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Erschienen in:
01.03.2015 | Schwerpunkt
Involvement of sleep spindles in overnight declarative memory stabilization
Effects of time of incidence and spindle type
Erschienen in: Somnologie | Ausgabe 1/2015
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Objective
Numerous studies point to the involvement of sleep spindles and slow waves in memory processes, particularly in hippocampus-dependent declarative memory. We have shown previously that the overnight change in recall performance in a declarative word pair association task correlates significantly with increased spindle activity during the night after learning compared to a control night. The current study re-evaluates this relationship in detail and explores whether the observed positive correlation of two spindle parameters measured during stage 2 (S2) sleep with overnight stabilization depend on the time of night (early vs. late) and spindle type (fast vs. slow).
Methods
The study included 24 healthy volunteers aged 20–30 years. Two counterbalanced nights served as either control condition without intentional learning or as an experimental condition including a declarative memory task in the evening. Performance was tested directly after learning and in the following morning. Spindle detection was based on a validated automatic algorithm, providing density and intensity of slow (≤ 13 Hz) and fast (> 13 Hz) spindles per minute S2 sleep. To obtain spindle measures during the course of the night, the entire recording period was segmented into five 90-min parts.
Results
Significant correlations were observed between changes (experimental night minus control night) in spindle density (r = 0.44, p < 0.05) and intensity (r = 0.52, p < 0.01), and overnight changes in memory performance during the night following the learning task. Changes in memory performance correlate significantly with changes in fast spindle intensity (parts 1–3) and density (part 2), but not significantly with any slow spindle parameter.
Conclusion
The present study confirms the involvement of fast sleep spindles during S2 in declarative memory processes. Significant correlations with spindle retention are only seen in early S2 sleep and not during late S2 sleep. It remains to be investigated whether this effect is due to circadian factors, homeostatic factors, or the time elapsed after initial learning.