Temporally Graded Retrograde Amnesia of Contextual Fear after Hippocampal Damage in Rats: Within-Subjects Examination

TLDR

We have shown that electrolytic lesions of the dorsal hippocampus produce a severe deficit in contextual fear when made 1 day after conditioning but not 28 days later, indicating a time‑limited role of the hippocampus in consolidation. Here, we examine retrograde amnesia of contextual fear produced by DH lesions in a within‑subjects design. Rats received 10 tone–shock pairings in one context (remote memory) and 10 in a distinct context 50 days later (recent memory), then received DH or sham lesions one day later. DH lesions spared remote contextual fear but severely impaired recent contextual fear, with no effect on tone freezing, supporting a time‑limited mnemonic role of the hippocampus.

Abstract

We have shown previously that electrolytic lesions of the dorsal hippocampus (DH) produce a severe deficit in contextual fear if made 1 d, but not 28 d, after fear conditioning (Kim and Fanselow, 1992). As such, the hippocampus seems to play a time-limited role in the consolidation of contextual fear conditioning. Here, we examine retrograde amnesia of contextual fear produced by DH lesions in a within-subjects design. Unlike our previous reports, rats had both a remote and recent memory at the time of the lesion. Rats were given 10 tone–shock pairings in one context (remote memory) and 10 tone–shock pairings in a distinct context (with a different tone) 50 d later (recent memory), followed by DH or sham lesions 1 d later. Relative to controls, DH-lesioned rats exhibited no deficit in remote contextual fear, but recent contextual fear memory was severely impaired. They also did not exhibit deficits in tone freezing. This highly specific deficit in recent contextual memory demonstrated in a within-subjects design favors mnemonic over performance accounts of hippocampal involvement in fear. These findings also provide further support for a time-limited role of the hippocampus in memory storage.