Temporal instability weakens entorhinal grid-like coding in older adults

As we grow older, the ability to remember where we placed our keys or to navigate a familiar neighborhood often becomes less reliable. This decline in spatial memory and navigation is a common part of aging, but it does not happen to everyone in the same way. Some older adults remain as sharp and oriented as they were in their youth, while others struggle significantly. Scientists have long suspected that a specific part of the brain, located deep within the temporal lobe, plays a central role in these abilities. This region contains specialized cells that act like an internal GPS, firing in a repeating, six-sided pattern as we move through space. In the brain, this pattern creates a mental map that helps us know where we are and how to get to where we want to go. When this system works well, we feel grounded; when it falters, we may feel lost.

For years, researchers believed that the decline in these navigation skills was simply because the brain's internal map was fading away or disappearing entirely with age. However, a new study suggests the problem might be more subtle. Instead of the map vanishing, it may be that the map is becoming unstable, drifting or wobbling over time. This instability could make it difficult for the brain to use that map consistently, even if the map itself is still there. By looking closely at how these brain signals behave in older adults, scientists have found a key reason why some people maintain their spatial skills while others do not, pointing to a specific type of reliability in how the brain holds onto its sense of direction.

To investigate this, a team of researchers at the German Center for Neurodegenerative Diseases in Magdeburg studied 85 healthy older adults, with an average age of 75. They asked these participants to perform a task inside an MRI scanner, a machine that takes detailed pictures of brain activity. The participants did not move their bodies; instead, they watched a video of a person walking through a square virtual room. As they moved, they had to remember the locations of five different objects placed around the room. After the walk, they were tested on how well they could recall where those objects were. The researchers repeated this process twice, with the objects placed in different spots each time, to see how the brain responded to the same environment on different occasions.

The scientists were particularly interested in a specific signal in the posterior-medial entorhinal cortex, a small area of the brain known to house the grid-like cells that form our internal map. In a healthy, functioning brain, this area lights up in a predictable, six-sided pattern as a person moves in different directions. The researchers measured how strong this pattern was in each participant. As expected, they found that the strength of this signal varied widely among the older adults. Some showed a clear, strong pattern similar to that of young people, while others showed a very weak or almost non-existent signal. Surprisingly, the researchers found that a person's age alone did not explain these differences. A 90-year-old could have a strong signal, while a 65-year-old could have a weak one, suggesting that something other than simple aging was at play.

The team then asked whether the problem was that the brain's map was simply missing, or if it was present but unstable. To find out, they looked at how consistent the brain's activity was from one run of the task to the next. They measured whether the brain produced the same pattern of activity for the same direction of movement each time. They discovered that the strength of the grid signal was closely tied to this stability. In the left side of the entorhinal cortex, participants whose brains produced a consistent, stable pattern of activity across the two runs also had the strongest grid signals. Those whose brain activity drifted or changed between the runs had weaker signals. This finding suggests that the issue is not necessarily that the brain has lost its ability to create a map, but that it is struggling to keep that map steady and reliable over time.

The researchers also looked at whether this stability mattered for real-world memory. They found that older adults with more stable brain patterns in the left entorhinal cortex were better at remembering where the objects were located in the virtual room. This connection was specific to the stability of the pattern; the sheer strength of the signal itself did not predict memory performance as clearly as the consistency did. This implies that having a reliable, unchanging reference point in the brain is crucial for recalling where things are, especially when we need to retrieve that information later.

To understand why some brains were more stable than others, the team examined the physical structure of the brain tissue in these regions. They looked at the volume of the tissue, the amount of myelin (a fatty coating that helps signals travel), and the integrity of the microscopic structure. They found no clear link between these physical features and the stability of the grid signal. A person with a perfectly healthy-looking brain structure could still have an unstable map, while another with slight structural changes could have a very stable one. This ruled out the idea that simple physical wear and tear was the direct cause of the instability.

The study then traced the source of this instability further back in the brain's network. The entorhinal cortex receives information about direction from other areas, including the retrosplenial cortex, a region near the back of the brain that helps us orient ourselves relative to our surroundings. The researchers measured how stable the orientation signals were in this retrosplenial area. They found that when the retrosplenial cortex provided a steady, reliable sense of direction, the entorhinal cortex was better able to maintain its stable grid pattern. In fact, the stability of the retrosplenial signals appeared to influence the memory performance of the participants indirectly, by helping to keep the entorhinal grid stable. This suggests a chain of events where a reliable sense of direction from the back of the brain helps anchor the internal map in the middle of the brain, preventing it from drifting.

These findings offer a new way of thinking about why spatial memory declines in some older adults but not others. It appears that the brain's internal GPS does not necessarily disappear with age. Instead, the system may become less reliable, with the map drifting or shifting slightly each time we try to use it. This instability seems to stem from the quality of the directional information coming into the system. If the brain cannot hold onto a steady sense of where it is facing, the internal map becomes shaky, making it harder to remember locations or navigate effectively. The study highlights that the key to maintaining spatial skills in later life may not be about preserving the map itself, but about ensuring the brain has a stable reference point to keep that map aligned with the world.