Sharp wave/ripple network oscillations and learning-associated hippocampal maps

Abstract

Sharp wave/ripple (SWR, 150-250 Hz) hippocampal events have long been postulated to be involved in memory consolidation. However, more recent work has investigated SWRs that occur during active waking behaviour: findings that suggest that SWRs may also play a role in cell assembly strengthening or spatial working memory. Do such theories of SWR function apply to animal learning? This review discusses how general theories linking SWRs to memory-related function may explain circuit mechanisms related to rodent spatial learning and to the associated stabilization of new cognitive maps.

Keywords: cognitive map; memory consolidation; place cell; sleep.

Figure 1.

(a) The movement pattern of the animal on the cheeseboard maze. The example shows the animal's movement path during learning trials and in memory probe sessions before and after learning. Animals learned the location of three food rewards during the learning sessions which changed every day. Animals were able to learn the task within a few trials and they frequently crossed the previously learned locations in the probe trials in which food was no longer available. (b) Goal-related remapping of place cells. The examples show the rate maps of place cells during the probe sessions and in blocks of consecutive learning trials. (Adapted from [39].) (Online version in colour.)

Figure 2.

(a) Examples of reactivation maps calculated during sleep SWRs. Reactivation maps quantified the similarity of SWR assembly patterns with the average waking rate patterns expected at different locations. Note that usually one of the reward locations (black dots) is reactivated. (b) Reactivation of goal locations during SWR patterns predicts future memory performance. The number of crossings in probe trials was predicted by the proportions of SWRs in which a goal location was reactivated. (c) Examples of averaged SWRs recorded in learning in drug free and under NMDA blockade (CPP). (d) The averaged in- and out-field SWR responses of pyramidal cells at new goal locations and at the start box (eSWR, exploration-related SWRs). (Adapted from [39].) (Online version in colour.)