As I understand grid cells, the thing that makes them “griddy” is that there is a global (in the sense of a single region or map in the entorhinal cortex) pattern that extends beyond the reach of the processes of any cell in any single mini-column. There is some overall coordinating mechanism that distributes whatever input is being represented (motion, location, …) so that small local clusters of cells respond in a phased and spatially repeating pattern.
This focus on the possible local phase mechanism seems to be ignoring the larger coordination that seems to be an important part of the grid cell story.
This cluster of questions are all looking at the same underlying question from different directions:
Q: Put plainly, why is that group of cells “way over there” responding in lockstep to what is happening locally? “Way over there” is so far away that no dendrite from any cell in this mini or macro-column is able to receive any direct input from that remote area.
Q: Though most of the hierarchy topology is maintained - any given part of a brain region/map projects to roughly the same part of the topologically related area in the next map. How do we get from a local part of a region/map to some region/map-wide global representation pattern that we are calling a grid? I am not aware of any “dispersion” mechanism in the hierarchy.
Q: When a critter enters a room and some grid pattern is established, what is the input to that grid, and is that part of the coordinating mechanism?
Q: If you are talking about the location signal in a single macro-column is it really correct to label that as having anything to do with grid cells if that area of the cortex does not display the signature grid pattern?
IIRC someone determined via lesions studies that the Entorihnal grid cells are primarily driven by Hippocampal place cells. When they cut the axons from the place cells to the grid cells, the grid cells stopped firing entirely.
I did not think they did that. When several grid cells respond to the same area, I did not think that those grid cells were always near each other. I assumed that the responding cells would be distributed throughout an area, not clustered together.
It may be that the individual grid cells are only responding to locally available information, but that information is likely strongly correlated with information local to other cells “way over there” due to the fundamental spatial and temporal structure of the sensed environment and the spatially correlated layout of the cells in the cortex. My current working theory for grid cells is that the patterns of activation from the input are parsed in such a way that features are filtered in both space and time.
Specific combinations of features are lighting up something like place cell SDRs. These place cell representations are encoding the presence of specific features and, through apical connections, their locations and/or orientations on the sensor or relative to other features on the sensor.
Features that are changing rapidly in time** would then activate cells that are part of modules with smaller length scales. This corresponds to grids that are responsive to objects that are physically closer to the subject. Similarly, features that are further away are changing more slowly** on the sensor would activate cells in grid modules with larger length scales.
While I’ve not worked out the specific dynamics yet, my intuition tells me that something like this is what gives rise to the grid cell like behavior.
** There may also be an additional mechanism that is able to distinguish between sensor updates due to self-movement and those due to independent external phenomena. I’m still working on that one too.
Hello! We started answering HTM forum questions at the beginning of our weekly research meetings. You can find Marcus and Subutai’s response here: https://youtu.be/UbxcyLqLfc0?t=197 (start at 3:17).