A role for terrain slope in orienting hippocampal place fields

The three-dimensional topography of the environment is a potentially important source of orienting information for animals, but little is known about how such features affect either navigational behaviour or the neural representation of place. One component of the neural place representation comprises the hippocampal place cells, which show location-specific firing that can be oriented by directional cues in the environment. The present study investigated whether a simple topographical feature, terrain slope, could provide such orienting information to place cells. Place cells were recorded as rats explored a tilted (30 degrees) square box located in the centre of a dark, curtained and visually symmetrical circular enclosure. The orientation of the tilted surface was varied, first in conjunction with that of a visible cue card (to stabilise the system) and then in the absence of the cue card, when the slope of the box was the only remaining stable polarising cue in the environment. In the latter condition, place fields continued to be reliably oriented by the slope. Thus, terrain slope provides sufficient orienting information to set and probably maintain the orientation of the hippocampal place system. This may explain previous behavioural observations that spatial orientation is improved when slope information is available.