It is possible to reduce people's fears by treating the fears while sleeping, a new study suggests.
The researchers conditioned 15 participants to be afraid of a specific image of a face by giving them an electric shock each time that face appeared among other face images they were viewing. Along with the face, the participants were also exposed to an odor, so they would associate the odor with their fear.
The participants then took a nap, during which they were exposed to the fear-triggering odor; however, this time they did not receive the associated shock. Once awakened, the results showed that people's fear of the face and odor they had learned to fear had diminished, even though they had no memory of smelling the odor while they slept, according to a study published Sunday (Sept. 22) in the journal Nature Neuroscience.
The experiment was repeated in different conditions, including one in which people did not sleep during the study, but the researchers didn't find the same reduction in fear.
The findings suggest that "sleep may constitute a unique state in which targeted fear memories can be selectively extinguished," the researchers wrote in their study.
Activity in certain brain regions during sleep is believed to be critical for learning and consolidating memories, especially emotional memories. It is thought that newer memories, which have not yet been strongly engrained, are more vulnerable to modification during sleep, the researchers said.
In the study, to control for the calming effects of sleep itself, the researchers also used another odor, and conditioned the participants to associate it with a different face. But this odor was not presented to the sleeping participants, and their fear response to the face was not as diminished once they woke up. [10 Things You Didn't Know About the Brain]
Using brain imaging (fMRI), the researchers found that participants' brain activity in response to the feared face was altered after exposure to the associated odor during sleep.
Comparing brain scans after sleep showed reduced activity in the hippocampus, a brain region involved in memory, as well as changes in the amygdala, which is responsible for recognizing emotional stimuli, when people saw the feared face again.
It is not clear whether the fear reduction was a result of "unlearning" the association between the odor and fearful event, or a result of a "new learning," the researchers said. However, judging from the brain activity patterns, the researchers said the latter is more likely — the participants learned a new memory for the odor while they were sleeping, one that wasn't fearful.