The dentate gyrus controls hippocampal representational drift
This study demonstrates that hippocampal representational drift is not an intrinsic property of neural populations but is actively controlled by upstream dentate gyrus inputs, which concentrate drift around episode transitions to differentiate repeated experiences into distinct episodic memories.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer
The brain is not a static recording device; it is a living, breathing archive that constantly rewrites its own entries. In the hippocampus, a seahorse-shaped region deep inside the brain, specialized neurons called place cells act as the brain's internal GPS. When an animal moves through a familiar space, these cells fire in specific patterns to map the location. For decades, scientists believed these maps were fixed, like a printed road map that never changes. However, recent observations have revealed a surprising truth: even when an animal runs the exact same path in the exact same environment day after day, the specific pattern of neurons firing slowly shifts over time. This phenomenon, known as representational drift, means the brain's internal map of a familiar place is never quite the same twice. The big question has been whether this drifting is a random glitch, a sign of the brain's natural instability, or if it is a deliberate, controlled process serving a specific purpose.
A team of researchers at the University of Chicago has now shown that this drifting is not a mistake, but a feature actively managed by a specific part of the brain's circuitry. They focused on the dentate gyrus, a gateway structure that feeds information into the hippocampus. Think of the dentate gyrus as a gatekeeper that helps the brain tell the difference between two very similar experiences, like walking down the same hallway on Tuesday versus Wednesday. The researchers wanted to know if this gatekeeper was the one pulling the strings behind the drifting maps. To find out, they used a precise technique to temporarily quiet the activity of the dentate gyrus in mice while the animals ran on a virtual track. By turning down the volume on this specific input, they could watch what happened to the maps in the downstream hippocampus without changing the mouse's actual experience or the environment it saw.
The results were clear and decisive. When the dentate gyrus was active, the brain's map of the familiar track changed noticeably from one run to the next, just as seen in previous studies. But when the researchers inhibited the dentate gyrus, the map became remarkably stable. The pattern of neurons firing on the second run looked almost identical to the first run, as if the brain had decided to stop updating the map. This proved that the drifting is not an unavoidable biological quirk of the hippocampus itself; rather, it is actively driven by signals coming from the dentate gyrus. The study also revealed that this control is highly specific. The dentate gyrus only drove this change when the mouse was in a familiar environment. When the mouse entered a completely new, novel environment, the dentate gyrus did not stop the brain from forming a new map, nor did it prevent the map from changing as the mouse learned the new space. This suggests the gatekeeper's job is not to stop all change, but to manage how the brain handles repeated, familiar experiences.
Perhaps the most striking discovery was when this change happens. Scientists had long assumed that the map drifted slowly and steadily throughout an entire session, like a clock hand moving inch by inch. The researchers found this was not the case. Instead, the shift in the map occurred almost entirely at the moment the mouse re-entered the familiar track. The brain seemed to make a sudden, discrete jump to a slightly different version of the map right at the start of the experience, and then held that new version steady for the rest of the run. This rapid shift allowed the brain to treat the second run as a distinct event, separate from the first, even though the physical path was identical. Without this mechanism, the brain might struggle to distinguish between two separate memories of the same place.
By blocking this process, the researchers showed that the brain lost its ability to tell these repeated episodes apart. In the control group, the brain could clearly distinguish between the first run and the second run, treating them as unique memories. In the group where the dentate gyrus was quieted, the brain began to confuse the two runs, seeing them as the same event. This confirms that the drifting is a functional tool for memory. It allows the brain to keep the general layout of a place stable while still creating a unique, separate record for every time we visit it. The dentate gyrus acts as the engine for this differentiation, pushing the brain's internal map just enough to ensure that every experience, even in a familiar world, is recorded as its own distinct chapter. This work transforms our understanding of memory from a static storage system into a dynamic, regulated process where change is essential for clarity.
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