Objects and landmarks: hippocampal place cells respond differently to manipulations of visual cues depending on size, perspective, and experience.

Human navigation studies show that landmarks are used for orientation, whereas objects contribute to the contextual representation of an environment. What constitutes a landmark? Classic rodent studies show that hippocampal place fields are controlled by distal, polarizing cues. Place fields, however, are also influenced by local cues. One difficulty in examining mechanisms by which distal and local cues influence the activity of hippocampal cells is that distant cues are necessarily processed visually, whereas local cues are generally multimodal. Here, we compared the effects of 90° rotations under different cue conditions in which cues were restricted to the visual modality. Three two-dimensional visual cue conditions were presented in a square open field: a large vertical cue on one wall, a large floor cue in a corner abutting two walls, and a smaller complex floor cue in a corner adjacent to two walls. We show that rotations of large distal cues, whether on the wall or floor, were equally effective in controlling place fields. Rotations of the smaller floor cues were significantly more likely to result in remapping, whether or not animals were also exposed to the distal polarizing cues. Responses of distal and local cues were affected differently by extended experience. Our data provide evidence that hippocampal place cell responses to visual cues are influenced by perspective, salience of the cue, and prior experience. The hippocampus processes visual cues either as stable landmarks useful for orientation and navigation or as nonstationary objects or features of the local environment available for associative learning or binding items in context.