Biology

Your memory keeps time

Deep in your brain sits the hippocampus, and inside it are neurons called time cells, first recorded in people in 2020. Each one fires at a particular moment of an experience, and firing one after another they lay a memory along a timeline. That is why an event's order and rough length return with no clock involved.

Sources & further reading

Science (AAAS)Internally Generated Cell Assembly Sequences in the Rat HippocampusThe study showing hippocampal cells fire in a set order during a plain wait, marking time with no outside cue.Neuron (Cell Press)Hippocampal "Time Cells" Bridge the Gap in Memory for Discontiguous EventsThe paper that named time cells, cells that each fire at their own moment to bridge a gap in a memory.Nature Reviews NeuroscienceTime Cells in the Hippocampus: A New Dimension for Mapping MemoriesA review arguing time cells map when a memory happened, alongside place cells that map where.NatureIntegrating Time from Experience in the Lateral Entorhinal CortexThe study showing a brain region feeding the hippocampus carries a slowly drifting signal that acts like a running timestamp.Proceedings of the National Academy of SciencesTime Cells in the Human Hippocampus and Entorhinal Cortex Support Episodic MemoryThe study that recorded time cells directly in people and found their firing predicts memory for the order of events.Brain ResearchThe Hippocampus as a Spatial Map: Preliminary Evidence from Unit Activity in the Freely-Moving RatThe 1971 discovery of place cells, brain cells that each fire in one spot to build a map of space.Journal of Neurology, Neurosurgery & PsychiatryLoss of Recent Memory After Bilateral Hippocampal LesionsThe report on patient H.M., who kept his old memories but could form no new ones after his hippocampus was removed.Journal of Verbal Learning and Verbal BehaviorReconstruction of Automobile Destruction: An Example of the Interaction Between Language and MemoryThe car-crash study showing a single leading word can change what people are sure they remember seeing.Science (AAAS)Reactivation of Hippocampal Ensemble Memories During SleepThe study showing a sleeping rat's brain replays the same cell pattern it ran while awake.Nature Reviews NeuroscienceThe Memory Function of SleepA review of the evidence that sleep after learning helps new memories stick, partly by replaying them.HippocampusThe Hippocampal Indexing Theory and Episodic Memory: Updating the IndexThe theory that memory content is stored across the cortex while the hippocampus keeps a compact index that points to it.