Absence of systems consolidation of fear memories after dorsal, ventral, or complete hippocampal damage

Abstract

We examined the effects of dorsal, ventral, or complete damage to the hippocampus on long-term retention of a Pavlovian conditioned fear response to a tone and a context paired with foot shock. Rats received a fear conditioning episode, in which a tone and context or context-alone were paired with foot shock. Two days or 12 weeks later, they received sham, dorsal, ventral, or complete NMDA-induced damage of the hippocampus. During a retention test conducted 2 weeks after surgery, the sham control rats exhibited high levels of freezing in the context and in the presence of the tone. Rats with dorsal, ventral, or complete hippocampal damage displayed very little freezing in the context at either learning-surgery intervals. Partial hippocampal damage tended to cause a smaller but consistent deficit in conditioned responding to context at the shorter (2 day) learning-surgery interval. Rats with hippocampal damage did not display less severe retrograde amnesia for more remote (12 weeks) memories. A similar pattern of results was observed for freezing to the tone. We find that the severity of retrograde amnesia for fear conditioning is related to the extent of the damage and that there is consistent and severe retrograde amnesia for remote contextual and cued fear memories. These findings support the idea that the hippocampal formation plays an essential and possibly permanent role in fear memories.