Differential ripple propagation along the hippocampal longitudinal axis

Abstract

Hippocampal ripples are highly synchronous neural events critical for memory consolidation and retrieval. A minority of strong ripples has been shown to be of particular importance in situations of increased memory demands. The propagation dynamics of strong ripples inside the hippocampal formation are, however, still opaque. We analyzed ripple propagation within the hippocampal formation in a large open access dataset comprising 267 Neuropixel recordings in 49 awake, head-fixed mice. Surprisingly, strong ripples (top 10% in ripple strength) propagate differentially depending on their generation point along the hippocampal longitudinal axis. The septal hippocampal pole is able to generate longer ripples that engage more neurons and elicit spiking activity for an extended time even at considerable distances. Accordingly, a substantial portion of the variance in strong ripple duration (R2= 0.463) is explained by the ripple generation location on the longitudinal axis. Our results are consistent with a possible distinctive role of the hippocampal septal pole in conditions of high memory demand.

Sensory cortical ensembles exhibit differential coupling to ripples in distinct hippocampal subregions

“…We focused on the analysis of hippocampal ripples and associated cortical activity during the spontaneous period. As reported previously (De Filippo & Schmitz, 2022; Nitzan et al, 2022; Patel et al, 2013; Sosa et al, 2020), we observed some ripples propagating along the longitudinal axis of CA1 and others localized to dCA1 or iCA1 (Fig. 1C-E).…”

“…Consistent with these proposals, ripples are coupled to brain-wide activity changes across cortical and subcortical areas (Logothetis et al, 2012; Karimi Abadchi et al, 2020; Liu et al, 2021; Chrobak & Buzsáki, 1996; Siapas & Wilson, 1998; Jadhav et al, 2016; Rothschild et al, 2017; Gomperts et al, 2015; Pennartz et al, 2004). Notably, ripples do not always occur over the entire longitudinal axis of the hippocampus; instead, localized ripples are frequently observed (De Filippo & Schmitz, 2022; Nitzan et al, 2022; Patel et al, 2013; Sosa et al, 2020). These localized ripples have different features such as amplitude, duration, and frequency (Nitzan et al, 2022), and recruit distinct populations of nucleus accumbens neurons (Sosa et al, 2020).…”

Hippocampal sharp wave ripples and coincident cortical ripples orchestrate human semantic networks

“…We speculate that this may be due to different subtypes of cortical ripples. Long-duration ripples are most associated with improved memory consolidation 6,61,62 , but shorter-duration ripples may contain more succinct neuronal firing that communicate smaller packets of information. It is also possible that inter-areal communication via coincident ripples requires heightened similarity in frequency and/or duration.…”

Brain-wide topographic coordination of traveling spiral waves

“…In studying the 2-8Hz frequency range, we have not captured traveling wave dynamics on faster 5,9,45 or slower 20,21 timescales. Whether dynamics in those frequency ranges follow the same principles is an open question, as some evidence suggests that cortical activity in disparate frequency ranges may be uncoupled 46 .…”