Persistence of neuronal representations through time and damage in the hippocampus.

Memories can persist for decades but how they are stably encoded in individual and groups of neurons is not known. To investigate how a familiar environment is encoded in CA1 neurons over time we implanted bilateral microendoscopes in transgenic mice to image the activity of pyramidal neurons in the hippocampus over weeks. Most of the neurons (90 %) are active every day, however, the response of neurons to specific cues changes across days. Approximately 40 % of place and time cells lose fields between two days; however, on timescales longer than two days the neuronal pattern changes at a rate of 1 % for each additional day. Despite continuous changes, field responses are more resilient, with place/time cells recovering their fields after a 10-day period of no task or following CA1 damage. Recovery of these neuronal patterns is characterized by transient changes in firing fields which ultimately converge to the original representation. Unlike individual neurons, groups of neurons with inter and intrahemispheric synchronous activity form stable place and time fields across days. Neurons whose activity was synchronous with a large group of neurons were more likely to preserve their responses to place or time across multiple days. These results support the view that although task-relevant information stored in individual neurons is relatively labile, it can persist in networks of neurons with synchronized activity spanning both hemispheres. One Sentence SummaryNeuronal representations in networks of neurons with synchronized activity are stable over weeks, even after lack of training or following damage.