Geon Memory Consolidation:
Generation of Sharp Wave Ripples in the Hippocampus


Memory consolidation refers to the process that converts short-term memory into very long-term memory (VLTM, > 1 month). It was thought that the memories initially acquired in the hippocampus were gradually transferred to the cerebral cortex for long-term storage. After being consolidated, VLTM becomes hippocampus-independent (Born et al., 2006). However, evidence is accumulating that for autobiographical (episodic) memories, the hippocampus is always required (Martin et al., 2005; Steinvorth et al., 2005; Tononi and Cirelli, 2014; Ocampo et al., 2017).

Winocur and Moscovitch (2011) suggested that the hippocampus might play a role in the index and binding of sparse cortical representations. Consistent with this notion, there is evidence for the dentate gyrus (DG) of the hippocampus to index time of events (Chapter 12). Furthermore, the DG granule cells are a type of spiny neurons, well-suited for memory storage. Experiments have demonstrated that activation of a set of DG neurons is sufficient to retrieve a particular fear memory (Liu et al., 2012), even for amnesic memory that cannot be retrieved by natural reminding cues (Ryan et al., 2015; Roy et al., 2017).


Figure 23-1. The hippocampal circuit. [Adapted from: Llorens-Martín et al., 2014]

Sharp wave ripples (SWRs), which underlie the mechanism of hippocampal replay (Jadhav et al., 2012), play a pivotal role in memory consolidation (Rothschild et al., 2017). While SWRs are generated in the CA3 subregion, DG has been shown to exert a significant influence (Buzsáki, 2015). The hippocampal replay is basically a spontaneous memory retrieving process which could be triggered by the neurotransmitters, acetylcholine and norepinephrine, through G-protein-coupled signaling that ultimately enhances PKA activity to cause dissociation of tubulin/CRMP2 from NMDA receptors (see Chapter 21 and Chapter 22).

SWRs occur not only in the quiet awake state, but also during slow wave sleep, which is characterized by slow oscillations (0.2 - 2 Hz). The neural activity of slow oscillations may effectively trigger SWRs (Sullivan et al., 2011).


Author: Frank Lee
First published: February, 2018