Definition: Offline memory refers to memory processing that occurs in the absence of external stimuli or active task engagement—typically during rest, sleep, or passive states. It involves reactivation, consolidation, and reorganization of previously encoded experiences.

• Occurs after learning, not during active encoding or recall.
• Involves spontaneous neural replay (e.g., hippocampal sharp-wave ripples) that strengthens or reorganizes memory traces.
• Often linked to systems consolidation, where memories are gradually transferred from the hippocampus to neocortical areas.

• Sleep stages:
  • Slow-wave sleep (SWS): Promotes consolidation of declarative memories.
  • REM sleep: Associated with emotional and procedural memory integration.
• Resting-state activity:
  • Observed during quiet wakefulness (e.g., post-task rest).
  • Increased connectivity between memory-related regions (e.g., hippocampus ↔ neocortex).

• Memory consolidation: Stabilization and integration of new memories into existing knowledge networks.
• Memory abstraction: Extraction of statistical regularities or general patterns (schema formation).
• Prediction and planning: Offline reactivation helps simulate future scenarios based on past experiences.

• Rodent studies: Hippocampal place cell sequences are replayed during rest, reflecting prior navigation paths.
• Human fMRI/iEEG: Post-learning rest shows reactivation of task-related brain patterns, predictive of later recall.
• Targeted Memory Reactivation (TMR): Sounds or cues during sleep can enhance specific memory consolidation.

• Sleep disorders (e.g., insomnia, sleep apnea) can impair offline memory consolidation.
• Neurodegenerative diseases may disrupt hippocampal-neocortical transfer during offline states.
• Understanding offline memory mechanisms is crucial for rehabilitation, learning optimization, and early detection of cognitive decline.