INERTIAL, SUBSTRATAL AND LANDMARK CUE CONTROL OF HIPPOCAMPAL CA1 PLACE CELL-ACTIVITY

Hippocampal 'place cells' discharge when a rat occupies a location tha t is fixed in relation to environmental landmarks. A principal goal of this study was to determine whether hippocampal place cell activity c ould be influenced by inertial cues. Water-deprived rats were trained in a square-walled open field in a dark room. The behavioural task req uired alternating visits to water reservoirs in the centre and in the four corners of the arena. The rat and arena were rotated in total dar kness through +/-90, 180 or 270 degrees. The next water reward was the n presented in the corner at the same position relative to the outside room as before the rotation. A cue card was later illuminated in this corner as a visual cue for the extra-arena (room) reference frame. fi fteen out of 97 recorded hippocampal CA1 complex spike cells had spati ally selective discharges in non-central parts of the arena. After are na rotations, the firing fields of three units shifted between corners of the arena to maintain a fixed orientation relative to the room. Th is indicates that the hippocampus updated its representation of the po sition and heading direction of the rat using vestibular-derived input s concerning rotation angle. Other spatially selective discharges were guided to landmark cues (cue card or position of the reward: two unit s) or arena-locked 'substratal' cues (eight units). In six cells, plac e cell activity suddenly ceased or appeared following rotations. These results provide evidence for contributions of inertial as well as sub stratal and landmark information to hippocampal spatial representation s.