How do we form memories? Researchers have always believed that the hippocampus is the main part of the brain responsible for making memories, but a new study shows that another brain region plays a critical role.
The human brain has the fascinating ability to store memories like we do books on a bookshelf. Most of the time we do not think about them, but whenever we want to access one, all we have to do is take them off the shelf.
Similarly, our brains keep records of places, events, and experiences in a memory bank, ready to access whenever we want - sometimes many years after the event took place.
But how is this actually made possible? Scientists have known for a while that the hippocampus is critical in reactivating spatial and episodic memories, while other brain regions were only thought to play a subordinate role.
However, new research from the Institute of Science and Technology (IST) in Austria suggests that there may be another part of the brain that has a crucial role in recalling memories.
The study examined the memory system in rodents, and the findings were published in Science, the journal of the American Association for the Advancement of Science.
How do we form memories?
When we experience an event, our brains form an episodic memory. An episodic memory is unique to each individual, and the physical location we were in at the time of the event plays an important role in forming it.
The brain's hippocampus is studded with neurons called place cells, and each place cell corresponds to a specific point in the surrounding physical environment.
"Reporting" to the hippocampus is also a region called the medial entorhinal cortex (MEC), which sends input to the hippocampus and contains so-called grid cells. These neurons also respond to specific locations in the surrounding physical space, but these locations are arranged in a triangular grid pattern.
We most likely consolidate our memories during sleep and when we take breaks from an activity. This, at least, is the case in animals, which have been observed to generate events in the hippocampus at a much more accelerated rate when they sleep or pause during a task.
These events are "replayed" in our brain by reactivating the same place cells we activate when having the experience for the first time. This occurs as a result of a highly synchronized neural firing, a brain activity known as "sharp wave-ripple events."
Despite the fact that the MEC also has cells that help with spatial location, the role of this part of the brain in memory formation has, until now, been underplayed. Researchers believed that in memory consolidation, the hippocampus starts the replay, while the MEC simply helps to spread the message to the rest of the brain.
Entorhinal cortex works independently of hippocampus
In this latest study, researchers led by IST's Prof. Jozsef Csicsvari examined the brain's activity both in the hippocampus and the superficial layers of the MEC (sMEC).
Prof. Csicsvari and team recorded the neural activity of rats while they were attempting to find their way out of a maze.
The scientists discovered that apart from the hippocampus, the sMEC was also firing neurons during sleep and waking states.
After decoding the spatial trajectory represented by the neural firings, researchers found them to match the actual trajectories of the maze.
Surprisingly, the sMEC neural firing sequences were observed to occur independently of the hippocampus. No replay firing was detected in the hippocampus at the time that sMEC was activated.
As Prof. Csicsvari explains, these results change our understanding of memory formation:
"Until now, the entorhinal cortex has been considered subservient to the hippocampus in both memory formation and recall. But we show that the medial entorhinal cortex can replay the firing pattern associated with moving in a maze independent of the hippocampus. The entorhinal cortex could be a new system for memory formation that works in parallel to the hippocampus."
"The hippocampus alone does not dominate how memories are formed and recalled. Despite being interrelated, the two regions may recruit different pathways and play different roles in memory," adds Joseph O'Neill, first author of the study.