Temporal sleep spindle clustering and slow-oscillation coupling in motor memory consolidation and generalization.

Sleep benefits memory consolidation through periodic sleep spindle activity and associated memory reactivations. The temporal organization of spindles in "trains" is considered a critical sleep mechanism for the timed and repeated reactivation of memories. Evidence suggests that a timely phase-locking between slow oscillations (SO) and spindles facilitates learning-related synaptic plasticity. Here, we investigated the contribution of spindles' clustering and coupling with SO in motor memory consolidation by promoting local synaptic depression in sensorimotor cortical regions through upper-limb immobilization following motor sequence learning. Our results reveal that the cluster-based organization of spindles is independent of daytime sensorimotor experience, while leading to distinct overnight behavioral outcomes. Interestingly, immobilization induced a phase shift in the SO-spindle coupling for spindles grouped in trains, but not when isolated outside trains. We demonstrate that spindle trains may promote skill-specific strengthening of motor memories, while isolated spindles may instead create memory-instability conditions leading to enhanced skill generalization.