Closed-loop acoustic stimulation promotes local slow oscillation train initiation and persistence in elderly adults

Slow oscillations (SOs) during non-rapid eye movement (NREM) sleep support memory consolidation and metabolic waste clearance. Aging is associated with reduced SO amplitude and density and disrupted temporal organization, characterized by more isolated events and shorter SO trains. We investigated whether closed-loop acoustic stimulation (CLAS) could enhance the initiation and persistence of rhythmic SO activity in elderly adults. Twenty-seven participants underwent a baseline night followed by an experimental night, during which online-detected SOs were either stimulated using CLAS (STIM; n = 16) or left unstimulated (SHAM; n = 11). We quantified the proportions of isolated and consecutive SOs, train-initiation probability, and SO train length. Analyses assessed local effects around online-detected SOs and global effects across NREM sleep using offline detection. Locally, CLAS reduced the proportion of isolated SOs and increased the proportion of consecutive SOs, train-initiation probability, and mean train length. Global overnight effects were weaker but directionally consistent, primarily reflecting a shift toward longer SO trains. These changes occurred without a significant between-group difference in the change in SO density, and local effects remained significant after adjustment for SO density. Thus, CLAS promoted sustained rhythmic activity beyond changes in SO occurrence. These findings suggest that CLAS can modify the temporal organization of SOs in elderly adults, supporting its potential to mitigate age-related disruptions in SO dynamics. Temporal organization measures may complement conventional amplitude and density measures when evaluating CLAS effects. Further studies should establish whether these changes translate into cognitive benefits or improvements in other aspects of brain health.