BINDING OF HIPPOCAMPAL CA1 NEURAL ACTIVITY TO MULTIPLE REFERENCE FRAMES IN A LANDMARK-BASED NAVIGATION TASK

The behavioral correlates of rat hippocampal CA1 cells were examined i n a spatial navigation task in which two cylindrical landmarks predict ed the location of food. The landmarks were maintained at a constant d istance from each other but were moved from trial to trial within a la rge arena surrounded by static background cues. On each trial, the rat s were released from a box to which they returned for additional food after locating the goal. The box also was located variably from trial to trial and was moved to a new location while the animals were search ing for the goal site. The discharge characteristics of multiple, simu ltaneously recorded cells were examined with respect to the landmarks, the static background cues, and the box in which each trial started a nd ended. Three clear categories of cells were observed: (1) cells wit h location-specific firing (place cells); (2) goal/landmark-related ce lls that fired in the vicinity of the goal or landmarks, regardless of their location in the arena; and (3) box-related cells that fired eit her when the rat was in the box or as it was leaving or entering the b ox, regardless of its location in the arena. Disjunctive cells with se parate firing fields in more than one reference frame also were observ ed. These results suggest that in this task a subpopulation of hippoca mpal cells encodes location in the fixed spatial frame, whereas other subpopulations encode location with respect to different reference fra mes associated with the task-relevant, mobile objects.