Complete or partial hippocampal damage produces equivalent retrograde amnesia for remote contextual fear memories

We examined the effects of partial or complete damage to the hippocampus on long-term retention of a Pavlovian conditioned fear response to context. Rats received a single contextual fear-conditioning episode and 1 week, 3 months or 6 months later they received sham, partial (dorsal) or complete NMDA-induced damage of the hippocampus. During a retention test conducted 2 weeks after surgery, the control rats exhibited high levels of freezing in the context, although their level of freezing was significantly lower with longer retention intervals. Rats with complete hippocampal damage displayed very little freezing in the context at each learning-surgery interval. Partial hippocampal damage caused a smaller but consistent deficit in conditioned responding, especially at longer learning-surgery intervals. Neither group of hippocampus-damaged rats showed less retrograde amnesia for more remote memories. We found that the severity of retrograde amnesia for contextual fear conditioning following hippocampal damage is related to the extent of the damage and that there is consistent and severe retrograde amnesia for even remote contextual fear memories. These findings support the idea that the hippocampal formation has an essential and long-lasting, possibly permanent, role in memory for contexts.