Do We Sleep to Remember or to Forget?: The Brain & Mind Series (Part 2 – Biological Psychology)

Do We Sleep to Remember or to Forget?

Part 2 of the Brain & Mind Series

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Lay Summary

Most people will have experienced the effects of a bad night’s sleep. These can include becoming forgetful, finding it hard to pay attention, and struggling to pick up new information. However, scientists still don’t fully understand why sleep negatively impacts the ability to learn and recall information. The current paper discusses why this is. It is argued that in order to be sure of how sleep affects memory, three criteria need to be fulfilled.

Firstly, researchers need to be certain that in experiments where individuals are deprived of sleep, that lack of sleep is directly impairing memory. For example, sleep may be indirectly impairing memory, by impairing the cognitive abilities that uphold memory (e.g. the ability to pay attention to a task) rather than memory itself.

Similarly, since sleep deprivation causes stress, and since stress can cause memory impairment, there may not be a direct relationship between sleep and memory. Therefore, the second criterion to be fulfilled is that there should be good reasons for thinking sleep may affect memory directly, and these should be tested.

Lastly, the biology of sleep and memory need to be known for us to fully understand their relationship. Until these criteria are fulfilled, we can’t be sure how exactly sleep may support the memory processes responsible for learning and recalling information. Finally, the likely role sleep may play in support memory according to popular theories is discussed. This is important to know, because memory is crucial in daily life and many don’t prioritise sleep.

science-2953886_1920.pngHere’s my informed opinion – what’s yours?

There is an abundance of high quality evidence unequivocally demonstrating that sleep supports memory function. It is clear from such evidence that lack of sleep disrupts our ability to form new memories and recall existing ones. Precisely how sleep does this though remains a mystery. Therefore, I would argue that while it is apparent that sleep plays a role in memory function, until further technological advancements enable us to examine what is occurring in the brain on a deeper level during sleep, we cannot say what exactly this role is and cannot be certain that sleep is directly affecting memory function. I am not yet fully convinced that it does. Nonetheless, there are interesting and radical theories concerning how it might. I would favour the forgetting role because it more readily explains a range of seemingly contradictory findings in the field.

The Paper:

What role may sleep play in supporting memory function?

Introduction

Many of us have experienced first-hand the deleterious effects of not getting enough sleep, such as becoming forgetful, finding it hard to pay attention and struggling to learn new information. Despite this, researchers are only beginning to understand the relationship between sleep and memory. This essay argues that in order to ascertain what role sleep may play in supporting memory function, a number of criteria need to be fulfilled. That is, research needs to have demonstrated that this relationship is causal, identified its precise nature, and have revealed the underlying biological mechanisms by which the hypothesised role may be carried out. This essay then evaluates two recently addressed popular theoretical perspectives by assessing their explanatory worth and weighting supportive evidence according to the extent to which the criteria just described are fulfilled. In light of this evaluation, the likely role sleep plays in supporting memory function is provided.

The Link between Sleep and Memory

According to a review by Born, Rasch and Gais (2006), a large body of evidence collectively indicates a link between sleep and memory. But in order to determine whether sleep plays a role in supporting memory function, we need to be certain that this is a causal relationship. Experiments have indeed indicated just this, since they have successfully manipulated sleep quantity and revealed its effect on memory function. For instance, sleep deprivation and restriction has been shown to have a detrimental effect on memory processes (Durmer & Dinges, 2005; Gais, Lucas & Born, 2006; Krause et al., 2017). However, there are two major methodological issues with these studies which will be referred to throughout this essay. These issues compromise the validity of conclusions drawn concerning how sleep may support memory function.

Two Major Methodological Issues

The first major methodological issue renders it difficult to identify whether there is a direct relationship between sleep and memory function. This becomes apparent when considering that sleep deprivation doesn’t just impact memory – it also impairs a host of cognitive processes that themselves support learning and memory retrieval. These include executive attention (Durmer and Dinges, 2005; Krause et al., 2017), global cognition and attentional processes (Killgore, 2010; Krause et al., 2017; Tomasi et al., 2008). In order to ascertain how sleep may support memory function, it is thereby of importance to determine experimentally whether sleep affects memory function directly, via these other cognitive processes, or whether it affects a number of cognitive processes including memory processes in concert. This has not yet been achieved. Furthermore, the effects of sleep deprivation on the neural networks underpinning these cognitive processes and their interactions remain unknown (Krause et al., 2017). The second major methodological challenge is that sleep disruptions typically cause elevated stress levels. This can prevent the precise role of sleep in memory processing being identified (Maquet, 2001). That is, manipulations of sleep have been shown to induce stress (Liu, Verhulst, Massar, & Chee, 2015), which may act as a confounding variable.

However, these two methodological challenges may be circumvented if an underlying biological mechanism is proposed whereby sleep might directly, and uniquely, influence memory function. In fact, the well-documented link between sleep and memory has inspired numerous explanatory theories proposing how sleep may support memory function.

The Consolidation Role of Sleep 

A popular theory that has been endorsed by Rasch and Born (2013) in their comprehensive review of thousands of studies in the area, is that sleep plays an active role in the consolidation of memories. More specifically, there is a vast array of evidence indicating that sleep deprivation and restriction impair consolidation (Maier & Nissen, 2017; Maquet, 2001; Prince & Abel, 2013). Another strength of this approach is that the hypothesised role has been specified both in terms of its nature and its underlying mechanisms in light of evidence. The account typically specifies that rapid eye motion (REM) sleep is involved in the consolidation of procedural and emotional memories, while slow wave sleep (SWS) is thought to be involved in the consolidation of declarative memories (Sara, 2017). This latter process is theorised to involve the transformation and re-organisation of memory representations, such that the “gist” is obtained from the newly encoded memory content and assimilated into long-term memory store networks (Rasch & Born, 2013). Finally, evidence has revealed some of the electrophysiological, neurochemical, and genetic mechanisms underpinning the consolidation role of sleep (Rasch & Born, 2013).

Nonetheless, while the hippocampal and molecular processes thought to underpin memory consolidation appear to be impacted by sleep quantity and quality (Prince & Abel, 2013), it is difficult to disentangle the effects of sleep and stress on such processes. This brings us back to the second described major methodological issue. In other words, the fundamental debate, as explicated by Maquet (2001), of whether memory consolidation during sleep can be explained by sleep-specific neural mechanisms, or by considering stress-related alterations in hormonal levels, remains. Alongside this, there is a further fundamental caveat here. That is, the neurophysiological mechanisms underpinning memory consolidation itself are still not fully understood, however advances in modern biotechnology are gradually allowing for it to be revealed (Sara, 2017). When this is fully achieved, it may be possible to bypass the two major methodological issues, and thus confirm that sleep does play an active role in memory consolidation, as evidence strongly suggests.

The Forgetting Role of Sleep 

However, a criticism of this theoretical account is that the consolidation role may be equivalently carried out as a result of continued waking practice and is thus not unique to sleep (Poe, 2017). According to Poe (2017), while sleep may play a role in consolidating memories, it also carries out the role of targeted forgetting that is unique to sleep. This involves a “clean-up” process, whereby information accumulated across the day is erased, and this is thought to be important for memory function (Poe, 2017). The hypothesised biological mechanism underpinning this process is the targeted erasure of synapses and this only occurs during sleep (Poe, 2017).

The notion that sleep plays a forgetting role has received some supportive behavioural and neuroscientific evidence (Hoedlmoser et al., 2015; Oyarzún, Morís, Luque, de Diego-Balaguer and Fuentemilla, 2017; Poe, Nitz, McNaughton & Barnes, 2000), but it has not been as widely supported by evidence as the consolidation role of sleep. On the other hand, the forgetting account arguably has superior explanatory worth since it more wholly explains the effect of sleep on memory function. For instance, sleep has been found to not only affect the consolidation of memories, but also the acquisition (Diekelmann, 2014) and encoding (Cousins, Sasmita & Chee, 2017; Poh & Chee, 2017) of new memories. The forgetting theory directly accounts for these findings. This is because it is hypothesised that targeted forgetting is essential for the efficiency of memory systems to be maintained (Poe, 2017). Additionally, this hypothesis itself has received supportive evidence (Anderson, 2003; Saletin, Goldstein & Walker, 2011). However, because the biological mechanisms thought to underlie the forgetting role of sleep have not received direct supportive evidence (Sara, 2017), the two major methodological issues aforementioned afflict research testing this theoretical account as well as that testing the consolidation account just discussed.

A further issue is that both the consolidation and the forgetting theories propose that REM sleep and SWS support distinct types of memory, but these claims are not backed-up by consistent evidence. The forgetting account proposes that SWS plays a role in supporting motor and procedural memory processing, while REM and transition-to-REM sleep support hippocampal and somatosensory memory processing (Poe, 2017). Yet, there is no clear evidence revealing the roles that REM sleep and non-REM sleep play (Sara, 2017), and there is inconsistent evidence for these sleep types supporting the processing of particular forms of memory. For instance, human studies have not found a consistent role of REM sleep in memory processing (Rasch & Born, 2013).

Alongside this, while the seminal paper by Plihal and Born (1997), cited by over 800 articles, is typically used as evidence for REM and non-REM sleep’s involvement in procedural and declarative memory consolidation respectively, its validity has been questioned. This is because it did not use an electroencephalogram (EEG) control for the waking time in participants, so the proportion of REM and non-REM sleep deprivation were only estimates, and crucially subsequent research using EEG monitoring has been unable to corroborate findings (Sara, 2017). This illustrates the importance of utilising recordings of brain activity in such a way to accurately reveal the precise relationship between sleep and memory. For instance, Poe (2017) recommends that future research should use longer portions of REM sleep, which would require EEG monitoring, to investigate the neurochemical mechanisms hypothesised to underlie the forgetting role. Sara (2017) suggests that additional functional magnetic resonance imaging studies are required to fully elucidate the relationship between brain regions for consolidation. Overall, while the role of sleep in organising the storage of memories is latent in the field of neuroscience, it has been challenging to experimentally demonstrate (Oyarzún et al., 2017).

Conclusion

Therefore, until further research can fully identify the biological mechanisms underpinning the relationship between sleep and memory function, it is not possible to be certain of the precise role sleep plays. This is especially pertinent because current research indicating that sleep plays a causal role in supporting memory function has not eliminated the effects of stress, nor accounted for the involvement of memory-related cognitive processes. As a result, it is only possible to conclude the following. Of the two popular theoretical accounts discussed, the hypothesised forgetting role of sleep has greater explanatory worth as it more wholly explains the effect of sleep on memory function, though the consolidation account possesses more supportive evidence. However, because these theories are not necessarily mutually exclusive, it is likely that sleep supports memory function by carrying out both consolidation and forgetting roles, with the latter role being unique to sleep.

Interested in taking part in research that could improve our understanding of the human brain? Visit http://www.ucl.ac.uk/news/students/022018/08022018-paid-research-subject-pool

References

Anderson, M. C. (2003). Rethinking interference theory: Executive control and the mechanisms of forgetting. Journal of Memory and Language49(4), 415-445.

Born, J., Rasch, B., & Gais, S. (2006). Sleep to remember. The Neuroscientist12(5), 410-424.

Cousins, J. N., Sasmita, K., & Chee, M. W. (2017). Memory encoding is impaired after multiple nights of partial sleep restriction. Journal of Sleep Research, 27(1), 138–145.

Diekelmann, S. (2014). Sleep for cognitive enhancement. Frontiers in Systems Neuroscience8, doi:10.3389/fnsys.2014.00046.

Durmer, J. S., & Dinges, D. F. (2005). Neurocognitive consequences of sleep deprivation. In K. L. Roos & A. Y. Avidan (Eds.), Seminars in Neurology (Vol. 25, No. 01, pp. 117-129). New York, NY: Thieme Medical Publishers.

Gais, S., Lucas, B., & Born, J. (2006). Sleep after learning aids memory recall. Learning & Memory13(3), 259-262.

Hoedlmoser, K., Birklbauer, J., Schabus, M., Eibenberger, P., Rigler, S., & Mueller, E. (2015). The impact of diurnal sleep on the consolidation of a complex gross motor adaptation task. Journal of Sleep Research24(1), 100-109.

Killgore, W. D. S. (2010). Effects of sleep deprivation on cognition. Progress in Brain Research, 185, 105-129.

Krause, A. J., Simon, E. B., Mander, B. A., Greer, S. M., Saletin, J. M., Goldstein-Piekarski, A. N., & Walker, M. P. (2017). The sleep-deprived human brain. Nature Reviews Neuroscience, 18, 404-418.

Liu, J. C., Verhulst, S., Massar, S. A., & Chee, M. W. (2015). Sleep deprived and sweating it out: the effects of total sleep deprivation on skin conductance reactivity to psychosocial stress. Sleep38(1), 155-159.

Maier, J. G., & Nissen, C. (2017). Sleep and memory: mechanisms and implications for psychiatry. Current Opinion in Psychiatry30(6), 480-484.

Maquet, P. (2001). The role of sleep in learning and memory. Science294(5544), 1048-1052.

Oyarzún, J. P., Morís, J., Luque, D., de Diego-Balaguer, R., & Fuentemilla, L. (2017). Targeted memory reactivation during sleep adaptively promotes the strengthening or weakening of overlapping memories. Journal of Neuroscience37(32), 7748-7758.

Plihal, W., & Born, J. (1997). Effects of early and late nocturnal sleep on declarative and procedural memory. Journal of Cognitive Neuroscience9(4), 534-547.

Poe, G. R. (2017). Sleep Is for Forgetting. Journal of Neuroscience37(3), 464-473.

Poe, G. R., Nitz, D. A., McNaughton, B. L., & Barnes, C. A. (2000). Experience-dependent phase-reversal of hippocampal neuron firing during REM sleep. Brain Research855(1), 176-180.

Poh, J. H., & Chee, M. W. (2017). Degradation of cortical representations during encoding following sleep deprivation. Neuroimage153, 131-138.

Prince, T. M., & Abel, T. (2013). The impact of sleep loss on hippocampal function. Learning & Memory20(10), 558-569.

Rasch, B., & Born, J. (2013). About sleep’s role in memory. Physiological Reviews93(2), 681-766.

Saletin, J. M., Goldstein, A. N., & Walker, M. P. (2011). The role of sleep in directed forgetting and remembering of human memories. Cerebral Cortex21(11), 2534-2541.

Sara, S. J. (2017). Sleep to remember. Journal of Neuroscience37(3), 457-463.

Tomasi, D., Wang, R. L., Telang, F., Boronikolas, V., Jayne, M. C., Wang, G. J., … & Volkow, N. D. (2008). Impairment of attentional networks after 1 night of sleep deprivation. Cerebral Cortex19(1), 233-240.

 

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