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Neurodegenerative damage reduces firing coherence in a continuous attractor model of grid cells

Grid cells in the dorsolateral band of the medial entorhinal cortex(dMEC) display strikingly regular periodic firing patterns on a lattice of positions in 2-D space. This helps animals to encode relative spatial location without reference to external cues. The dMEC is damaged in the early stages of Alzheimer's Disease, which affects navigation ability of a disease victim, reducing the synaptic density of neurons in the network. Within an established 2-dimensional continuous attractor neural network model of grid cell activity, we introduce damage parameterized by radius and by the strength of the synaptic output for neurons in the damaged region. The proportionality of the grid field flow on the dMEX to the velocity of the model organism is maintained, but when we examine the coherence of the grid cell firing field in the form of the Fourier transform (Bragg peaks) of the grid lattice, we find that a wide range of damage radius and strength induces an incoherent structure with only a single central peak, adjacent to narrow bands of striped (two additional peaks), which abut an orthorhombic pattern (four additional peaks), that abuts the undamaged hexagonal region (six additional peaks). Within the damaged region, grid cells show no Bragg peaks, and outside the damaged region the central Bragg peak strength is largely unaffected. There is a re-entrant region of normal grid firing for very large damage area. We anticipate that the modified grid cell behavior can be observed in non-invasive fMRI imaging of the dMEC.

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