Rapid eye movements and head direction cells recording reveal a possible cognitive procedure during REM sleep

Title: A cognitive process occurring during sleep is revealed by rapid eye movements

Journal: SCIENCE(2022) 377(6609):999–1004. Link: https://www.science.org/doi/10.1126/science.abp8852

Comments: The Rapid Eye Movement (REM) sleep is a sleeping stage characterized by rapid eye movements from which it gets its name and it has been associated to dreaming. In this phase, the brain is active, has perception and the movements are commanded but inhibited. Also, there is an alternative activation of different nucleus of the brain stem. Disruptions in this sleeping stage were described in some neuropsychiatric disorders such as depression. These patients do not have REM sleep phase. Thus, it is necessary to learn more about the sleeping stages. In this study, the researchers focused on the neuronal activity in a part of the brain, principally, the head direction (HD) system of the mouse located in the anterodorsal nucleus of the thalamus and compared its activity between different stages (awake or REM sleep). Simultaneously to the HD cells recording, they monitored both eye movements using a head-mounted camera to determine the relationship between head and eye turnings. Additionally, they created an algorithm correlating the HD cells activity and rapid eye movements in awake mice to predict the directions of the movements, be the virtual heading or rapid eye movements. After that, they proceeded to record HD cells activity and rapid eye movements in REM sleep. They observed that both eye movements are coordinated and move in the same direction. Also, the eye movements were correlated to the head direction in the virtual heading. Using the same algorithm created from awake mice, they observed a similarity in the firing activity of the HD cells and rapid eye movements between the sleeping and awake animals. Thus, these results suggest that some cognitive procedures may be happening during the REM sleep and that there are coordinated activities among different systems while sleeping.