Abstract
While grid cells in the medial entorhinal cortex (MEC) of rodents have multiple, regularly arranged firing fields, place cells in the cornu ammonis (CA) regions of the hippocampus mostly have single spatial firing fields. Since there are extensive projections from MEC to the CA regions, many models have suggested that a feedforward network can transform grid cell firing into robust place cell firing. However, these models generate place fields that are consistently too small compared to those recorded in experiments. Here, we argue that it is implausible that grid cell activity alone can be transformed into place cells with robust place fields of realistic size in a feedforward network. We propose two solutions to this problem. Firstly, weakly spatially modulated cells, which are abundant throughout EC, provide input to downstream place cells along with grid cells. This simple model reproduces many place cell characteristics as well as results from lesion studies. Secondly, the recurrent connections between place cells in the CA3 network generate robust and realistic place fields. Both mechanisms could work in parallel in the hippocampal formation and this redundancy might account for the robustness of place cell responses to a range of disruptions of the hippocampal circuitry.
Highlights
Place cells in the cornu ammonis (CA) regions of the hippocampus [1] and grid cells in the medial entorhinal cortex (MEC) [2] are important components of the navigation system in mammals [3]
The general problem is to find a weight vector w that divides the set of input population vectors (PVs) into two groups, in-field PVs and out-of-field PVs
We revealed that it is unlikely that robust place fields with realistic sizes are generated in a feedforward network driven by grid cell inputs alone [75], because of its structured spatial autocorrelation
Summary
Place cells in the CA regions of the hippocampus [1] and grid cells in the medial entorhinal cortex (MEC) [2] are important components of the navigation system in mammals [3]. Whereas place cells have just one or a few place fields, grid cells fire spikes in many fields that are arranged on a hexagonal grid. Both cell types are dependent on landmarks and boundaries of the environment. They exhibit stable firing patterns during repeated visits to the same environment [4], are robust to the removal of some environmental cues [2, 5], mostly preserve their firing maps in darkness [6, 7], rotate their spatial firing maps in concert with displaced landmarks [2, 8], rescale the size of the place fields when the environment is expanded [9, 10], and remap their.
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