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Neurobiol Learn Mem. 2018 Sep;153(Pt A):40-56. doi: 10.1016/j.nlm.2018.03.006. Epub 2018 Mar 10.

Hippocampal representations as a function of time, subregion, and brain state.

Author information

1
Department of Psychology, University of Toronto, Toronto, ON M5S 3G3, Canada. Electronic address: duncan@psych.utoronto.ca.
2
Department of Psychology, University of Toronto, Toronto, ON M5S 3G3, Canada. Electronic address: schlichting@psych.utoronto.ca.

Abstract

How does the hippocampus represent interrelated experiences in memory? We review prominent yet seemingly contradictory theoretical perspectives, which propose that the hippocampus distorts experiential representations to either emphasize their distinctiveness or highlight common elements. These fundamentally different kinds of memory representations may be instantiated in the brain via conjunctive separated codes and adaptively differentiated codes on the one hand, or integrated relational codes on the other. After reviewing empirical support for these different coding schemes within the hippocampus, we outline two organizing principles which may explain the conflicting findings in the literature. First focusing on where the memories are formed and stored, we argue that distinct hippocampal regions represent experiences at multiple levels of abstraction and may transmit them to distinct cortical networks. Then focusing on when memories are formed, we identify several factors that can open and maintain specialized time windows, during which the very same hippocampal network is biased toward one coding scheme over the others. Specifically, we discuss evidence for (1) excitability-mediated integration windows, maintained by persistently elevated CREB levels following encoding of a specific memory, (2) fleeting cholinergically-mediated windows favoring memory separation, and (3) sustained dopaminergically-mediated windows favoring memory integration. By presenting a broad overview of different hippocampal coding schemes across species, we hope to inspire future empirical and modeling research to consider how factors surrounding memory formation shape the representations in which they are stored.

KEYWORDS:

Differentiation; Episodic memory; Hippocampus; Memory integration; Neuromodulation; Pattern separation

PMID:
29535044
DOI:
10.1016/j.nlm.2018.03.006

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