Interactions between the orbitofrontal cortex and the hippocampal memory system during the storage of long-term memory.

It has been proposed that long-term declarative memories are ultimately stored through interactions between the hippocampal memory system and the neocortical association areas that initially processed the to-be-stored information. One association neocortex, the orbitofrontal cortex (OFC) is strongly and reciprocally connected with the hippocampal memory system and plays an important role in odor recognition memory in rats. We will report data from two studies: one that examined the firing of neurons in a task dependent on the parahippocampal region (PHR; including the perirhinal, postrhinal, and entrorhinal cortices), and one examined the firing of OFC neurons performing a task that is presumably dependent on the hippocampus. In the first study, we examined the role of OFC neurons in the continuous odor-guided nonmatching to sample task. While the firing of neurons in the PHR and OFC are similar in this task, there are several notable differences that are consistent with the idea that OFC is a high-order association cortex which interacts extensively with the PHR to store declarative memories. In the second study, we characterized the firing patterns of neurons in the OFC rats performing a passive, 8-odor-sequence memory task. Most interesting were neurons that fired selectively in anticipation of specific odors. We found that hippocampal lesions abolished the anticipatory firing in OFC, suggesting that these anticipatory responses (memory) were in fact dependent on the hippocampus, further supporting the view that the OFC interacts with the hippocampal memory system to store long-term, declarative memories.