Five decades ago, scientists uncovered a fascinating group of cells in the brain’s hippocampus that are responsible for storing memories of specific locations. These “place cells” are not just limited to spatial memory; they also play a critical role in recording life’s moments, known as episodic memories. While the process of how these cells encode spatial information has been well-documented, the mystery of how they store episodic memories has remained unsolved—until now.
Researchers at MIT have recently introduced a groundbreaking model that sheds light on how place cells can be repurposed to form episodic memories, even when the memories themselves lack a spatial element. The model proposes that place cells, alongside grid cells in the entorhinal cortex, act as a scaffold. This framework helps anchor memories, linking them together in a coherent sequence.
“This model is a first-draft model of the entorhinal-hippocampal episodic memory circuit. It’s a foundation to build on to understand the nature of episodic memory. That’s the thing I’m really excited about,” explains Ila Fiete,a professor of brain and cognitive sciences at MIT and the senior author of the study.
The Framework of Memory
Place cells and grid cells work in tandem to encode spatial information. Grid cells, a unique type of neuron, fire in multiple locations arranged in a geometric pattern of repeating triangles. Together, they form a lattice that represents physical space. But their role extends beyond navigation and spatial recall. These circuits are also essential for forming episodic memories,which frequently enough include spatial elements but primarily focus on events,like your last birthday party or yesterday’s lunch.
“The same hippocampal and entorhinal circuits are used not just for spatial memory, but also for general episodic memory,” Fiete notes. “The question you can ask is what is the connection between spatial and episodic memory that makes them live in the same circuit?”
Two theories have attempted to explain this overlap. One suggests that the circuit evolved to store spatial memories because they are crucial for survival, such as remembering where to find food or avoid danger. Under this view,episodic memories are a byproduct of this spatial function.The other theory posits that the circuit is optimized for episodic memory, with spatial details being just one aspect of these recollections.
Fiete and her team proposed a third viewpoint: the unique structure of grid cells and their interaction with the hippocampus are equally vital for both types of memory. To build this model, they drew on computational frameworks developed over the past decade, which simulate how grid cells encode spatial data.
“We reached the point where I felt like we understood on some level the mechanisms of the grid cell circuit, so it felt like the time to try to understand the interactions between the grid cells and the larger circuit that includes the hippocampus,” Fiete says.
In this new model, grid cells and hippocampal cells work together to create a scaffold for storing both spatial and episodic memories. Each activation pattern within the grid forms a “well,” spaced at regular intervals. These wells serve as anchor points for memories, linking them into a cohesive narrative.
Unlocking the Secrets of Memory: How the Brain Encodes and retrieves Experiences
Imagine your brain as a vast libary, where every memory is a book stored in its shelves. But instead of keeping the entire book in one place, the brain uses a clever indexing system. Researchers have discovered that the hippocampus—a small but mighty region of the brain—acts as a “pointer network,” directing where memories are stored in the sensory cortex. This process allows us to recall experiences with remarkable accuracy, even from fragmented cues.
“Conceptually, we can think about the hippocampus as a pointer network,” explains Fiete. “It’s like an index that can be pattern-completed from a partial input,and that index then points toward sensory cortex,where those inputs were experienced in the first place. The scaffold doesn’t contain the content, it only contains this index of abstract scaffold states.”
This mechanism doesn’t just store individual memories—it links them together. Sequential events are connected in a way that ensures they’re recalled in the right order, almost like a mental timeline. such as, remembering the steps of a recipe or the sequence of a story relies on this intricate interplay between the hippocampus and the sensory cortex.
The Pitfalls of Traditional Memory Models
Traditional models of memory, such as Hopfield networks, suggest that every memory is recalled in perfect detail until the brain’s capacity is reached. At that point, no new memories can form, and attempting to add more erases all prior ones.This “memory cliff” theory, however, doesn’t align with how the biological brain actually functions. In reality, the brain gradually forgets older details while continuously adding new ones—a process that’s far more fluid and efficient.
The Science Behind Memory Palaces
One fascinating application of this discovery lies in understanding the technique known as the ”memory palace.” Used by memory champions,this strategy involves assigning pieces of information—like a sequence of cards—to specific locations in a familiar environment,such as a childhood home. When it’s time to recall the information, the individual mentally walks through the space, visualizing each item in its designated spot.
What’s counterintuitive is that adding the extra layer of associating information with locations actually strengthens recall. The MIT team’s computational model has successfully replicated this process, showing that memory palaces leverage the brain’s natural strategy of using spatial scaffolds in the hippocampus. By repurposing long-acquired memories stored in the sensory cortex as scaffolds for new information, the brain can store and retrieve far more items in sequence than previously thought possible.
Future directions in Memory Research
The MIT researchers aren’t stopping here. They’re now exploring how episodic memories—specific experiences tied to a particular time and place—can transform into semantic memories, which are facts disconnected from their original context. For example, knowing that Paris is the capital of France is a semantic memory, even if you can’t recall when or where you learned it. This research could also pave the way for integrating brain-like memory models into modern machine learning systems, opening new frontiers in artificial intelligence.
funding and Support
This groundbreaking work was made possible through funding from the U.S. Office of Naval Research, the National Science Foundation under the Robust Intelligence program, the ARO-MURI award, the Simons Foundation, and the K. Lisa Yang ICoN Center.
Source: Massachusetts Institute of Technology
Journal Reference: Chandra, S., et al. (2025) Episodic and associative memory from spatial scaffolds in the hippocampus. Nature. doi.org/10.1038/s41586-024-08392-y.
