The brain needs navigational instruments to get around. We need to remember where we have been previously, what the context cues of the given location were and how to get there. This is all a part of our declarative memory system – or memories that we actively and consciously recall. The study of memory was pioneered by Herman Ebbinghaus (1885), who investigated associative memory, the ability to recall the relationship between two unrelated items. When navigating, one must associate where they are located with their memory of the place. The medial temporal lobe region of the brain has been shown to be vital for declarative memory. Cells in two parts of the medial temporal lobe - the hippocampus and the entorhinal cortex have been linked to declarative memory. More specifically, neurons termed ‘place cells’ and ‘grid cells’ in these regions have been found to be responsible for the navigational aspect of declarative memory.
Through a series of behavioral tasks, Tolman (1948) found that animals have cognitive maps, or internal representations of their environment. These representations are not “thin strip maps,” as he called them, but rather “broad strip maps”- maps that are not only tied to the physical environment, but also include context cues and previous memories. Thus, it is generally understood that animals update their location on an internal map based off of their “sighting” and their “dead reckoning.” The latter refers to an ability to navigate without external cues and to update your location based off of processing both your speed and your direction. The grid cells and the place cells in the entorhinal cortex and the hippocampus allow for this navigational functioning. Together, they form a positioning system. The discovery of these cells won Edvard and May-Britt Moser and John O’Keefe the Nobel Prize in Physiology or Medicine in 2014.
In 1971, John O’Keefe found the place cells within the hippocampus. These cells fire selectively at one or a couple of locations within an environment. They respond specifically to the current location of the animal. Different place cells were found to have different firing locations, or place fields, (O’Keefe, 1976). Thus, by looking at which neurons are firing at certain points in time, researchers are able to predict the animal’s location. Numerous studies have observed the neuron firings of animals, such as a rat, while filming the rat with a camera, to show this relationship. As can be seen in figure 2, neurons of a particular color will only fire in a specific location along the maze that the rat is in. Thus, certain neurons labeled in pink will only fire when the rat is in the upper right hand of the maze. The combination of all of the place fields creates an internal cognitive map that accounts for the behavioral data that Tolman found in 1948.
Representations also change with learning and experience. As the rat learns the environment that it’s in and navigates it more and more, the corresponding place cell neurons will begin to fire in specific locations. Damage to the hippocampus leads to an inability to map space.
30 years later, Edvard and May-Britt Moser found grid cells in the entorhinal cortex. This system created the “dead reckoning system,” meaning that as the animal moves, the grid cells update their distance and direction. While the grid cells are like the place cells and also fire when the animal is at specific places, they fire in more fields. The multiple firing fields of the neurons here form a grid-like pattern, hence their name.
While there is still much to be understood about our internal navigation system, one general hypothesis is that the hippocampal place cells are generated by input from the grid cells. The place cells respond to specific navigation and location inputs, while the grid cell systems provides the path integration. By synapsing onto the place cells, grid cells help with learning the environment. They provide the signaling distances and help draw connections between locations, while the hippocampus picks up on the specific environmental cues. If there were to be damage to either of these regions, our internal GPS system would seriously malfunction.
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