The Role of Hippocampus in Contextual Fear Conditioning: Beyond Simple Association
Date Issued
2010
Date
2010
Author(s)
Chang, Shih-Dar
Abstract
Configural learning refers to a process integrating elemental features into a unitary representation, which is often conceived to rely on the hippocampus. In past years, contextual fear conditioning in rodents has often been used to study neural mechanisms underlying configural learning. Many researchers conceive that contextual fear conditioning involves two learning processes: binding multiple cues in the context to form a unitary representation relying on the hippocampus, and then associating this representation to a shock relying on the amygdala. In light of evidence that the hippocampus is involved in binding multiple stimuli temporally or spatially related, it is proposed in this dissertation that the hippocampus is engaged not only in coding contextual information but also in integrating context and shock into a configural fear memory. To test this hypothesis, this dissertation adopted a two-phase training paradigm and examined the effects of manipulating the hippocampus at each phase independently. Suppressing the dorsal hippocampus (DH), but not its ventral part, by infusing 4% lidocaine during each training phase impaired conditioned freezing. Intra-DH infusion of DL-2-amino-5-phosphonovaleric acid, a N-methyl-D-aspartic acid antagonist prior to context-shock learning yielded the same results, suggesting that the DH is involved in both context coding as well as context-shock learning. Results further showed that context-shock training forges in the DH a new representation of frightful context representation by integrating environmental and shock cues altogether, which was readily retrieved through direct or mediated pattern completion, a kernel property of configural representation. Finally, intra-DH infusion of the GABAA receptor agonist muscimol immediately after context coding impaired contextual fear conditioning but had no effect if given after context-shock leaning. In contrast, intra-DH infusion of the muscarinic receptor antagonist scopolamine impaired contextual fear conditioning only if given immediately after context-shock learning but not given after context coding. These findings, taking together, suggest that in contextual fear conditioning the DH through different types of neurochemical operation may not only incorporate static environmental cues to form a configural memory for a neutral context but also incorporate shock-generated cues into a neutral context and transform it into a configural fear memory. These findings provide theoretical implications for studying underlying mechanisms of human learning and memory.
Subjects
configural learning
emotional memory
pattern completion
GABAA receptor
muscarinic receptor
conditioned freezing
amygdala
Type
thesis
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