Modulating overnight memory consolidation by acoustic stimulation during slow wave sleep – a systematic review and meta-analysis

Study objectives: The low-frequency high-amplitude oscillations of slow wave sleep are considered to promote the consolidation of episodic memory. Previous research suggests that sleep slow waves can be entrained and enhanced by presenting short acoustic stimuli to the up-states of endogenous waves. Several studies have investigated the effects of these increases in slow wave activity on overnight memory consolidation, with inconsistent results. The aim of this meta-analysis was to evaluate the accumulated evidence connecting acoustic stimulation during sleep to episodic memory consolidation. Methods: A systematic literature search was conducted in October 2020 using Pubmed, Web of Science and PsycInfo. Main study inclusion criteria were the application of acoustic slow wave enhancement in healthy participants and an assessment of pre- and post-sleep episodic memory performance. Effect sizes were pooled using a random effects model. Results: Ten primary studies with 11 experiments and 177 participants were included. Results showed a combined effect size (Hedges' g) of 0.25 (p=0.07). Subgroup models based on young adults (n = 8), phase-locked stimulation approaches (n = 8) and their combination (n = 6) showed combined effect sizes of 0.31 (p=0.051), 0.36 (p=0.047) and 0.44 (p=0.01), respectively. There was no indication of publication bias or bias in individual studies. Conclusions: Acoustic enhancement of slow wave sleep tends to increase the overnight consolidation of episodic memory but effects remain small and - with the exception of subgroup models - at trend levels. Currently, the evidence is not sufficient to recommend the use of commercially available devices.