Local coupling between sleep spindles and slow oscillations supports the stabilization of motor memories

Recent studies from us and others suggest that traditionally declarative structures (e.g., hippocampus) mediate some aspects of the encoding and consolidation of procedural memories. This evidence points to the existence of converging physiological pathways across memory systems. Here, we examined whether the coupling between slow oscillations (SO) and spindles, a mechanism well established in the consolidation of declarative memories, is relevant for the stabilization of human motor memories. To this aim, we conducted an EEG study in which we quantified various parameters of these oscillations during a night of sleep that took place immediately after learning a visuomotor adaptation task. We hypothesized that if this coupling is instrumental to motor memory consolidation then spindles locked to the active phase of a slow oscillation would predict long-term memory. We found that visuomotor adaptation increased the overall density of fast (≥12 Hz) but not slow (<12Hz) spindles during NREM3. This modulation was manifested rather locally, over the hemisphere contralateral to the trained hand. Although motor learning did not affect the density of SOs, it substantially enhanced the number of fast spindles locked to the active phase of SOs. The fact that only coupled spindles of the left hemisphere predicted long-term memory overnight, points to the precise phase relationship between these oscillations as a fundamental signature of motor memory consolidation. Our work provides evidence in favor of a common mechanism at the basis of the stabilization of declarative and non-declarative memories. Significance Statement Ever since the discovery of memory systems, the study of the mechanisms supporting the consolidation of declarative and procedural memories has progressed somewhat in parallel. In the last few years, however, this framework is starting to change. We now know that structures originally thought of as purely declarative, such as the hippocampus, participate in the consolidation of procedural tasks. Here, we show that sleep modulates the stabilization of motor memories through a mechanism involved in the consolidation of declarative memories, based on the local synchrony between fast sleep spindles and slow oscillations. The fact that only coupled –but not uncoupled- spindles of the contralateral hemisphere predicted long-term memory supports a role of this association in the consolidation of motor memories.