Monthly Archives: April 2014

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The Hippocampus Plays a Selective Role in the Retrieval of Detailed Contextual Memories (Aijing Gao)

AUTHORS: Brian J. Wiltgen, Miou Zhou, Ying Cai, J. Balaji, Mikael Guzman Karlsson, Sherveen N. Parivash, Weidong Li, and Alcino J. Silva

ABSTRACT: Background: It is widely believed that the hippocampus plays a temporary role in the retrieval of episodic and contextual memories. Initial research indicated that damage to this structure produced amnesia for newly acquired memories but did not affect those formed in the distant past. A number of recent studies, however, have found that the hippocampus is required for the retrieval of episodic and contextual memories regardless of their age. These findings are currently the subject of intense debate, and a satisfying resolution has yet to be identified.

Results: The current experiments address this issue by demonstrating that detailed memories require the hippocampus, whereas memories that lose precision become independent of this structure. First, we show that the dorsal hippocampus is preferentially activated by the retrieval of detailed contextual fear memories. We then establish that the hippocampus is necessary for the retrieval of detailed memories by using a context-generalization procedure. Mice that exhibit high levels of generalization to a novel environment show no memory loss when the hippocampus is subsequently inactivated. In contrast, mice that discriminate between contexts are significantly impaired by hippocampus inactivation.

Conclusions: Our data suggest that detailed contextual memories require the hippocampus, whereas memories that lose precision can be retrieved without this structure. These findings can account for discrepancies in the literature—memories of our distant past can be either lost or retained after hippocampus damage depending on their quality—and provide a new framework for understanding memory consolidation.

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Synaptic encoding of fear extinction in mPFC-amygdala circuits (Dekel Taliaz)

AUTHORS: Cho JHDeisseroth KBolshakov VY.

ABSTRACT: Retrieval of fear extinction memory is associated with increased firing of neurons in the medial prefrontal cortex (mPFC). It is unknown, however, how extinction learning-induced changes in mPFC activity are relayed to target structures in the amygdala, resulting in diminished fear responses. Here, we show that fear extinction decreases the efficacy of excitatory synaptic transmission in projections from the mPFC to the basolateral nucleus of the amygdala (BLA), whereas inhibitory responses are not altered. In contrast, synaptic strength at direct mPFC inputs to intercalated neurons remains unchanged after extinction. Moreover, priming stimulation of mPFC projections induced heterosynaptic inhibition in auditory cortical inputs to the BLA. These synaptic mechanisms could contribute to the encoding of extinction memory by diminishing the ability of projections from the mPFC to drive BLA activity while retaining the ability of intercalated neurons to inhibit the output nuclei of the amygdala.

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Hippocampal Activity Patterns Carry Information about Objects in Temporal Context (Matt Tran)

AUTHORS: Liang-Tien Hsieh, Matthias J. Gruber, Lucas J. Jenkins,and Charan Ranganath

ABSTRACT: The hippocampus is critical for human episodic memory, but its role remains controversial. One fundamental question concerns whether the hippocampus represents specific objects or assigns context-dependent representations to objects. Here, we used multivoxel pattern similarity analysis of fMRI data during retrieval of learned object sequences to systematically investigate hippocampal coding of object and temporal context information.Hippocampal activity patterns carried information about the temporal positions of objects in learned sequences, but not about objects or temporal positions in random sequences. Hippocampal activity patterns differentiated between overlapping object sequences and between temporally adjacent objects that belonged to distinct sequence contexts. Parahippocampal and perirhinal cortex showed different pattern information profiles consistent with coding of temporal position and object information, respectively. These findings are consistent with models proposing that the hippocampus represents objects within specific temporal contexts, a capability that might explain its critical role in episodic memory.

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Functional Connectivity between Amygdala and Cingulate Cortex for Adaptive Aversive Learning (Chen Yan)

AUTHORS: Oded Klavir, Rotem Genud-Gabai, and Rony Paz

ABSTRACT: The ability to switch flexibly between aversive and neutral behaviors based on predictive cues relies on learning driven by surprise or errors in outcome prediction. Surprise can occur as absolute value of the error (unsigned error) or its direction (signed errors; positive when something unexpected is delivered and negative when something expected is omitted). Signed and unsigned errors coexist in the brain and were associated with different systems, but how they interact and form across large networks remains vague. We recorded simultaneously in the amygdala and dorsal anterior cingulate cortex (dACC) of monkeys performing a reversal aversive-conditioning paradigm and quantified changes in interregional correlations when contingencies shift. We report that errors exist in different magnitudes and that they differentially develop at millisecond resolution. Our results support a model where unsigned errors first develop in the amygdala during successful learning and then propagate into the dACC, where signed errors develop and are distributed back to the amygdala.