Smells could be used to calm fears - while people are asleep, a new study suggests.
A fear memory was reduced in people by exposing them to the memory over and over again while they slept.
It's the first time that emotional memory has been manipulated in humans during sleep, researchers said.
The finding potentially offers a new way to enhance the typical daytime treatment of phobias through exposure therapy by adding a nighttime component.
Exposure therapy is a common treatment for phobia and involves a gradual exposure to the feared object or situation until the fear is extinguished.
"It's a novel finding," said Katherina Hauner, a postdoctoral fellow in neurology at Northwestern University Feinberg School of Medicine.
"We showed a small but significant decrease in fear. If it can be extended to pre-existing fear, the bigger picture is that, perhaps, the treatment of phobias can be enhanced during sleep," said Hauner.
In the study, 15 healthy human subjects received mild electric shocks while seeing two different faces. They also smelled a specific odourant while viewing each face and being shocked, so the face and the odourant both were associated with fear.
They received different odourants to smell with each face such as woody, clove, new sneaker, lemon or mint.
Then, when a subject was asleep, one of the two odourants was re-presented, but in the absence of the associated faces and shocks.
This occurred during slow wave sleep when memory consolidation is thought to occur. Sleep is very important for strengthening new memories, said Hauner.
"While this particular odorant was being presented during sleep, it was reactivating the memory of that face over and over again which is similar to the process of fear extinction during exposure therapy," Hauner said.
When the subjects woke up, they were exposed to both faces. When they saw the face linked to the smell they had been exposed to during sleep, their fear reactions were lower than their fear reactions to the other face.
Fear was measured in two ways: through small amounts of sweat in the skin, similar to a lie detector test, and through neuroimaging with functional magnetic resonance imaging (fMRI).
The fMRI results showed changes in regions associated with memory, such as the hippocampus, and changes in patterns of brain activity in regions associated with emotion, such as the amygdala.
These brain changes reflected a decrease in reactivity that was specific to the targeted face image associated with the odorant presented during sleep.
The study was published in the journal Nature Neuroscience.