Delineating memory reactivation in sleep with verbal and non-verbal retrieval cues

Sleep supports memory consolidation via the reactivation of newly formed memory traces. One way to investigate memory reactivation in sleep is by exposing the sleeping brain to auditory retrieval cues; a paradigm known as targeted memory reactivation (TMR). To what extent to acoustic properties of memory cues influence the effectiveness of TMR, however, has received limited attention. We addressed this question by exploring how verbal and non-verbal memory cues affect oscillatory activity linked to memory reactivation in slow-wave sleep. Fifty-one healthy adult males learned to associate visual stimuli with spoken words (verbal cues) and environmental sounds (non-verbal cues). Subsets of the verbal and non-verbal cues were then replayed during sleep, alongside previously unheard control cues. For a subset of the participants, the voice of the verbal cues was mismatched between sleep and learning. Memory cues (relative to control cues) prompted an increase in theta/alpha and spindle power, which have been heavily implicated in sleep-associated memory processing. Moreover, verbal memory cues were associated with a stronger increase in spindle power than non-verbal memory cues. There were no significant differences between the matched and mismatched conditions when analysing verbal memory cues in isolation. Our findings suggest that verbal memory cues may be more effective than non-verbal memory cues for triggering memory reactivation in sleep, as indicated by an amplified spindle response.