Scientists at the Department of Neurology of the University of Bern and University Hospital Bern identified how the human brain triages emotions during dream sleep to consolidate the storage of positive emotions while dampening the consolidation of negative ones. The findings of the study were published in the journal Science. REM or Rapid eye movement sleep is a unique and mysterious sleep state during which most of the dreams occur together with intense emotional content.
Prof Antoine Adamantidis from the Department of Biomedical Research (DBMR) at the University of Bern and the Department of Neurology at the Inselspital, University Hospital of Bern, said, “Our goal was to understand the underlying mechanism and the functions of such a surprising phenomenon.” How and why these emotions are reactivated is unclear. The prefrontal cortex integrates many of these emotions during wakefulness but appears paradoxically quiescent during REM sleep.
The researchers first conditioned mice to recognize auditory stimuli associated with safety and others associated with danger (aversive stimuli). The activity of neurons in the brain of mice was then recorded during sleep-wake cycles. In this way, the researchers were able to map different areas of a cell and determine how emotional memories are transformed during REM sleep.
The results obtained showed that cell somas are kept silent while their dendrites are activated. "This means a decoupling of the two cellular compartments, in other words, soma wide asleep and dendrites wide awake", explains Adamantidis. It was found that the brain favours the discrimination of safety versus danger in the dendrites, but blocks the over-reaction to emotion, in particular danger.
According to the researchers, the coexistence of both mechanisms is beneficial to the stability and survival of the organisms: "This bi-directional mechanism is essential to optimize the discrimination between dangerous and safe signals", says Mattia Aime from the DBMR, first author of the study. If this discrimination is missing in humans and excessive fear reactions are generated, this can lead to anxiety disorders. The findings are particularly relevant to pathological conditions such as post-traumatic stress disorders, in which trauma is over-consolidated in the prefrontal cortex, day after day during sleep.
Breakthrough for sleep medicine
These findings pave the way to a better understanding of the processing of emotions during sleep in humans and open new perspectives for therapeutic targets to treat maladaptive processing of traumatic memories, such as Post Traumatic Stress Disorders (PTSD) and their early sleep-dependent consolidation. “We hope that our findings will not only be of interest to the patients but also to the broad public”, says Adamantidis.
