A study published in *Nature Neuroscience* (2013) investigated the long-term dynamics of CA1 hippocampal place codes in freely behaving mice. Using calcium imaging, researchers tracked thousands of pyramidal cells over weeks, revealing that while the ensemble representation of an environment changed daily, a subset of cells retained stable place fields, sufficient to maintain accurate spatial representations over weeks. This suggests that CA1 place cells, crucial for spatial memory, do not necessarily retain stable place fields over long periods, but a dynamic aspect of place coding allows for distinct memory traces of events in the same environment. The study combined a viral vector to express GCaMP3 in pyramidal cells, a chronic mouse preparation for time-lapse imaging, and a miniaturized microscope for calcium imaging in hundreds of cells. Over 45 days, they tracked 515–1040 pyramidal cells in individual mice as they explored a familiar track. The data showed that up to 740 cells exhibited calcium excitation in single fields of view, with calcium dynamics generally displaying quiescent periods interrupted by prominent transients. Place fields were defined by statistically significant mutual information between a cell's calcium excitation events and the mouse's location. Approximately 20% of cells had place fields for left, right, or both running directions. The set of place fields fully covered the track, with the ends represented more densely than the interior. The mean place field size was about 27% of the 84-cm track, consistent with published data for mice. Across days 5–35, the percentages of cells with place fields for right or left motion did not vary significantly. The distributions of place fields' locations or sizes also remained consistent. No discernible changes were observed in cells' morphologies or mean calcium transient amplitudes or baseline fluorescence. The study found that while the overlap between different days' coding ensembles was about 15–25%, this sufficed to retain a stable spatial representation. Bayesian decoding techniques confirmed that a decoder trained on data from 30 days prior could accurately reconstruct the mouse's trajectory. The study also showed that place fields' invariant locations, combined with the slowly declining overlap in place-coding ensembles, led to spatial representations that retained a clear resemblance while decaying over time. The findings suggest that CA1 coding has day-to-day dynamism at the cellular level while preserving spatial information in the 15–25% overlap between coding ensembles from any two days. This supports prior reports of individually stable place fields but shows that CA1 coding is dynamic, with each episode in a familiar arena having a unique signature via the 75–85% of cells that do not overlap when comparing coding ensembles from any two sessions.A study published in *Nature Neuroscience* (2013) investigated the long-term dynamics of CA1 hippocampal place codes in freely behaving mice. Using calcium imaging, researchers tracked thousands of pyramidal cells over weeks, revealing that while the ensemble representation of an environment changed daily, a subset of cells retained stable place fields, sufficient to maintain accurate spatial representations over weeks. This suggests that CA1 place cells, crucial for spatial memory, do not necessarily retain stable place fields over long periods, but a dynamic aspect of place coding allows for distinct memory traces of events in the same environment. The study combined a viral vector to express GCaMP3 in pyramidal cells, a chronic mouse preparation for time-lapse imaging, and a miniaturized microscope for calcium imaging in hundreds of cells. Over 45 days, they tracked 515–1040 pyramidal cells in individual mice as they explored a familiar track. The data showed that up to 740 cells exhibited calcium excitation in single fields of view, with calcium dynamics generally displaying quiescent periods interrupted by prominent transients. Place fields were defined by statistically significant mutual information between a cell's calcium excitation events and the mouse's location. Approximately 20% of cells had place fields for left, right, or both running directions. The set of place fields fully covered the track, with the ends represented more densely than the interior. The mean place field size was about 27% of the 84-cm track, consistent with published data for mice. Across days 5–35, the percentages of cells with place fields for right or left motion did not vary significantly. The distributions of place fields' locations or sizes also remained consistent. No discernible changes were observed in cells' morphologies or mean calcium transient amplitudes or baseline fluorescence. The study found that while the overlap between different days' coding ensembles was about 15–25%, this sufficed to retain a stable spatial representation. Bayesian decoding techniques confirmed that a decoder trained on data from 30 days prior could accurately reconstruct the mouse's trajectory. The study also showed that place fields' invariant locations, combined with the slowly declining overlap in place-coding ensembles, led to spatial representations that retained a clear resemblance while decaying over time. The findings suggest that CA1 coding has day-to-day dynamism at the cellular level while preserving spatial information in the 15–25% overlap between coding ensembles from any two days. This supports prior reports of individually stable place fields but shows that CA1 coding is dynamic, with each episode in a familiar arena having a unique signature via the 75–85% of cells that do not overlap when comparing coding ensembles from any two sessions.