EVIDENCE OF SPATIAL PERIODIC FIRING IN THE SUBICULUM OF MICE
The subiculum is a critical node of the hippocampal formation, integrating multiple circuits--including thalamic inputs and afferents from CA1 and medial entorhinal cortex--and projecting broadly to cortical and subcortical targets. Yet its contribution to spatial coding remains incompletely understood. We recorded single units in freely moving mice using two complementary approaches: (i) chronic tetrodes targeting CA1 and dorsal SUB, and (ii) 64-channel linear silicon probes targeting dorsal SUB. In addition to place cells, boundary-vector cells (BVC) and corner cells (CC), we identified a subset of SUB neurons that exhibited spatially periodic (grid-like) firing. This phenomenon was replicated across recording technologies indicating that periodic coding is a consistent feature of mouse subiculum. Compared with CA1 place cells, SUB spatial neurons showed lower spatial information and reduced within-session stability, suggesting distinct coding regimes across these hippocampal subregions. Sampling along the proximodistal axis with probe arrays further revealed that burst propensity correlated positively with spatial information at more distal recording sites, consistent with known physiological gradients in subiculum and echoing relationships seen in CA1. Together, these results expand the repertoire of identified spatial codes in SUB and support a view in which subiculum contributes to geometry- and periodicity-based representations that complement CA1 and entorhinal spatial representation, thus, shaping downstream computations in cortico-subcortical circuits. Significance StatementSpatial information and memory emerge from interactions among hippocampal and entorhinal circuits with diverse, spatially tuned neurons. Here we provide the first evidence in mice that pyramidal neurons in the subiculum exhibit grid-like, spatially periodic firing, replicated across tetrodes and high-density probes. These findings suggest that the subiculum contributes to computations beyond simple relay/integration of CA1 inputs, adding a periodic component to subicular spatial coding that may shape downstream cortico-subcortical circuits.