Engrams and circuits crucial for systems consolidation of a memory

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Optogenetic stimulation of a hippocampal engram activates fear memory recall

Xu Liu, Steve Ramirez, Petti T. Pang … (2012)

A specific memory is thought to be encoded by a sparse population of neurons 1,2 . These neurons can be tagged during learning for subsequent identification 3 and manipulation 4,5,6 . Moreover, their ablation or inactivation results in reduced memory expression, suggesting their necessity in mnemonic processes. However, a critical question of sufficiency remains: can one elicit the behavioral output of a specific memory by directly activating a population of neurons that was active during learning? Here we show that optogenetic reactivation of hippocampal neurons activated during fear conditioning is sufficient to induce freezing behavior. We labeled a population of hippocampal dentate gyrus neurons activated during fear learning with channelrhodopsin-2 (ChR2) 7,8 and later optically reactivated these neurons in a different context. The mice showed increased freezing only upon light stimulation, indicating light-induced fear memory recall. This freezing was not detected in non-fear conditioned mice expressing ChR2 in a similar proportion of cells, nor in fear conditioned mice with cells labeled by EYFP instead of ChR2. Finally, activation of cells labeled in a context not associated with fear did not evoke freezing in mice that were previously fear conditioned in a different context, suggesting that light-induced fear memory recall is context-specific. Together, our findings indicate that activating a sparse but specific ensemble of hippocampal neurons that contribute to a memory engram is sufficient for the recall of that memory. Moreover, our experimental approach offers a general method of mapping cellular populations bearing memory engrams.

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Why there are complementary learning systems in the hippocampus and neocortex: insights from the successes and failures of connectionist models of learning and memory.

James McClelland, Bruce L. McNaughton, Randall C O'Reilly (1995)

Damage to the hippocampal system disrupts recent memory but leaves remote memory intact. The account presented here suggests that memories are first stored via synaptic changes in the hippocampal system, that these changes support reinstatement of recent memories in the neocortex, that neocortical synapses change a little on each reinstatement, and that remote memory is based on accumulated neocortical changes. Models that learn via changes to connections help explain this organization. These models discover the structure in ensembles of items if learning of each item is gradual and interleaved with learning about other items. This suggests that the neocortex learns slowly to discover the structure in ensembles of experiences. The hippocampal system permits rapid learning of new items without disrupting this structure, and reinstatement of new memories interleaves them with others to integrate them into structured neocortical memory systems.

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Interplay of hippocampus and prefrontal cortex in memory.

Alison R Preston, Howard Eichenbaum (2013)

Recent studies on the hippocampus and the prefrontal cortex have considerably advanced our understanding of the distinct roles of these brain areas in the encoding and retrieval of memories, and of how they interact in the prolonged process by which new memories are consolidated into our permanent storehouse of knowledge. These studies have led to a new model of how the hippocampus forms and replays memories and how the prefrontal cortex engages representations of the meaningful contexts in which related memories occur, as well as how these areas interact during memory retrieval. Furthermore, they have provided new insights into how interactions between the hippocampus and prefrontal cortex support the assimilation of new memories into pre-existing networks of knowledge, called schemas, and how schemas are modified in this process as the foundation of memory consolidation. Copyright © 2013 Elsevier Ltd. All rights reserved.

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Title: Science

Abbreviated Title: Science

Publisher: American Association for the Advancement of Science (AAAS)

ISSN (Print): 0036-8075

ISSN (Electronic): 1095-9203

Publication date Created: April 06 2017

Publication date (Print): April 06 2017

Volume: 356

Issue: 6333

Pages: 73-78

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DOI: 10.1126/science.aam6808

PMC ID: 5493329

PubMed ID: 28386011

SO-VID: 8ecaa8d1-a61c-49f1-9eec-63a6dbde12b0

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Engrams and circuits crucial for systems consolidation of a memory

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Most cited references 37

Optogenetic stimulation of a hippocampal engram activates fear memory recall

Why there are complementary learning systems in the hippocampus and neocortex: insights from the successes and failures of connectionist models of learning and memory.

Interplay of hippocampus and prefrontal cortex in memory.

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