Independent coding of connected environments by place cells

Place cells are hippocampal neurons that have a strong location-specific firing activity in the rat's current environment. Collectively, place cells also provide a signature of the rat's environment as their ensemble activity is markedly different when recorded in distinct apparatuses. This phenomenon, referred to as 'remapping', suggests that each environment activates a different hippocampal map. In this study, we sought to determine the independence of such maps. In Experiment 1, we used a cylinder apparatus that was divided into two equal halves by a central barrier with an aperture allowing the rat to freely commute between the two sides. A local change in one side failed to induce field remapping in the changed side, thus precluding any significant conclusion to be drawn. We therefore designed Experiment 2 in which place cells were first recorded while rats explored three distinct high-walled boxes. Most cells had distinctive firing fields in each box. A runway was then added to connect two initially unrelated boxes. This manipulation altered the firing of some cells but the fields in each box were still clearly distinguishable. The final manipulation consisted of changing one box and allowing the rat to commute freely between the changed and unchanged boxes. While the firing fields remapped in the changed box, they were most usually unaltered in the unchanged box. These results suggest that the hippocampus holds a set of independent maps for each box, and that each specific map is activated mainly according to the rat's current sensory environment.