Slow wave sleep supports the reorganisation of episodic memory networks

Abstract

Models of memory consolidation propose that newly acquired memory traces undergo reorganisation during sleep. To test this idea, we recorded high-density electroencephalography (EEG) during an evening session of word-image learning followed by immediate (pre-sleep) and delayed (post-sleep) recall. Polysomnography was employed throughout the intervening night, capturing time spent in different sleep stages. Using source-reconstructed time-frequency analysis, we first replicated the effect of alpha power decreases for successful relative to unsuccessful recall, emerging between 700 and 1500 ms after cue onset and spanning medial and lateral temporal lobe regions as well as posterior parietal cortex. Directly contrasting successful post-sleep vs. pre-sleep recall revealed a shift of alpha power decrease from parietal towards anterior temporal lobe (ATL) after sleep. Critically, time spent in slow wave sleep (SWS) during the intervening night not only predicted the extent of memory retention, but also correlated with the shift to ATL recall effects. Finally, brain-wide functional connectivity profiles during successful recall pointed to a marked overnight reorganisation of memory networks, with the extent of reorganisation again predicted by time spent in SWS. Together, these findings link SWS to the consolidation and functional reorganisation of episodic memory networks.