Sleep is an essential function in our lives. Sleep promotes cognitive performance, both mental and physical health, development, immune system functions, etc. Sleep also has shown to have vast benefits for memory consolidation, as well as promotes learning. During sleep, the body is able to repair itself and prepare itself for the following day. The effects sleep has on our learning/memory capabilities are still a subject of debate as determining the extensive working anatomy of memory and how sleep correlates to them.
While we associate things like cellular repair, physical development, etc with sleep, we still are left to understand how sleep promotes memory consolidation, as well as what mechanisms are at play during sleep. One such hypothesis is that sleep supports memory consolidation, perhaps when hippocampal neurons replay patterns of firing that were experienced during learning. In the article, How the brain consolidates memory during deep sleep”, researchers at the University of California were able to use a computational model looking at electrical activity in the brain during slow-wave sleep (deep sleep). The model displayed that patterns of slow oscillations in the cortex are influenced by hippocampal sharp-wave ripples, which determined synaptic changes in the cortex. Going off the theory of hippocampal neurons replaying patterns can be applied here as the model showed these synaptic changes affected patterns of slow oscillations, which replays a specific firing sequence of cortical neurons. Yina Wei explained how the inputs from the hippocampus determined the spatial and temporal pattern of the slow oscillations. She also explained that “by influencing the nature of these oscillations, this hippocampal input activates selective memories during deep sleep and causes a replay of specific memories.” These findings can be connected to other experiments and research into memory consolidation as well as further understanding mechanisms of sleep and memory.
This research conducted by Dr. Yina Wei and the team at UC Riverside can be used alongside the research of Laura Shanahan and Jay A. Gottfried. In their review article, “Scents and Reminiscence: Olfactory Influences on Memory Consolidation in the Sleeping Human Brain”, they referenced research regarding targeted memory reactivation (TMR), where research by Rasch et al. 2007 found that odor stimulation could be used to enhance consolidation of declarative memories. They had subjects learn the location of several card pairs. During this, they were exposed to a rose odor (phenylethyl alcohol), and then they would go to sleep. During sleep, they would be re-exposed to the odor, and then upon waking they were asked to recall the locations of the items. They found that during the slow-wave sleep showed to have the greatest response to the odor and enhancement of recall. As displayed by the research of the UC Riverside team they also saw that there was activation during deep sleep (slow-wave sleep). They also employed fMRI on hippocampal activation. The results showed that the rose odor activated the hippocampus to a greater extent during deep sleep than it did during wakefulness, which mirrors that computational model used by the UC Riverside group.
Both these studies identify non-REM sleep (slow-wave sleep) to be the most active and likely area of memory activation, but a recent study done at the University of Tsukuba provides insight into the possible role REM sleep plays on memory consolidation. The research article, “Memory consolidation during REM sleep”, found that adult-born neurons in the hippocampus may be responsible for memory consolidation during REM sleep. The researchers exposed mice to a context-specific fear memory task. They then recorded activity in the adult-born neurons across the stages of memory. They found that these neurons were most active during REM sleep after the memory task. They also found consolidation of contextual fear memories was impaired upon optogenetic silencing of young ABNs. This can be used alongside the research talked about in the Shanahan article about how odors can promote fear extinction during sleep. This is important as the other research points back to memory consolidation taking place in the slow-wave sleep while this research points to memory consolidation in REM sleep.
The three research articles all cover a different aspect of sleep and memory and have building blocks upon each other to further research into how sleep affects our memory system. The computational model supports many findings of increased activation during slow-wave sleep which is believed to be where memory-enhancing effects take place. While the odor experiment did not stimulate memory-enhancing effects during REM sleep, it is not fully conclusive that memory consolidation or activation can not take place during REM sleep. The REM sleep article offers insight into the role of adult-born neurons aiding in memory consolidation during REM, which could be further studied and applied to previous experiments. The hypothesis of hippocampal neurons replaying patterns of firing from learning also gains much support from the articles and experiments discussed. There is still much research to do on sleep and memory, but looking into ABNs effects with odor stimuli could provide interesting results to further our understanding, as well as identifying the specific tasks taking place in different stages of sleep.
Shanahan, Laura K., and Jay A. Gottfried. "Scents and Reminiscence: Olfactory Influences on Memory ..." Web. 6 May 2021.
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