightmares and flashbacks to traumatic experiences they never actually experienced.
Because this pathological fear is socially transmitted to children, the team says it is "of clinical concern."
Building on what scientists have already learned about fear circuitry in the brain, psychiatrists have been able to develop treatments for humans with phobias and anxiety disorders. For example, the team explains that exposure therapy can help a patient overcome a fear by gradually facing the source of the fear.
Dr. Debiec hopes that further investigations into this topic will help human patients. Though it is still too early to know if the same odor-transmission effect happens between human mothers and their babies, he notes that it is already known that a mother's scent plays a role in calming human babies.