Spatial memory, grid cells and…honeycomb?!
Remember when we talked about memory? One very important aspect you should never forget is that there are many types of memory (stored in different areas of the brain). Today I’d like to discuss a particular and very important type of memory, known as spatial memory. As a matter of fact, I’m going to be even more specific: this article deals with one of the several types of cells involved in spatial memory, which were recently discovered. These were called grid cells (the name is very suggestive, as you’ll see in a few moments).
You may wonder why I chose this topic. Obviously, it is of great interest for many scientists, but to be honest, my own subjectivity played a role in here as well. For those who haven’t met me, you should know this: there is no one in the world less capable of spatial self-orientation than myself! Therefore, when I heard that a young Norwegian couple (the Mosers) won the Noble Prize in 2014 for discovering some neurones specialised in location memory, it came as no surprise that I became extremely curious about their research.
As I mentioned before, grid cells are not the only neurons involved in spatial memory. In the 1970, John O’Keefe discovered navigation in the brain along with the cells that generate this process, known as place cells. These neurons are located in the hippocampus (a structure of the temporal lobe associated with long-term and spatial memory) and appear to code for location, by creating a spatial map of the environment. They are dependent on the motion’s speed and direction.
Grid cells represent a type of place cells, although unlike place cells, they are predominantly found in the medial entorhinal cortex. Some grid cells were also found in the prefrontal cortex, which is involved in forming episodic memory. A quite peculiar characteristic about these neurons is that they fire whenever the subject is at certain locations and these signalling locations form a hexagonal pattern, very similar to a honeycomb (which explains the rather odd image I chose for this article).
This honeycomb-like pattern is not only mathematically precise, but it is also very efficient as it uses a minimum number of grid cells to achieve the highest-possible resolution, thus saving energy. It was also discovered that the firing patters remain constant during the subject’s movement, regardless its speed or direction. Moreover, unlike the response of place cells, which is based on visual stimuli, the grid cells’ firing is maintained even in the absence of sensory stimuli, thus showing an algorithm based on self-motion. In an interview for Deutsche Welle Magazine, Edvard Moser says: “It is thought that these [grid cells] are part of an internal map that is based on our own movement, so that these cells signal the distances we move and the directions we take”.
The discovery raises scientists’ hopes up in treating neurodegenerative diseases linked to memory, such as Alzheimer’s disease, as it has been demonstrated that the first symptoms of dementia appear in the entorhinal cortex. At the same time, for other people, like me, grid cells may offer an explanation as to why they still can’t find their best friend’s house, after having been there several times.
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Image created by Isuru Priyaranga