Slow Oscillations Modulate Overnight Brain Changes in Working Memory Function

Working memory (WM), the transient storage and manipulation of information, is a cognitive function that can improve with training. Accumulating evidence suggests that sleep, particularly slow oscillations (SOs) that occur during nonrapid eye movement sleep, supports this improvement. Yet, how sleep and SOs in particular relate to neural processes supporting WM remains poorly understood. In this study, we investigated how WM-related neural activity evolves across nocturnal sleep and how these changes relate to neural processing during SOs. Participants performed a WM task during fMRI before and after sleep, with simultaneous EEG-fMRI capturing neural activity during the first 2.5 hr of sleep. Our results showed significant overnight changes in WM-related activity, characterized by reduced recruitment of the dorsal precuneus during WM encoding/maintenance phases and frontal regions during the retrieval phase of WM. In addition, sleep increased item-specific reinstatement in a sensory processing region during WM. This pattern of results suggests a combined influence of sleep on enhancing sensory reinstatement while reducing potential top-down control (i.e., reduced parietal and frontal activity). Critically, SO-related activity was directly linked to overnight activity changes: Stronger SO activation in the premotor cortex and ventromedial pFC predicted greater overnight reductions in WM-related activity, and multivoxel analyses in the ventral attention network revealed a parallel relationship. These findings suggest that SOs play a critical role in WM function by facilitating the reorganization of WM-related processing.