Hippocampal activity represents many behaviorally important variables, including context, an animal’s location within a given environmental context, time, and reward. Using longitudinal calcium imaging in mice, multiple large virtual environments, and differing reward contingencies, we derived a unified probabilistic model of CA1 representations centered on a single feature—the field propensity. Each cell’s propensity governs how many place fields it has per unit space, predicts its reward-related activity, and is preserved across distinct environments and over months. Propensity is broadly distributed—with many low, and some very high, propensity cells—and thus strongly shapes hippocampal representations. This results in a range of spatial codes, from sparse to dense. Propensity varied ∼10-fold between adjacent cells in salt-and-pepper fashion, indicating substantial functional differences within a presumed cell type. Intracellular recordings linked propensity to cell excitability. The stability of each cell’s propensity across conditions suggests this fundamental property has anatomical, transcriptional, and/or developmental origins.
Keywords : hippocampus, place cell, place field, memory, calcium imaging, propensity, gain modulation, sparse coding, intracellular recording, excitability