Visuomotor adaptation modulates the clustering of sleep spindles into trains
Sleep spindles are thought to promote memory consolidation. Recently, we have shown that visuomotor adaptation (VMA) learning increases the density of spindles and promotes the coupling between spindles and slow oscillations, locally, with the level of spindle-SO synchrony predicting overnight memory retention. Yet, growing evidence suggests that the rhythmicity in spindle occurrence may also influence the stabilization of declarative and procedural memories. Here, we examined if VMA learning promotes the temporal organization of sleep spindles into trains. We found that VMA increased the proportion of spindles and spindle-SO couplings in trains. In agreement with our previous work, this modulation was observed over the contralateral hemisphere to the trained hand, and predicted overnight memory retention. Interestingly, spindles grouped in a cluster showed greater amplitude and duration than isolated spindles. The fact that these features increased as a function of train length, provides evidence supporting a biological advantage of this temporal arrangement. Our work opens the possibility that the periodicity of NREM oscillations may be relevant in the stabilization of procedural memories. CONTRIBUTION STATEMENTEver since the discovery of memory systems, the study of the mechanisms supporting the consolidation of declarative and procedural memories has progressed somewhat in parallel. We now know, however, that structures originally thought of as purely declarative such as the hippocampus, participate in the consolidation of procedural tasks. Recently, we showed that sleep predicts long-term motor memory through the local synchrony between fast sleep spindles and slow oscillations, a mechanism initially described for the consolidation of declarative memories. Novel evidence has linked the rhythmicity in the occurrence of spindles to memory stabilization. This framework proposes that temporally clustered spindles into trains of two or more separated by 3-6 seconds, may favor the reinstatement and subsequent reprocessing of previously acquired memories. This temporal arrangement may facilitate mnemonic replay and neocortical integration. In the present study, we show that motor learning promotes the organization of spindles into trains, locally, over the contralateral hemisphere, and that this modulation predicts overnight memory retention. Spindle grouping also augmented the proportion of spindle-SO couplings in trains. Importantly, spindles in a cluster increased their duration and amplitude as a function of train length, pointing to a physiological benefit of this temporal organization.