The hippocampus of a mouse brain, after the making of a memory. The cells glowing in red contain the memory of a particular environment and have been genetically tricked to be capable of being turned on with just brief pulses of light. (Courtesy of Xu Liu and Steve Ramirez)

It can take only an instant for fear to take hold in the brain: a fear of snakes after being bitten by one, or anxiety around bodies of water after witnessing a drowning.

Overcoming that fear can take a long time, but now researchers are saying it can be done in your sleep. Scientists at Northwestern University say they have lowered levels of fear in people by using certain odors to trigger and rechannel frightening memories into harmless ones during a deep slumber.

“Sleep sort of stamps memories in more strongly,” said neurologist Jay Gottfried, senior author of the study, which was scheduled to be published online Sunday by the journal Nature Neuroscience. “That’s when a lot of memory formation can take place.”

The researchers first created a fear of a certain face in their subjects by using conditioning — making them link a face and an odor in their minds with a painful electric shock. After some trials, the participants became afraid of the face, and the smell acted as a cue associated with that face.

The researchers then used the smell to trigger fear memories during sleep as a way to enable patients to adjust without the stress of conscious terror.

Gottfried and his colleagues have not attempted this on preexisting fears, but theoretically it could be done — by creating a connection between a phobia and a distinct odor. He said the first type of patients who could be helped by this process would be those who already have an odor associated with their fear — for example, the smell of gunpowder.

“From a clinical perspective, this can be a new approach to try and treat stressful or traumatic memories,” Gottfried said.

Exposure therapy

He said that fear, a type of emotional memory, is often learned at a young age from experience or observation. A child who was bitten by a dog grows up afraid of dogs, but so does a child who sees his father attacked by one.

“Across all species, one thing is true: The learning of a fear occurs much more quickly than the fear extinction process,” said lead researcher and neurologist Katherina K. Hauner.

In exposure therapy, repeated exposure to the stimulus forms a new, safe memory.

For example, Hauner said, a fear of spiders can be quelled by the person initially looking at photos of spiders, then a real spider housed in a faraway cage, next by petting one with a thick glove, and finally by holding one.

“Extinction learning is typically known to be a new memory that essentially overwrites the fear memory,” said New York University’s Elizabeth Phelps, a psychologist and fear-conditioning researcher who was not involved in the study.

Hauner previously studied how slow and steady contact with spiders can change the brains of arachnophobes. As a postdoctoral researcher under olfactory expert Gottfried, she thought about using odors to trigger fear memories during sleep as a way to alter the emotional memory.

Odor association

The researchers showed 15 healthy subjects a photo of a different face every few seconds, but for two faces they received foot electric shocks. They eventually learned to fear both faces, as measured by increased levels of sweat.

Each face was also associated with its own odor in the subjects’ minds by a scent being pumped out whenever that specific face popped up — for instance, roses for one and lemons for the other.

Then the subjects napped for a couple of hours. Whenever they fell into a deep phase known as slow-wave sleep, they were given the rose scent to call up one face, but without the accompanying jolts. This was a subconscious version of having someone pet a spider without it harming them to help create a safe memory of spiders.

After the subjects awoke, they went through the same prenap process of being shown the series of different faces. This time, they weren’t as afraid of the face with the rose scent — but were still just as fearful of the other, whose associated odor they hadn’t been exposed to during sleep.

Also, those who smelled roses for the longest total time while sleeping were less fearful of the face than those who had smelled roses only for a short time.

The subjects did not know what happened during their sleep.

‘Interesting question’

Sleep plays a key role in memory consolidation, which involves parts of the brain replaying the events of the day and choosing which parts to store for safekeeping and which to forgo.

This experiment “asks a very interesting question, whether there is a unique period during memory consolidation as we sleep in which such memories could be, let’s say, updated,” said Rutgers University psychologist Mauricio Delgado, who was not involved in the study.

“It’s novel. I think it’s a really clever idea,” said Phelps, who believes that the research could one day be used for clinical applications to help those with phobias and post-traumatic stress disorder.

While exposure therapy is the most effective treatment available for post-traumatic stress disorder, in general PTSD is much more complex than traditional phobias.

“People are very quick to jump on the PTSD train, and it would be wonderful if it could be used in that way,” said Hauner, but she stressed that more research would be required before it could be used as a treatment.

Meeri Kim is a freelance science journalist based in Philadelphia.