Sleeping time is sometimes considered unproductive time. This raises the question of whether the spent sleep could be used more productively – e.g. for learning a new language? This sleeping research was focused on the stabilization and consolidation of memories that were formed during previous wakefulness. However, learning during sleep has rarely been examined. There is considerable evidence for wake-learned information undergoing recapitulation by replay in the sleeping brain. The replay during sleep strengthens the still fragile memory traces and embeds newly acquired information in the preexisting store of knowledge.
If re-play during sleep improves the storage of wake-learned information, then first-play – i.e., initial processing of new information – should also be feasible during sleep, possibly carving out a memory trace that lasts into wakefulness. This was the research question of Katharina Henke, Marc Zust and Simon Ruch of the Institute of Psychology and of the Interfaculty Research Cooperation "Decoding Sleep" at the University of Bern, Switzerland. These investigators have now shown for the first time that new foreign words and their translation words could be associated during a midday nap with associations stored in wakefulness. After waking, participants could reactivate the sleep-formed associations to access word meaningings when represented with formerly sleep-played foreign words. The hippocampus, and the brain structure essential for wake-up associative learning, also supported the retrieval of sleep-formed associations. The results of this experiment are published in the scientific journal Open Access Current Biology.
The brain cells' active states are central to sleep-learning
The research group of Katharina Henke examined whether a sleeping person is able to form new semantic associations between played foreign words and translation words during active states of the brain cells, the so-called "Up states". When we reach deep sleep stages, our brain cells progressively coordinate their activity. During deep sleep, the brain cells are usually active for a short period of time before they jointly enter into a state of brief inactivity. The active state is called "Up-state" and the inactive state "Down-state." The two states alternate about every half-second.
Semantic associations between sleep-played words of an artificial language and their German translation words were only coded and stored, if the second word of a pair was repeatedly (2, 3 or 4 times) played during an Up-state. Eg, when a sleeping person heard the word pairs "tofer = key" and "guga = elephant," then after waking they could categorize with a better-than-chance accuracy whether the sleep-played foreign words denominated something large (" Guga ") or small (" Tofer "). "It was interesting that language areas of the brain and the hippocampus – the brain's essential memory hub – were activated during the wake retrieval of sleep-learned vocabulary because these brain structures normally mediate the wake learning of new vocabulary," says Marc Zust, co-first-author of this paper. "These brain structures appear to mediate memory formation independently of the prevailing state of consciousness – unconscious during deep sleep, conscious during wakefulness."
Memory formation does not require consciousness
Besides its practical relevance, this new evidence for sleep-learning challenges current theories of sleep and theories of memory. The concept of sleep as an encapsulated mental state, in which we are detached from the physical environment is no longer tenable. "We could disprove that sophisticated learning be impossible during deep sleep," says Simon Ruch, co-first-author. Current results underscore a new theoretical notion of the relationship between memory and consciousness that Katharina Henke published in 2010 (Nature Reviews Neuroscience). "And how far and with which consequences deep sleep can be used for the acquisition of new information will be a topic of research in the coming years," says Katharina Henke.
The research group of Katharina Henke is part of the Interfaculty Research Cooperation "Decoding Sleep: From Neurons to Health & Mind" (IRC). Decoding Sleep is a large, interdisciplinary research project financed by the University of Bern, Switzerland. Thirteen research groups in medicine, biology, psychology, and informatics are part of the IRC. The aim of these research groups is to gain a better understanding of the mechanisms involved in sleep, consciousness and cognition.
The reported study was carried out in collaboration with Roland Wiest, who is affiliated with the Support Center for Advanced Neuroimaging (SCAN) at the Institute of Diagnostic and Interventional Neuroradiology, Inselspital, University of Bern. Both research groups also belong to the BENESCO consortium, which consists of 22 interdisciplinary research groups specialized in sleep medicine, epilepsy and research on altered states of consciousness.
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