Study of CA1 place cell activity and exploratory behavior following spatial and nonspatial changes in the environment.

Changes in the spatial arrangement or identity of objects inside a familiar environment induce reexploration. The present study looks at modifications of place cell activity during such renewed exploration. Hungry rats foraged for food in a cylinder with a salient cue card attached to the wall and with two distinct objects at fixed positions on the floor relative to each other and to the cue card. Once a set of CA1 place cells was recorded in this standard configuration, additional sessions were done after two kinds of manipulation. In the first, the two objects were rotated as a rigid set 90 degrees counterclockwise around the cylinder center while leaving the cue card in place; this was considered a spatial change. The effects of rotating the objects were different for fields near the objects (near fields) and fields far from the objects (far fields). Object rotation altered most near fields in complex ways, including remapping and cessation of firing. Near fields that remained intact after object rotation underwent unpredictable rotations that frequently departed considerably from the expected value of 90 degrees CCW. In contrast, the only change induced in far fields was a reduction of discharge rate on day 1, but not day 2, exposures of the rat to the rotated objects. The effects on both near and far fields were reversed when the objects were returned to their standard position. In the second manipulation, substitution of one of the two familiar objects with a novel object, a nonspatial change, had no detectable effect on place cell activity, regardless of field location. The sensitivity of hippocampal place cells to spatial changes but not to nonspatial changes is in agreement with earlier results showing that hippocampal lesions abolish reexploration after spatial but not after nonspatial object manipulations. The fact that reexploration is accompanied by place cell changes after spatial but not nonspatial changes reinforces the role that the hippocampus is believed to play in navigational computing and is perfectly compatible with the idea that another brain structure, likely perirhinal cortex, is responsible for object recognition.