Have you ever thought back to a recent day and seen all the events as a continuous film, while your vacation from a year ago appears as distinct flashes? A recent study explains this phenomenon through a surprising brain mechanism that literally "welds" our temporally close memories together.
Illustration image Pixabay
Researchers have discovered that our brain uses radically different strategies to store recent and old experiences. This work, published in Nature Neuroscience, reveals how neurons create physical links between new memories but not between older ones.
The brain's glue: how recent memories bind
When we experience multiple events in a short time frame, our neurons activate a remarkable process of "synaptic welding." Dendrites—the extensions of neurons that receive information—develop small protrusions called dendritic spines. These have the unique ability to cluster tightly together, forming privileged zones of connection between temporally close memories.
This clustering phenomenon only occurs within a precise time window, typically a few hours to a few days depending on the type of memory. Experiments on mice showed that two events separated by less than 5 hours systematically activate the same dendritic clusters, while a 24-hour gap already leads to the formation of new, distinct groups. This timing explains why we naturally link events from the same day.
The hippocampus acts as the conductor of this process. Not only does it identify which memories to associate, but it also sends specific chemical signals to promote cluster formation. Imaging studies reveal that the hippocampus is particularly active when we experience similar events a few hours apart, as if our brain anticipates the need to create links between them.
This mechanism perfectly illustrates neural plasticity—the brain's ability to constantly reconfigure itself. Each new cluster formed slightly alters the architecture of our neural networks. Remarkably, the more emotionally charged or repetitive the experiences, the more stable and long-lasting these synaptic clusters become, explaining why certain moments in our lives remain so vividly connected in our memory.
Why old memories stay isolated
Over time, the brain gradually "unsticks" these dendritic clusters. The spines don't disappear but lose their privileged connections. Each old memory then becomes a separate island in your mental landscape. This difference is explained by evolution: linking recent events helps us make quick decisions ("this path has smelled bad since yesterday, let's avoid it"), while older, more stable memories serve as general references ("this park is pleasant in summer").
Thus, neuroscientists have found that the older a memory is, the more it relies on specialized circuits in the neocortex. Unlike the dynamic interactions of recent memories in the hippocampus, these old memory traces consolidate into more stable but less interconnected representations. This is why recalling an event from several years ago activates more localized brain patterns that are less likely to associate with other memories.
This gradual separation of old memories serves an adaptive purpose: it allows us to clearly distinguish distant events while maintaining flexibility for recent experiences. Studies show that patients with deficient mechanisms (as in some cases of epilepsy) often suffer from temporal confusion, mixing events separated by years. A healthy brain naturally creates this temporal hierarchy to optimize our access to information.
Article author: Cédric DEPOND