Mechanisms underlying delay fear conditioning in which conditioned stimuli (CS) are paired and co-terminated with unconditioned stimuli (US), have been extensively characterized, thus expanding knowledge concerning learning and memory. However, trace fear conditioning in which CS and US are separated by trace interval periods, has received much less attention though it involves cognitive processes including timing and working memories. Various brain regions including the hippocampus are known to play an important role in memory acquisition and/or retrieval of trace fear conditioning. However, neural correlates, which are specific for the discrete steps in trace fear conditioning, have not been characterized thoroughly. Here, we investigated the network activities between the dorsal and ventral hippocampi at different stages of memory processing after trace fear conditioning. When fear memory was retrieved successfully, theta synchronization between the two regions was enhanced relative to preconditioning levels. The enhancement in theta synchronization was observed only during the trace interval period but not during CS presentation or after the trace interval period. Thus, the enhanced theta synchronization between the dorsal and ventral hippocampi may underlie a cognitive process associated with the trace interval period when fear memory is retrieved successfully.
