It has been known for some time now that the part of the brain responsible for long-term memory of significant events (episodic memory) in humans is the hippocampus. But we do not yet fully understand the process — why, and how, are some memories rejected, others retained?
The hippocampus is an interesting area. In the 1970s, it was shown it is also responsible for the ability to recognise a location in space. When rats, implanted with electrodes, were allowed to move around, it was observed that individual neurons in this area would get activated at specific locations. Each “place cell”, as these neurons are known, would fire at a unique location. This is similar to GPS mapping.
So,we know that hippocampus is responsible for episodic memory and we know that it is sensitive to spatial coordinates as well. Could there be a link?
The most commonly accepted and prevalent idea right now is a cognitive map. This stores information about what happened in the context of where it happened. For example, if a lizard falls on our head while being seated at our study table, that experience gets associated with the location. What happens in the brain is that the neurons that represent that particular location in space, our study table, get modified in their activity, so that every time they fire, when we come to that location, we are reminded of our experience.
Hippocampus thus needs to put together information about what happened, and where it happened. Entorhinal cortex acts as a gateway for information flowing into the hippocampus. There are two sub-regions in the cortex: the lateral and medial entorhinal cortex (LEC and MEC for short). While LEC gets stronger sensory inputs, MEC gets stronger inputs from regions involved in representing movement through space.
Our work at the Centre for Neuroscience is starting to show how information from these two regions helps the hippocampus in forming a cognitive map. MEC uses path integration — a process of keeping track of how fast you are moving and where you are heading. If you make small mistakes in estimating your speed or direction, over time, where you think you are will be farther and farther away from where you actually are.
This is where LEC comes in. It can tell you you’ve already reached your favourite panipuriwala when you thought you had to walk a little further to get to him. And remember that lizard? It was probably LEC that told the hippocampus to remind you to look up before sitting on that study table.