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Memory Consolidation during REM Sleep

By Jillian Kunze

Researchers generally agree that a good night’s rest is important for improving and maintaining one’s health, mood, and energy, among other things. Though sleep is also essential for memory processing, the mechanism for memory consolidation during rapid eye movement (REM) sleep remains poorly understood. A suitable model of the relationship between REM sleep and memory could help researchers better comprehend memory-related mental health conditions, such as post-traumatic stress disorder (PTSD). 

During a minisymposium presentation at the 2020 SIAM Conference on the Life Sciences, which took place virtually in June, Sujith Vijayan of Virginia Tech spoke about a framework for memory consolidation during REM sleep with implications for PTSD. Vijayan explained how one can differentiate the stages of sleep based on the muscle activity, brain activity, and eye movements that occur during this period. Tools like an electroencephalogram (EEG) measure brain activity during sleep and show that brainwaves—oscillations in the brain’s electrical signal—are present at various frequencies during different sleep stages.

Previous studies have demonstrated that people can remember things better if they learn them right before going to sleep, thus indicating that sleep is especially important for memory consolidation. Most of this research has explored the relationship between non-REM sleep and memory. For instance, scientists have found that memories are transferred from the hippocampus (which stores short-term memories) to the neocortex (which stores long-term memories) during non-REM sleep. 

Vijayan, however, is more interested in REM sleep, which is thought to play a role in consolidating both procedural and emotional memories. In order to study brain activity during REM sleep, his research team obtained data from epileptic patients undergoing invasive monitoring that was intended to determine the focus point of their seizures. Electrodes were placed in the patients’ brains to monitor brain activity, which provided a rich source of data for the researchers. By using FreeSurfer, a brain imaging software that analyzes brain magnetic resonance imaging (MRI) scans, Vijayan and his team assigned anatomical labels to each electrode’s location in the patients’ brains. 

Vijayan compared the patterns of beta activity in different regions of the brain.
 These electrodes clearly showed the occurrence of beta oscillations—brainwaves with frequencies between 12.5 and 30 Hz—during REM sleep. All of the electrodes also recorded theta activity, or brainwaves between 4 and 7 Hz, in the dorsolateral prefrontal cortex (DLPFC), while many also registered beta activity in that specific region. In addition, all the electrodes documented bursts of beta activity in the anterior cingulate cortex (ACC). However, the middle temporal gyrus featured much less beta activity, thus indicating that these brainwaves were sparse in the brain’s temporal region and more concentrated frontally. 

Interestingly, the electrodes recorded practically simultaneous beta activity in the ACC and DLPFC during REM sleep, despite the distance between those two areas. Vijayan believes that the beta activity may be transmitted by ponto-geniculo-occipital (PGO) waves, which propagate between different regions of the brain. The electrodes recorded an increase in beta activity after PGO waves occurred, so these waves might carry the beta activity between the ACC and DLPFC. 

Scientists suspect that emotional memories are consolidated during REM sleep, when their emotional charge is reduced. The network that Vijayan described between the ACC and DLPFC is a candidate for the system that works through memories during REM sleep. This framework of a memory pathway could be important for understanding certain mental health issues. Disruption to this network could cause the strength of certain memories to remain the same, triggering the brain’s continual reevaluation of these memories. This phenomenon might produce the symptoms of certain mental health conditions, such as PTSD. By further researching the way in which memories are processed during REM sleep, scientists may be able to better understand and treat these difficult conditions. 

   Jillian Kunze is the associate editor of SIAM News.