The Role of the Hippocampus in Representations of Emotional Memory

Although the hippocampus has long been implicated in contextual fear learning, the exact function of this brain structure is unclear. It is generally thought to encode a spatial context with which a fear memory can be associated, but how it may accomplish this and whether it plays a role in emotional memory is largely unknown. It is also unclear whether the hippocampus acts as a single unitary structure, or whether the dorsal and ventral poles, which exhibit differential connectivity to other brain regions, function independently. This dissertation examines the involvement of the hippocampus in emotional learning. A contextual fear conditioning paradigm using a predator odor as an ethologically relevant fearful stimulus was developed and lesions and immunohistochemistry were used to examine differential involvement of the dorsal and ventral hippocampus in response to fear learning. Long-term physiological recordings of dorsal place cells were then conducted to determine the effects of fear conditioning and also fear extinction on contextual representations in the hippocampus. Additionally, cells in the ventral hippocampus were assessed for responses to visuospatial manipulations and changing odor cues of varying emotional valence. It was found that the dorsal and ventral hippocampal regions are both independently required for contextual fear conditioning, and neurons in each region are differentially activated in response to fear learning. Furthermore, place cells in the dorsal hippocampus remapped in response to fear conditioning and stabilized those new fields in the long term. Extinction training caused many place cells to remap once again, suggesting that the dorsal hippocampus encodes varying representations of `fearful\u27 and `safe\u27 contexts. Finally, cells in the ventral hippocampus exhibited stronger responses to anxiogenic contextual cues compared to dorsal cells. In conclusion, these data suggest that the hippocampus is involved in emotional learning and that its function may vary along its longitudinal axis