DIRECTIONAL CONTROL OF HIPPOCAMPAL PLACE FIELDS

Pyramidal cells in the rat hippocampus fire whenever the animal is in a particular place, suggesting that the hippocampus maintains a repres entation of the environment. Receptive fields of place cells (place fi elds) are largely determined by the distance of the rat from environme ntal walls. Because these walls are sometimes distinguishable only by their orientation with respect to the outside room, it has been hypoth esised that a polarising directional input enables the cells to locate their fields off-centre in an otherwise symmetrical environment. We t ested this hypothesis by gaining control of the rat's internal directi onal sense, independently of other cues, to see whether manipulating t his sense could, by itself, produce a corresponding alteration in plac e field orientation. Place cells were recorded while rats foraged in a rectangular box, in the absence or presence of external room cues. Wi th room cues masked, slow rotation of the rat and the box together cau sed the fields to rotate accordingly. Rotating the recording box alone by 180 degrees rarely caused corresponding field rotation, while rota ting the rat alone 180 degrees outside the environment and then replac ing it in the recording box almost always resulted in a corresponding rotation of the fields. This shows that place field orientation can be controlled by controlling the internal direction-sense of the rat, an d it opens the door to psychophysical exploration of the sensory basis of the direction sense. When room cues were present, distal visual cu es predominated over internal cues in establishing place field orienta tion.