Fears could be passed down from mothers to newborn babies through the odor the mothers give off when they feel fear, a new U.S. study said on July 28.
In the first direct observation of this kind of fear transmission, a team of University of Michigan Medical School and New York University studied mother rats who had learned to fear the smell of peppermint and showed how they "taught" this fear to their babies in their first days of life through their alarm odor released during distress.
Their findings in rats may help explain a phenomenon that has puzzled mental health experts for generations: how a mother's traumatic experience can affect her children in profound ways, even when it happened long before they were born.
"Our research demonstrates that infants can learn from maternal expression of fear, very early in life," lead author Jacek Debiec of the University of Michigan Medical School said in a statement.
"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," Debiec said.
In the new study, the researchers first taught female rats to fear the smell of peppermint by exposing them to mild, unpleasant electric shocks while they smelled the scent, before they were pregnant.
Then after they gave birth, the team exposed the mothers to just the minty smell, without the shocks, to provoke the fear response. Another group of female rats that didn't fear peppermint was used as a control group.
The team exposed the pups of both groups of mothers to the peppermint smell under many different conditions with and without their mothers present.
The study showed that a mother's expression of fear can activate the infant rat amygdala, a brain region involved in fear conditioning.
Interestingly, the newborns could learn their mothers' fears even when the mothers weren't present. Just the piped-in scent of their mother reacting to the peppermint odor she feared was enough to make them fear the same thing.
And when the researchers gave the baby rats a substance that blocked activity in the amygdala, they failed to learn the fear of peppermint smell from their mothers.
The study suggested that there may be ways to intervene to prevent children from learning irrational or harmful fear responses from their mothers, or reduce their impact, Debiec said.
The findings were published in the U.S. journal Proceedings of the National Academy of Sciences.
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