Strong and Weak Episodic Memories Are Shaped by Multiple Cycles of NREM Spindles and REM Alpha Bursts

Prior research has built up the notion that non-rapid eye movement sleep (NREM) and rapid eye movement sleep (REM) are separately important for specific types of memory. However, growing evidence suggests that these sleep stages work in conjunction to jointly shape memory. The sequential hypothesis proposes that the ordered sequence of NREM followed by REM is critical for memory consolidation. Additionally, NREM spindles and REM alpha bursts have been independently implicated in episodic memory, yet it is unknown how these oscillatory events may shape episodic memory together. We hypothesised that NREM spindles and REM alpha bursts play complementary roles in episodic memory, with spindles associated with memory retention and REM alpha bursts with forgetting. Participants (n = 24, 18-35 years) completed a Face-Name Association task before and after sleep; interference was introduced to a subset of the face-name pairs prior to sleep. We then examined relations between sleep oscillatory events and overnight change in performance for memory with and without interference. Participants exhibited forgetting for both memory types (p < 0.05), with no significant difference between types (p > 0.05); however, the underlying sleep neurophysiology differed. Spindle density predicted memory improvement exclusively for interfered-with memories (p < 0.05). REM alpha burst power predicted forgetting for memories with and without interference (p < 0.05), with a stronger effect for memories without interference. These findings indicate distinct, yet complementary, roles for NREM spindles and REM alpha bursts in levelling the strength of newly acquired memories, consistent with the recently proposed REM Refine and Rescue Hypothesis.