Have you ever been rocked to sleep as a child? Might this soothing motion that could calm us down as children have an effect on quality of sleep and memory consolidation?  In 2020, a team of researchers, motivated by this question, mounted a bed on a moving platform and asked participants to sleep on it. They gently rocked the bed during an afternoon nap and found that this low-intensity rocking led to an increased density of slow oscillations in the brain [1]. Meanwhile, another group of researchers connected electrodes to participants' heads and stimulated their brains while they slept, aiming to understand the role of sleep spindles in cognitive processes, particularly memory consolidation [2]. From egg-shaped sleep pods to robotic pillows designed by students, the exploration of sleep science has inspired some rather unconventional attempts to improve sleep outcomes. However, one promising technique stands out for its convenience: acoustic stimulation. This non-invasive method could potentially stabilize our sleep and enhance our memory [3][4].

While acoustic stimulation has shown success in improving declarative memory (the type of memory that involves explicit knowledge, such as facts or events), less research has focused on its potential to enhance procedural memory [5]. Procedural memory refers to skills and tasks we perform, such as playing an instrument or typing on a keyboard. Interestingly, previous studies suggest that the more sleep spindles we experience, the more memory is consolidated after we wake up [6]. Furthermore, when researchers increased the frequency of sleep spindles using electrical stimulation, procedural memory was notably improved when tested the following day [2]. Researchers also propose that sleep spindles, coupled with slow oscillations, play a crucial role in memory consolidation [7]. For instance, disrupted coupling of slow oscillations and sleep spindles in older adults has been linked to memory problems [8].

To explore whether acoustic stimulation could enhance procedural memory consolidation, a team of South Korean researchers studied the effects of closed-loop acoustic feedback targeting sleep spindles [9]. The researchers aimed to boost the sleep spindle, a burst of brain activity associated with memory consolidation, by using acoustic stimulation during naps. In their study, 16 volunteers were divided into three treatment groups. The first was a SHAM group, which received no acoustic stimulation, serving as a control for the placebo effect. The second group, the SPIN group, received a 60 dB pink noise every time the researchers detected a sleep spindle using an EEG. Finally, the RAND group received acoustic stimulation at random intervals to control for the effect of general acoustic stimulation on memory consolidation. The participants were trained on a finger-tapping task before the nap, which lasted 90 minutes, and then tested again on the same task.

All groups performed better on the finger-tapping task after the nap than they had before [9]. However, the SPIN group outperformed both the SHAM and RAND groups. Additionally, the SPIN group had a lower wake time after sleep onset compared to the other groups. Since the SPIN group had fewer awakenings than the RAND group, the researchers concluded that acoustic stimulation specifically targeted at sleep spindles helped stabilize naps and improve procedural memory consolidation [9]. The results of this study add to the growing body of research on the use of acoustic stimulation to enhance sleep spindle activity. While other studies have failed to produce significant results or have not adequately designed experiments to isolate the effects of acoustic stimulation, this study provides strong evidence that spindle-targeted acoustic stimulation can improve nap quality and enhance procedural memory consolidation [9][10][11]. As a result, we may soon see the development of goal-specific sleep-aid technologies, such as wearable headbands designed to improve sleep quality and work performance. These technologies could be particularly beneficial for employees seeking to boost performance and engagement [12].

However, there are still questions that need further investigation. For example, this study did not collect data on whether acoustic stimulation affects specific sleep stages, such as REM sleep. Future studies should examine how different sleep stages, particularly REM, interact with acoustic stimulation to optimize memory consolidation. Moreover, while the study confirms that spindle-targeted acoustic stimulation can improve procedural memory, more research is needed to understand how various sleep components work together to influence memory outcomes. As the development of acoustic stimulation techniques progresses, we may see more than just marginal improvements in procedural memory. This research may lead to more effective methods for enhancing memory consolidation during sleep, which could have significant implications for cognitive performance, especially for individuals dealing with age-related cognitive decline or neurodegenerative disorders.

In recent years, neuroscience has made major breakthroughs in areas like brain surgery for cancer treatment, advanced brain imaging for disease detection, and new Alzheimer’s treatments to improve quality of life. However, sleep remains an often-overlooked area of study, despite its profound impact on our lives. Recent studies demonstrate that even seemingly simple interventions, such as acoustic stimulation targeting sleep spindles, can significantly enhance procedural memory consolidation and improve sleep quality. These findings open the door to potential sleep-aid technologies that could be used to boost cognitive performance in everyday life and in professional settings, particularly for patients experiencing memory loss due to aging or neurodegenerative diseases. As sleep-enhancing technologies continue to develop, they could provide powerful interventions to help individuals optimize their sleep and cognitive performance, potentially transforming the way we approach memory improvement and cognitive health. Future research should delve deeper into how different sleep stages interact with these stimulation techniques, offering the possibility of tailored interventions that could further enhance memory consolidation and provide new treatments for cognitive disorders.

References: 

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[2] Lustenberger C, Boyle MR, Alagapan S, Mellin JM, Vaughn BV, Fröhlich F. Feedback-Controlled Transcranial Alternating Current Stimulation Reveals a Functional Role of Sleep Spindles in 

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[4] Vroon, J., Zaga, C., Davison, D., Kolkmeier, J., & Linssen, J. (2017, March). Snoozle--A robotic pillow that helps you go to sleep: HRI 2017 Student Design Competition. In Proceedings of the Companion of the 2017 ACM/IEEE International Conference on Human-Robot Interaction (pp. 399-400).

[5] Stanyer EC, Baniqued PDE, Awais M, et al. The impact of acoustic stimulation during sleep on memory and sleep architecture: A meta-analysis. J Sleep Res. 2022; 31:e13385. https://doi.org/10.1111/jsr.13385

[6] Schabus, M., Gruber, G., Parapatics, S., Sauter, C., & Klimesch, W. (2004). Sleep spindles and their significance for declarative memory consolidation. Sleep, 27(6), 1295–1303. https://doi.org/10.1093/sleep/27.6.1295

[7] Staresina BP, Bergmann TO, Bonnefond M, van der Meij R, Jensen O, Deuker L, Elger CE, Axmacher N, Fell J. Hierarchical nesting of slow oscillations, spindles and ripples in the human hippocampus during sleep. Nat Neurosci. 2015 Nov;18(11):1679-1686. doi: 10.1038/nn.4119.  Epub 2015 Sep 21. PMID: 26389842; PMCID: PMC4625581.

[8] Muehlroth, B. E., Sander, M. C., Fandakova, Y., Grandy, T. H., Rasch, B., Shing, Y. L., & Werkle-Bergner, M. (2019). Precise slow oscillation–spindle coupling promotes memory consolidation in younger and older adults. Scientific reports, 9(1), 1940.

[9] Choi, J., & Jun, S. C. (2022). Spindle-targeted acoustic stimulation may stabilize an ongoing nap. Journal of Sleep Research, 31, e13583. https://doi.org/10.1111/jsr.13583

[10] J. Choi, K. Won and S. C. Jun, "Acoustic Stimulation Following Sleep Spindle Activity May Enhance Procedural Memory Consolidation During a Nap," in IEEE Access, vol. 7, pp. 56297-56307, 2019, doi: 10.1109/ACCESS.2019.2913457.

[11] Bryan S Baxter, Dimitrios Mylonas, Kristi S Kwok, Christine E Talbot, Rudra Patel, Lin Zhu, Mark Vangel, Robert Stickgold, Dara S Manoach, The effects of closed-loop auditory stimulation on sleep oscillatory dynamics in relation to motor procedural memory consolidation, Sleep, Volume 46, Issue 10, October 2023, zsad206, https://doi.org/10.1093/sleep/zsad206

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