Wednesday, November 21, 2007

Place cells and Grid Cells: Part II

You do not have to dig very deep to find good descriptions of place cells. (It is starting to become a bit more clear to neuroscientists how the brain works together as an entirety, to conduct "thought". No more dualism, please.)

It's interesting that both place and grid cells are at/in the hippocampus. György Buzsáki discusses hippocampal "theta" rhythm as fundamental to all brain function in his book "Rhythms of the Brain".

Grid cells were discovered more recently and are not quite as famous yet. Blakeslee discusses grid cells next: These map space too, but differently - they do not use external markers to orient you - they let you know where you are in space based on your own movements. These were discovered in 2005 by scientists in Norway.

The discoverers say,
Imagine coming up from an unknown subway station. You immediately look for a landmark to figure out directions and your position. The moment you find it, your cognitive map is calibrated, and things fall into place,” explain Edvard and May-Britt Moser. They reveal the secrets of memory.

Grid cells do this calibration. Blakeslee says,
"Located just one step higher in the cortical hierarchy from place cells, in a region called the entorhinal cortex, each grid cell acts as though the surface of your local environment had a triangular grid painted all over it... A grid cell is active when you are at the vertex of any of the triangles in the field in front of you but inactive for locations between the vertices. The grid persists like graph paper spread as far as you can see, or like the Holodeck on Star Trek before scenes are projected onto it. When you move through space, grid cells mark your position independent of context. Place cells "say" I am in the store, I am in my house, I am in a strange plaza. Grid cells keep track of where you are in all contexts, in all kinds of places, as if they were a property of the environment itself and not cells in your brain.... Moser, when asked, is willing to venture a guess that great athletes have highly developed place cells and grid cells. Yes, they need fast reflexes, trained muscles, great eyesight, and developed brain networks to compare different trajectories; but when Ronaldinho looks down a soccer field, he is mapping the entire field in his brain. He has an effortless, innate sense of where he is in space and time, thanks to how well his brain maps that space. Every time he takes a step, an entire new geometry of action is created within his brain. In ten seconds, Ronaldinho will see at least one hundred alternatives and will make choices that draw on cells and grid cells."

I've done some traveling, enough to know that I get lost easily, turned around, hardly know up from down, don't have a clue which way is west without a good map. I've traveled with others who effortlessly (maddeningly) "know" where to find a site. They stand in the middle of a foreign city, gaze around for about 3 seconds and say, "over there" - and take off toward the place we've decided to go to. They are almost always right. My strategy when alone is to use a map, and double check my progress by stopping local people, asking them, to "feel" sure I'm going the right way.

No comments: