The role of the hippocampus in memory for the temporal order of a sequence of odors

Memory for the temporal order of a sequence of odors was assessed in rats. A sequence of 5 odors mixed in sand was presented in digging cups, 1 at a time, to each rat in a sequence that varied on each trial. A reward was buried in each cup. After the 5th odor, 2 of the previous 5 odors were presented simultaneously; to receive a reward, the rat had to choose the odor that occurred earliest in the sequence. Temporal separations of 1, 2, or 3 represented the number of odors that occurred between the 2 odors in the sequence. Once a preoperative criterion was reached, each rat received a hippocampal (HIP) or cortical control lesion and was retested on the task. On postoperative trials, the HIP group was impaired relative to controls. However, the HIP group could discriminate between the odors. The data suggest that the hippocampus is involved in separating sensory events in time so that 1 event can be remembered separately from another event.