can learn very early in life to fear something that frightened their
mothers even before they were born. Scientists have known for some time
that trauma can ripple through generations. New research on fear
transmission may help explain how that happens.
team of researchers from the University of Michigan Medical School
taught a group of female rats to fear the smell of peppermint by
repeatedly accompanying the smell with mild but unpleasant electric
shocks. That was before they were pregnant. After the rats became
pregnant and gave birth, the team exposed them to the peppermint smell
again, without the shocks, to induce the fear response again.
A story on the university website by Kara Gavin
explains that the babies of fearful mother rats, and a comparable group
of rat pups whose mothers had no fear of peppermint, were exposed to
the smell under many conditions with and without their mothers. When
babies were separated from their mothers and exposed to the minty smell
along with air piped to them from a nearby container occupied by their
frightened mothers, they quickly learned to fear the smell. The trigger
for learning apparently was the scent the mothers give off when they are fearful.
"Our research demonstrates that infants can learn from maternal expression of fear very early in life," said Jacek Debiec,
MD, PhD, the psychiatrist and neuroscientist who led the research.
"Before they can even make their own experiences, they basically acquire
their mothers' experiences. Most important, these maternally
transmitted memories are long-lived, where other types of infant
learning, if not repeated, rapidly perish."
In a paper published in Proceedings of the National Academy of Science, Debiec and colleague Regina Marie Sullivan
PhD, describe how brain imaging, studies of the genetic activity of
individual brain cells, and monitoring blood levels of cortisol, the
stress hormone, were used to examine the working of fear in the brain.
They found a brain structure called the lateral amygdale was the key
location for learning fears, and when they gave baby rats something that
blocked activity in that region, they did not learn their mothers'
fear. That could help explain why some offspring of traumatized mothers
don't inherit fears. The authors hope the work will aid understanding of
post-traumatic stress and other mental ills in humans.
recalls working with adult children of Holocaust survivors who had
nightmares and flashbacks related to experiences they had not endured
themselves. Rachel Yehuda,
a psychiatrist at Mount Sinai Hospital in New York, has studied
descendants of Holocaust survivors and the children of women who were
pregnant and in or near the World Trade Center on 9/11/2001. She found
evidence of intergenerational trauma transmission that could not have
occurred through storytelling. She was not involved in Debiec's work,
but she told Arielle Duhaime-Ross of Verge
magazine that the study is valuable because it provides molecular
analysis that would not be possible in living human brains. She said
understanding the brain changes that occur with intergenerational
transmission could help people understand the long-term impact of
parental experiences. "Your fears are not only a response to your
personal experiences," Yehuda told Verge, "but those that your parents
had as well."