The role of grid cell replay in navigation and memory consolidation

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Experimental evidence links the hippocampus and related brain regions with memory for places and events. Investigations in animals provided a neural basis for these functions; neurons with spatially tuned receptive fields. The most widely recognized of which being hippocampal place cells and entorhinal grid cells, both of which appear to signal an animal's self-location. Questions remain. It is unclear how these neurons are engaged when an animal is planning routes to travel in the immediate future. Furthermore, it is known that hippocampal damage produces severe mnemonic deficits but that old memories are often spared, suggesting their content is duplicated in the brain; an effect known as consolidation. Again the role of place and grid cells in this process are unclear. None-the-less, a potential mechanism underlying both navigational planning and consolidation is replay; a phenomenon identified in place cells, during which the cells 'simulate' trajectories through the environment. We showed that grid cells are also recruited during replay, with their activity typically lagging behind place cells. This supports the notion of relay involvement in consolidation, suggesting the grid cells convey hippocampal information to the cortex.. We also proposed a computational model suggesting that grid cell replay may be a mechanism by which routes are calculated; a simple prediction being that replay should occur when animals make navigational choices. However, replay analyses are demanding and require large numbers of neurons to accurately decode the information content. The aims of this project are to use two photon microscopy to record replay in larger populations of grid cells than have previously been accessible, this will enable better quantification of the involvement of grid replay in navigation and consolidation. Secondarily it will enable us to distinguish the engagement of cells from different layers of the entorhinal cortex, which have distinct connectivity patterns with the hippocampus.