11 Feb Memories Can Be Stored During Some Unconscious Sleep States
MedicalResearch.com Interview with:
Marc Züst, PhD
University of Bern
Department of Psychology
Division of Experimental Psychology and Neuropsychology
MedicalResearch.com: What is the background for this study? What are the main findings?
Response: Slow wave sleep (deep sleep) is known to be very important for memory reorganization. The brain goes through the memory traces that were created during wakefulness and strengthens the important ones, while unimportant ones are weakened or deleted to make room for new learning the next day. This happens during the peaks of the eponymous slow waves, also called up-states, where the brain is highly active and interconnected. Up-states last for about 0.5 sec before transitioning into down-states, where the brain is relatively silent.
Based on these findings, we hypothesized that up-states constitute windows of opportunity to learn new information during slow wave sleep: The “channels are open”, and the brain is already performing memory functions.
The results of our study support this hypothesis. We found that, if we repeatedly managed to synchronize presentation of word pairs with up-states, memory for these pairs was best. Moreover, we find a dose-response function: The more often word pairs hit up-states, the better the memory. On top of that, fMRI during the retrieval test suggests that the same brain regions are involved in sleep learning as are involved in learning during wakefulness.
MedicalResearch.com: What should readers take away from your report?
Response: That humans are capable of sophisticated information processing without consciousness. Sleep-formed memory traces endure into the following wakefulness and can influence how you react to foreign words like “biktum” even though you think you’ve never seen that word before. It’s an implicit, unconscious form of memory – like a gut feeling.
However, if you try putting on headphones and listening to a language-learning-tape while asleep, you will probably not get much out of it. Timing is key. Not all of slow wave sleep is conductive to sleep learning. We really need to hit those up-states.
MedicalResearch.com: What recommendations do you have for future research as a result of this work?
Response: We currently don’t know if you can explicitly learn new information quicker during wakefulness if that information has already been presented during sleep. That is something we’ll look into in a future study. If an initial, unconscious bout of sleep-learning is beneficial to conscious learning later on, sleep-learning could find an application in people with learning difficulties, people with attention deficits that have trouble consciously focusing on explicit learning, or older people suffering from cognitive decline.
MedicalResearch.com: Is there anything else you would like to add?
Response: This is basic research, so this study was not intended for clinical application or to make lifestyle-recommendations. We do see that it is possible to encode new information during sleep, but there may be hidden costs. Sleep, memory, and the brain as a whole are intricately balanced systems that have undergone millions of years of optimization. So if we nudge the sleeping brain towards learning new information, we could be impeding whatever function it was already performing. We didn’t test our intervention against just getting a good night’s sleep. But from what we know about the importance of a good night’s sleep, I wouldn’t bet my French exam on listening to a tape under your pillow (even if your tape somehow manages to only talk to you during up-states). Rather, I recommend learning it well consciously and then going to bed early, and let the brain do its thing.
But it’s definitely good to know that you aren’t completely shut-off from your surroundings while asleep, especially if you’re prone to falling asleep in front of your TV. Imagine listening to commercials all night. Some of that information could stick and you would never know.
Marc Alain Züst, Simon Ruch, Roland Wiest, and Katharina Henke. Implicit Vocabulary Learning during Sleep Is Bound to Slow-Wave Peaks. Current Biology, 2019 DOI: 10.1016/j.cub.2018.12.038
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