Vocabulary memory formed during deep sleep persists at least for 36 hours, if word pairs are encoded during a slow-wave down-state

We recently published an experiment showing that vocabulary learning during deep sleep is possible (Züst et al., 2019). Here, we present the results of a follow-up study, in which we investigated (1) for how long sleep-formed lexical-semantic associations persist in memory and (2) how their formation is neurophysiologically mediated. Pairs of pseudowords and German words were played to 30 sleeping participants for their unconscious formation of semantic associations between pseudowords and translation words. Sleep-generated memory formation was probed 12 and 36 hours after sleep-learning. We were particularly interested in the role of the slow oscillation (SO) phase that is most conducive to sleeplearning. Using an EEG-acoustic stimulation closed-loop algorithm we aligned the onset of word presentations to the up- (15 participants) or the down-state (15 participants) of SO. The new semantic word associations, formed during slow-wave sleep were stored into wakefulness and influenced decision-making at retrieval testing up to at least 36 hours following sleep-learning. Participants retrieved significantly more associations if the word pairs had been played during a SO down-phase versus a SO up-phase. Hence, the coincidence of word presentation with a particular phase of the ongoing SO is critical in determining whether the played information is stored long-term or not. Findings suggest that hippocampally mediated semantic paired-associative learning and long-term storage is feasible during human slow-wave sleep