A new study reveals that while we sleep, our brains quietly sort through the day’s experiences, keeping what we intend to remember and letting emotion-driven moments drift away.
Study: Top-down instruction outweighs emotional salience: nocturnal sleep physiology indicates selective memory consolidation. Image credit: Gorodenkoff/Shutterstock.com
A recent study published in Frontiers in Behavioral Neuroscience examined the interaction of emotional content and intentional memory instruction as salience cues that prompt memory tasks.
Sleep is key to memory consolidation, converting newly coded information to established long-term memories. Many sleep theories suggest that it is also central to choosing what to save or discard, which is important in avoiding interference with needed memories. But little is known about how this happens.
Introduction
Early on, sleep was found to be central in transforming new data into durable memories. Later, it was found to be linked with selective memory consolidation, based on the motivational or cognitive relevance. This is decided by the cues present at the time of the event, which indicate its emotional or reward value, or its potential usefulness.
Emotional salience, cues that drive emotional reactions, and top-down cognitive instructions (consciously choosing to remember or forget) are among the most important signals for this process. Emotional salience might help people remember dangerous or rewarding events and adapt their behavior accordingly. Automatic interactions between memory and emotion processing brain areas, like the amygdala and hippocampus, drive this.
A second pathway is top-down control, where the individual decides what to forget or remember. This can occur in combination with emotional cues or compete with them.
Associations with sleep stages
Various sleep stages correlate with the type of memory stored. Sleep spindles, characteristic bursts of EEG wave patterns occurring during non-rapid eye movement (NREM) sleep, were associated with increased recall of goal-directed and emotionally loaded memories. Theta wave activity during rapid eye movement (REM) sleep is linked to the spreading out of negative emotions from a specific cue to related cues, probably because of the association of similar cues.
Thus, memory consolidation cues include competing emotional salience and instructional cues. If memories are tagged by neurotransmitters like dopamine or norepinephrine, they are more likely to be reactivated (and consolidated) during sleep. Some theories suggest that NREM sleep, especially with sleep spindles, promotes the reactivation of some memories by selectively reinforcing the synaptic pathways they use.
REM theta waves are linked to the storage of emotional memories or associated content and can thus alter the nature of the final memory.
Prior nap-based studies suggested that instructional cues dominate over emotional salience in memory consolidation. However, these findings may not be true of night-time sleep, with its different structure and longer duration. It comprises both EM and REM cycles as well as more deep sleep (slow-wave sleep or SWS), and REM theta waves. These respond differently to various memory cues.
The current study explored the interaction of emotional and top-down instruction cues in choosing what to retain or discard during sleep. The aim was to see if individuals could consciously forget emotionally negative content. Secondly, would this directed forgetting be strengthened or overridden during sleep?
About the study
The researchers conducted two studies. Both studies followed identical testing procedures but in different settings: online and laboratory. The study involved about 100 participants (45 in the online study and 53 in the laboratory study).
Participants in the second study had overnight electroencephalogram (EEG) monitoring for brain activity during their sleep using a wearable headband at home. Fifteen participants provided usable EEG data. This allowed investigators to record sleep architecture, correlating sleep phases and the effect of sleep with memory performance.
In each study, the participants were shown a hundred words on a screen, one at a time, with each followed by a semi-randomly assigned Remember or Forget cue. Fifty of the words were negatively loaded, and the rest were neutral.
An immediate recognition task followed. The participants were shown 100 words, of which 25 each were previously shown remember-cued or forget-cued words. The remaining 50 were neutral or negatively charged new words.
Momentary mood was also assessed using validated questionnaires. The participants’ circadian rhythms and sleep quality were also assessed to help correlate memory performance with their chronotype and mood.
The delayed recall test was performed after 12 hours of sleep or wakefulness. The participants recalled all the words they could remember from the original list.
Study findings
In the online study, participants remembered the remember-cued words more accurately in the immediate recognition task. Emotionally charged content was also remembered more accurately than neutral words. There was no significant difference when tested after either nighttime sleep or wakefulness.
In the second study, emotional content was remembered more accurately when it was remember-cued compared to neutral remember-cued words. Thus, the conscious decision to remember a word played the dominant role, though it was enhanced if the word was negatively loaded.
In both studies, with delayed recall, the remember-cued words were recalled more often than the ones they were instructed to forget. No overall behavioural differences were found between the sleep and wake groups. There was also no significant main effect of emotional valence on delayed recall.
However, negatively charged foils shown in the immediate recognition test were falsely recalled during the delayed recall test more often than neutral ones. Even though they had not been learned, they left a greater imprint on the memory.
Sleep phase markers were correlated with memory performance. Sleep spindles were positively correlated with increased recall of negative remember-cued words. In contrast, more slow-wave sleep (SWS) and higher delta power (the amount of delta-frequency electrical activity in this phase) were negatively correlated with total recall.
This suggests that the need for deep sleep is balanced against the ability to store and retrieve memories.
Theta power (the amount of theta-frequency waves during REM sleep) was positively correlated with a higher false recall of negative foils. This suggests that REM is linked to emotional memory generalization.
The study extends earlier nap-based sleep research that showed the importance of sleep in selective memory consolidation. Importantly, the authors showed that specific sleep phases tracked which memories were selectively consolidated, even though sleep itself did not change behavioural performance.
Conclusions
“Overall, results underscore the primacy of top-down instruction over emotional salience in shaping memory.”
The authors emphasize that specific aspects of sleep physiology, rather than the mere presence of sleep, were linked to selective recall and emotional memory distortion.
Sleep studies remain key to understanding how the brain selects memories for long-term storage and recall.
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