CONTINUOUS 3D CURVED-SURFACE NAVIGATION: AN ESSENTIAL ROLE FOR RSC AND ITS IMPACT ON MEC 3D CODING

Animals live in three-dimensional (3D) environments, yet how the brain maintains stable directional and spatial representations across multiple 3D surfaces remains poorly understood. Here we established a navigation paradigm in which mice freely explore a cylinder consisting of a flat top platform and a continuous curved sidewall, and recorded neuronal activity in the retrosplenial cortex (RSC) and medial entorhinal cortex (MEC) using miniature two-photon imaging. In the RSC, we found that many neurons follow the dual-axis (DA) rule, integrating head rotation and body-axis rotation relative to gravity. These neurons dynamically shift their preferred firing directions (PFDs) along the cylindrical side while maintaining coherent tuning on the top platform, thereby exhibiting global directional tuning. In addition to DA cells, we identified a distinct neuronal population, tangent-plane (TP) cells, that maintains stable directional tuning along the curved surface, is independent of visual input, and is primarily driven by gravitational cues. Next, we found that RSC→MEC projection neurons were enriched in these 3D directional signals. Chemogenetic silencing of RSC markedly disrupted MEC spatial representations on the continuous surface in darkness, particularly along the circumferential axis. Moreover, by increasing cylinder radius, we found that MEC place fields were anchored to consistent rotation-angle positions, whereas RSC inhibition significantly reduced cross-radius field correspondence. Together, our results highlight that RSC is essential for 3D navigation and strongly shapes MEC 3D coding.