Scientists generally define fear as a negative emotional state triggered by the presence of a stimulus that has the potential to cause harm.
Certainly, fear is a vital response to physical and emotional danger — if we didn't feel it, we couldn't protect ourselves from legitimate threats. But often we fear situations that are far from life or death and thus hang back for no good reason.
It is an exciting time for the science of fear. Scientists are actively researching about fear as well as other basic emotions, such as why some children of traumatized or depressed mothers are affected differently than their siblings.
Scientists have always felt that fear is transmitted across generations, although how this transmission occurs has not been well understood.
What they have known, and what is apparent in animal behavior, is that genes can be turned on or off in a semipermanent manner with molecular changes to the DNA, known as epigenetic marks — changes that can be maintained across generations.
Traumatic or stressful experiences in animals typically have been associated with effects on the emotional behavior of later generation offspring through epigenetic change.
A new study suggests that mothers can, in fact, pass on specific fears to newborn babies through their sense of smell. Jacek Debiec, a Polish-born psychiatrist and neuroscientist from the University of Michigan Medical School and New York University, decided to test the role of smell in fear transfer.
They conditioned female rats to fear the smell of peppermint by delivering mild, unpleasant shocks in tandem with the odor before the rats were pregnant.
The researchers then exposed the rats’ pups to the peppermint smell and tested their lateral amygdala, an area that detects and plans response to threats.
What they discovered was that the pups learned to fear the smell of peppermint even when their mothers weren’t present, replaced by a piped-in scent of them reacting to the peppermint odor.
Even with a single exposure, these newborn pups seem to create lasting memories. On the other hand, when researchers gave the pups a substance that blocked activity in the amygdale, the baby rats did not learn the fear of peppermint smell from their mothers.
Some mother rats tried to plug the tubing so that the smell wouldn’t come through, a behavior that warrants further studies.
This study has important implications for next steps, such as conducting human studies to test how infants respond to maternal distress. Importantly, this and future studies will help mental health experts find ways to prevent children from learning certain fear responses from their mothers. That would be a breakthrough.
References
Egger G, Liang G, Aparicio A et al. Epigenetics in human disease and prospects for epigenetic therapy. Nature. 2004; (429):457-463.
Debiec J, Sullivan RM. Intergenerational transmission of emotional trauma through amygdale-dependent mother-to-infant transfer of specific fear. PNAS. 2014; published ahead of print July 28, 2014, doi:10.1073/pnas.1316740111.
