Mechanisms of associative memory consolidation during sleep

Petzka, Marit (2022). Mechanisms of associative memory consolidation during sleep. University of Birmingham. Ph.D.

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Abstract

How are transient memories transformed into lasting ones? While previous research has established the significance of sleep for consolidating memories, the intricate brain mechanisms underlying sleep-dependent memory consolidation are yet to be explored. This thesis investigates the mechanistic role of two cardinal brain oscillations during sleep, sleep spindles and slow oscillations, for consolidating associative memories. In a first study, a new memory paradigm operationalising multiple aspects of memories, precisely temporal and spatial features, is introduced. The results indicate that the paradigm indeed captures memory aspects that are consolidated during sleep. By combining this paradigm with electrophysiological brain recordings, a second study demonstrates that sleep spindles are most pronounced over learning-related cortical areas.
The extent to which spindles track these learning-related cortical areas predicts behavioural measures of memory consolidation. Thereby, the second study provides evidence supporting a mechanistic function of sleep spindles for memory consolidation. That is, sleep spindles specifically occur in encoding relevant cortical areas to facilitate consolidation, presumably by inducing long-lasting changes (plasticity) in these areas. In a third and fourth study, the interplay between the two cardinal sleep oscillations (sleep spindles and slow oscillations) and memory reactivation is investigated. Besides inducing plasticity, memory reactivation has been suggested as a potential mechanism underlying sleep-dependent memory consolidation. In the third study, we tested for a synchronisation of sequential memory reactivation by slow oscillations. To this end, we employed a sequential memory paradigm together with novel analysis techniques enabling the tracking of sequential memory reactivation. Results represent first evidence of sequential memory reactivation in humans and support the hypothesis that reactivation of sequential memories is synchronised by slow oscillations. Applying the same analysis techniques in a fourth study together with an associative memory paradigm, the importance of slow oscillation and sleep spindle coupling for memory reactivation has been tested. Results of study four reveal memory reactivation during slow oscillation-sleep spindle complexes and moreover, that the temporal precision of slow oscillation-sleep spindle coupling predicts memory reactivation strength. Study three and four corroborate a timing function of cardinal sleep oscillations in service of memory consolidation, suggesting the temporal coordination of memory reactivation as a potential mechanistic function of slow oscillations and slow oscillation-sleep spindle complexes. The final chapter provides a contextualised overview of the work and discusses the interplay between brain oscillations during sleep and the proposed mechanisms, induction of plasticity and memory reactivation. Together, this thesis provides further insights into the mechanisms subserving associative memory consolidation during sleep.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):