Sleep reduces fear learning activity in the brain
New evidence suggests that habitual sleep can reduce subsequent fear learning activity in the brain.
One important function of sleep is memory retention. This extends to retaining fearful memories after a distressing experience. This process, called fear learning, can lead to the development of post traumatic stress disorder (PTSD).
In particular, rapid-eye movement (REM) sleep aids the fear learning process by helping to consolidate fearful memories. There is a theory that sleep deprivation may actually be beneficial following a traumatic experience, reducing the severity of the fear learning as well as avoiding nightmares which occur during REM sleep.
Less research has focused on the question of whether sleep plays a modulating role prior to fear learning. Scientists from Rutgers University have been trying to fill this data gap. Publishing in The Journal of Neuroscience, Itamar Lerner, Shira Lupkin and colleagues report evidence that time in REM sleep prior to fear learning, is associated with reduced activity in brain regions where fear learning occurs.
The researchers measured the sleep of participants at home over the course of a week using a headband that measured brainwave activity. After one week of sleeping at home, participants came into the lab to take part in the fear conditioning phase of the experiment inside a magnetic resonance imaging (MRI) machine. This allowed the researchers to see what was happing inside the brain as the participants learned to associate an image with a mild electric shock.
The team found that more time in REM sleep over the week was correlated to less activity in brain regions related to fear learning. “The fact that there was less connectivity between the amygdala, which is the fear centre of the brain and the hippocampus which is known for memory formation, would suggest that the memory was being encoded, perhaps, with less emotional tone to it, so the memory was being recalled but with less fear”, explained study author Shria Lupkin.
The researchers then replicated their finding in a follow up experiment where sleep was measured in a sleep lab over one night using polysomnography, which is the gold standard method of measuring brainwave activity during sleep. The results showed a similar correlation, albeit weaker, between time in REM sleep during one night and brain activity in the parts of the brain where fear learning occurs.
Despite this finding, the researchers found no correlation between time in REM sleep and skin conductance responses, which could be considered a more direct measure of fear. This suggests that REM sleep may indirectly affect fear learning. "REM sleep may be resetting the amygdala ... and allowing it to have more apprioapriate and accurate responses to fear the next day", explained lead author Itamar Lerner.
To test whether there is a direct causal relationship between REM sleep and the fear learning process, the researchers would need to conduct an experiment where REM sleep is manipulated to see the subsequent effect on fear learning.
The current finding paves the way for future studies to investigate the modulating role of sleep prior to fear learning. This research may have application in the military, where sleep may be a useful biomarker indicating resilience or susceptibility to developing PTSD following a traumatic experience.