The regular and repeated triangular spatial firing pattern of entorhinal grid cells suggests a strong path-integration (idiothetic) input. At the same time, trial-to-trial repeatability and lack of intra-trial drift suggest location-specific sensory drive. Our basic model of hippocampal circuitry involves a reciprocal relation between hippocampal place cells and grid cells: grid cells provide idiothetic input to place cells and place cells provide location-specific sensory input to grid cells.
Our model for the contribution of grid cells to navigation follows from our earlier work on heading-vector navigation between pairs of familiar locations (landmarks). Here we are suggesting that hippocampal place cells identify start-points and endpoints with unique real-world locations, while the grid-cell universal map provides the connecting heading vector. More specifically, start-point place cells point to a firing vector of grid cells (the start-point grid-cell set). Similarly, goal place cells point to a second firing vector of grid cells (the goal grid-cell set). Each grid-cell set includes cells of numerous scales, but, likely, a single orientation. We are interested in three questions: (1) knowing the start-point and goal grid-cell sets, can the heading vector connecting these be computed? (2) How can the computation be performed? (3) Is this computation implemented in the rat brain?
Potential models for heading-vector computation depend heavily on the organizational structure of entorhinal cortex. Specifically, different types of models apply if the DV distribution of grid scale is step-like or continuous. Additionally, one class of models depends on grid cells of a fixed scale having highly constrained scale and “rigid” spatial relations between pairs of cells across all locations and all environments. These models will be described and explored. We have begun recording grid cells in a large rectangular enclosure (1.8m x 1.4m) using a 3D tracking system to eliminate optical and parallax distortion. Preliminary findings from 8 cells recorded from one rat support the discrete-step organizational pattern of grid cell scale: 7 of 8 cells had virtually the same scale, while the 8th was larger by a factor of 1.45. Additionally, all 8 cells exhibited virtually identical grid orientations. These findings suggest that each set of grid cells with identical scale form a rigid network in that movement of any direction and distance from one firing vector of grid cells will predict a unique firing vector.
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