The Role of Sleep Spindles, Slow Oscillations, and Slow Oscillation-Spindle Coupling in Episodic Memory Consolidation

Non-rapid eye movement (NREM) sleep is known to play a critical role in episodic memory consolidation. Sleep spindles, events in the 12-15 Hz range that are characteristic of NREM stage 2 sleep, are often implicated in this process. However, spindles also occur during slow wave sleep (SWS), and it remains unclear whether stage 2 sleep is sufficient to observe spindle-related memory benefits, if obtaining SWS is necessary as well, or if the temporal coupling of spindles and lower frequency oscillations in SWS (i.e., slow oscillations, < 1 Hz) is essential for memory consolidation. To address these possibilities, the present study compared episodic memory retention after a period of active wakefulness, a nap containing stage 2 but not SWS, and a nap containing both stage 2 and SWS. This study also examined relationships between spindles, slow oscillations, and their cross- frequency coupling in relation to episodic memory. The results indicated that, while there were no significant group differences in memory performance, slow oscillations in SWS significantly predicted the retention of episodic information across a 90-minute nap. However, memory retention was not significantly predicted by spindles in either stage 2 or SWS, nor was it predicted by the temporal coupling of slow oscillations and spindles in SWS. Although these findings suggest that a daytime nap may affect episodic memory consolidation via SWS-specific mechanisms, further research is needed to understand the role of spindles, slow oscillations, and their cooccurrence for episodic memory consolidation.