Le rôle du stade 2 du sommeil non-paradoxal et des fuseaux de sommeil dans la consolidation de la mémoire motrice séquentielle

From infancy to old age, motor learning is part of our everyday life. After learning a new motor skill, our brain continues to reinforce its memory trace. This process is known as “memory consolidation”, in which sleep is thought to be a key period. However, there is still no consensus concerning the sleep stage when consolidation of a motor sequence memory trace takes place, although there is increasing evidence pointing towards stage-2 non-rapid eye movement (NREM2) sleep. Some of this evidence suggests that sleep spindles, hallmark features of NREM sleep, are associated with improvement in consolidation. Yet, these reports are only correlational in nature. Hence, the aim of this thesis is to identify the sleep stage during which motor sequence consolidation occurs and define the involvement of sleep spindles in this process. To tackle these objectives, we used a finger tapping task paired to an olfactory targeted memory reactivation (TMR) paradigm. Participants were trained to a motor sequence learning (MSL) task while being exposed to an odor. The same olfactory stimulus was used during sleep as a cue to reactivate the memory trace of the newly learned motor task. In the morning, consolidation was assessed through a retest session of the same task. The first study probed the role of sleep stages by comparing motor performance gains between groups cued in NREM2 and rapid eye movements (REM) sleep. It also assessed the effect of cuing on sleep spindles characteristics. Results showed that participants cued during NREM2 sleep showed greater gains in performance at retest than those cued during REM sleep or those not conditioned but exposed during NREM2 sleep. Furthermore, they demonstrated that cuing led to increases in frequency and amplitude of parietal sleep spindles only. Finally, we found that cue-related changes in specific spindle frequency bands predicted the overnight gains in performance. In the second study, building on our previous findings, we investigated how parietal NREM2 sleep spindles interact with other frequency bands locally and between cortical regions (connectivity). Results showed that TMR stimulation instigated increases of concomitant activity in lower frequency bands (delta [1-4 Hz], theta [4-8 Hz]) preceding and following spindles, and increases within the high-beta (25-30 Hz) band during spindles. These changes were found particularly over motor and associative areas. We also reported enhanced connectivity between task-related regions in the sigma band as well as widespread increases in the delta and theta bands. Together, both studies demonstrated the importance of sleep and its constituents, namely NREM2 sleep and spindles, in the consolidation of motor sequence memories. Indeed, they showed that the newly learned memory trace can be reinforced during NREM2 by the use of an olfactory stimulus leading ultimately to an enhanced motor performance in the morning over and above normal behaviour, while the same effect does not appear to occur with REM sleep. They also demonstrated that sleep spindles are instrumental to the consolidation process although its effect relies on a complex interaction between multiple frequency bands, including delta, theta and high-beta, both locally and inter-regionally. Overall, this thesis underlines the significance of NREM2 sleep spindles’ contribution to motor sequence consolidation. Importantly, building on decades of work, it lays the groundwork for studying the role of sleep spindles in memory consolidation within their temporal, topographical and frequential context.

Keyphrases: Consolidation, Sleep, Sleep spindles, Sequence motor memory, Memory