7/14/2023 0 Comments Micro grids map![]() ![]() The new recordings showed, within animals, that the gradient in grid scale (grid spacing) along the dorsoventral axis of MEC progressed in clear steps rather than as a continuum. By combining novel and established experimental approaches, we were able to record an unprecedented number of grid cells-up to 186 cells from the same animal-which finally allowed us to determine that the grid map is a conglomerate of sub-maps or modules (Stensola et al. 2007), but it was unknown from the first reports if the entire grid map functioned as a coordinated whole or if it was fractioned into sub-units that displayed a capacity for independent function. Locally, grid cells behave as a coherent ensemble (Fyhn et al. The next section will address the possibility of a modular functional organization of the grid-cell circuit.ĭiscretization of the Entorhinal Grid Map A major objective, based on this possibility, has therefore been to determine if grid cells within the same grid circuit perform separate operations on the same inputs. The existence of multiple orientation configurations across multiple levels of grid scale highlights a basic question: is the grid map composed of smaller sub-maps or does it act as one coherent representation of space, but with variable geometric features such as spacing and orientation? A grid map with independently functioning sub-maps may produce unique population-pattern combinations for every environment, resulting in unique input patterns to place cells and, in turn, unique hippocampal output (Fyhn et al. Later work has shown that multiple orientation configurations may be present in the same animal (Krupic et al. 2007), co-localized cells always had a similar grid orientation (orientation of grid axes), suggesting there was only one shared orientation in the entire circuit. In the first reports of grid cells (Hafting et al. It was necessary to record with minimal discontinuity in the tissue so that steps in spacing could be discerned reliably from discontinuities in sampling of a smooth topography. To answer this question, it was essential to record large numbers of grid cells over considerable dorsoventral distances within animals, so as to sample a sufficient range of grid spacing. 2007), it remained unclear after the first studies whether grid scale distributed within animals as a scale-continuum or instead progressed in steps. Despite initial reports based on low cell numbers (Barry et al. Grid spacing is organized topographically along the dorsoventral axis of MEC, with average grid spacing increasing from dorsal to ventral (Fyhn et al. Models that describe possible grid-to-place transforms are dependent on how the grid map is organized at several functional levels. Shown are grid patterns of four distinct scales recorded within the same animal. Action potentials ( black) superimposed on the movement path ( gray) reveal a periodic spatial activity pattern. Grid cell firing patterns bird’s eye view. 2013) soon pointed to grid cells as prime candidates in conferring spatial selectivity to place cells in downstream hippocampus. 2007) and multiple lines of experimental evidence (Brun et al. ![]() Several computational models (O’Keefe and Burgess 2005 Fuhs and Touretzky 2006 McNaughton et al. At more ventral MEC locations, with increasing distance from the dorsal MEC border, the scale of the grid pattern expanded (Fyhn et al. Dorsally in MEC, grid patterns typically had small fields packed densely together. Each grid cell had a slightly different set of x, y-coordinates in the environment, so that the entire environment could be covered collectively by a small number of grid cells. It turned out that the firing fields of the spatial cells in MEC formed a near-perfect hexagonal grid tessellating the entire space available to the animal (Hafting et al. 2005), although MEC neurons typically had several firing fields in environments where place cells had only a single field. In pursuing this possibility, we observed that neurons in MEC were also spatially selective (Fyhn et al. This observation suggested that place responses in CA1 originated from an alternative source of excitatory input to CA1: the medial entorhinal cortex (MEC). An important clue was the experimental demonstration that place cells in CA1 could sustain place characteristics after ablation of all input from CA3 (Brun et al. In trying to understand which incoming signals could take part in generating location-specific responses in place cells, both experimental and theoretical suggestions have been presented. t indicates tetrode number, c cell number. Bird’s eye view of firing locations of three place cells, with firing locations shown as red dots on the path of the rat ( black). Place cells recorded in hippocampal subarea CA3. ![]()
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