Sleep can decrease the intensity of fearful memories, according to researchers from Northwestern University in a recent article published in Nature.
Previous studies have shown that slow-wave sleep can both modify and consolidate memories relating to events in someone’s life or procedures on how to do something. Sleep can also affect emotional memories, but scientists were unaware if those memories could be actively modified during sleep and which brain areas would be involved in that change. Understanding sleep’s role in specific emotional memories presents a huge window of opportunity for understanding pathological fear and the ability of memory to accommodate new information.
The research team, led by Katherina Hauner, elicited a fearful memory in volunteers by associating a target odor with face images paired with an electric shock. In anticipation of a shock, volunteers would begin to fear the image and the odor. To determine how afraid each volunteer was, researchers measured the skin-conductance response, which shows how much someone sweats — the higher the response, the more sweat is produced, which illustrates increased fear.
After the volunteers were conditioned to fear the target odor, they all went to sleep. During slow-wave sleep, researchers continuously delivered only the target odor, hoping that the volunteers would re-conjure the fearful memory of the shock. They simultaneously measured the skin-conductance response.
During the first half of slow-wave, volunteers demonstrated a high amount of sweating. However, the second half led to a significant decrease in the skin-conductance response. This demonstrated that re-exposure to the target odor while asleep led to a decrease in fear.
After the nap, volunteers continued to be less fearful of the odor. The longer the subjects were re-exposed to the odorant during sleep, the greater the reduction in skin-conductance response and the less fearful they became.
Scientists conducted the experiment one more time with patients who were awake to see if they could replicate their findings or if sleep was unique in its ability to decrease fear. After volunteers were conditioned to fear the odorant, they all watched a documentary instead of taking a nap. During the documentary, volunteers were exposed to the target odorant without the shock and the skin-conductance response was then measured. No significant decrease in skin-conductance response occurred, which showed that wakefulness does not decrease fear even if presented with an odor without a shock.
Now that the scientists knew being asleep (and only asleep) while re-exposed to the fear-inducing odor could lessen fear, they had to figure out why. Unsure as to whether or not reduced sweating was due to unlearning the fear (no longer associating the odor with the shock) or forming a new memory (associating the odor with a lack of shock), scientists analyzed brain activity.
After sleep, there was a decline in hippocampal activity, which is involved in fear conditioning and memory consolidation. The patterns of activity in the amygdala, responsible for emotions, did not decrease like activity in the hippocampus did, but rather changed. The change in amygdala patterns supported scientists’ latter hypothesis that forming a new memory led to a lower skin-conductance response.
Only one other major question remained – why didn’t those who were awake also experience decreased fear when re-exposed to the target odorant? One hypothesis was that the decreased hippocampal activity in slow-wave sleep prevented recent memories from being fully encoded and subsequently stored.
Sleep therapy possesses many potential future benefits, especially those regarding pathological fears such as phobias and post-traumatic stress disorder. Understanding the mechanisms by which memories are consolidated and modified during sleep could lead to better treatment for fear.
Hauner, K. K., Howard, J. D., Zelano, C. & Gottfried, J. A. Nature Neuroscience. http://dx.doi.org/10.1038/nn.3527 (2013).