Experience reorganizes content-specific memory traces in macaques

Memory formation relies on the reorganization of neural activity patterns during experience that persist in subsequent sleep. How these processes promote learning while preserving established memories remains unclear. Here we recorded neural ensemble activity from the hippocampal and associated regions in freely moving macaques as they recalled item sequences presented that day ('new'), 1 day earlier ('recent') or more than 2 weeks earlier ('old'). Cell assemblies biased for old sequences showed less drift, greater network connectivity and stronger sleep reactivation than new-biased assemblies. Pairs of old and recent assemblies formed persistent task-to-sleep coupling ('metassemblies'), unlike new assembly pairs, and these assembly pairs showed longer interactions over time than the new pairs. In the hippocampus, the propensity for superficial and deep CA1 pyramidal cells to form integrated assemblies increased with memory age. These findings reveal rapid organization and stabilization of neural activity in the primate brain, suggesting potential mechanisms for balancing learning with memory linking and durability.