Some memories from adolescence don’t just vanish. The brain may simply lock them behind a door.
It does this while rebuilding the very system that holds those experiences. New research in mice shows a surprising wave of structural changes in a brain region critical for long-term memory. During late adolescence, protective structures around key neurons temporarily weaken. This makes recalling earlier life events surprisingly difficult. Those memories often return in adulthood, but with a catch: some have lost their original sharpness.
Published in PLOS Biology by researchers at Albert Einstein College of Medicine results offer a biological reason why access to older memories shifts as we mature. It challenges the old idea that adolescence is just a brief teenage phase. Instead, it suggests a prolonged period where neural circuits are constantly reorganizing.
The Retrospinal Cortex Under Stress
The study focused on the retrosplenial cortex, or RSP. This area is often overlooked compared to the hippocampus. But the RSP helps connect personal experiences with specific places, contexts, and stored information. It’s a hub for retrieving established memories.
In mice, the team noticed something distinct. During late adolescence, perineuronal nets in the Rsp declined sharply. These mesh-like structures surround certain neurons and stabilize connections that preserve information. Without them, memory circuits become more flexible. But flexibility comes with a cost. It makes accessing older memories harder.
This instability wasn’t seen in the nearby hippocampus. The hippocampus handles the formation of new memories. So the changes are not universal across the entire memory system. Only specific circuits undergo this temporary fragility.
Why Access to Old Memories Changes
“We’ve known for years that the brain is still developing through adolescence,” says senior author Jelena Radulovic.
Her team found that the reorganization in the RSP seems to prioritize adult-formed memories. It does so at the expense of early adolescent ones. This shift might help the brain adapt to adult life stages and challenges. The brain is essentially clearing space for new, relevant information by temporarily suppressing the old.
“We do not yet fully understand all the consequences,” Radulovic notes.
The researchers are now investigating what happens if those fluctuations are prevented. Would stopping the remodeling cause issues later in life? It remains an open question.
Adolescence Is a Long Process
Older studies suggested memory circuits matured by early adolescence. These new findings contradict that view. A key part of the system enters a period of instability later in the teenage years. It only settles again in early adulthood.
This timing matters because the human brain continues developing well beyond the teen years. Planning, impulse control, and emotional regulation can keep maturing into the mid-20s. The brain is essentially under construction for longer than previously thought.
“The behavior matched the biology,” says lead author Hui Zhang.
As the stabilizing structures in the retrosplenial cortex declined, access to older memories became less reliable. The brain was changing its priority list.
Hidden, Not Erased
Did the mice forget the lesson entirely? No.
To test this, researchers exposed mice to a mild foot shock in a specific chamber. The mice froze upon return—a clear sign of fear memory.
When the researchers returned to check weeks later, many adolescent mice had stopped freezing. Adult mice, however, retained the response. Their memories seemed more stable.
But the adolescent memories were still there. When researchers reminded the mice of the shock in a different setting, the fear returned. The information was stored. It was just temporarily inaccessible. The retrieval path was blocked, not destroyed.
Restoring the Protective Nets
The team linked this loss of recall to reduced levels of proteins needed for perineuronal nets. TGFβ2, a growth factor supporting these structures, also dropped during this period.
Crucially, the team intervened. When they strengthened the nets or restored TGF2 activity, the mice could again retrieve those early memories. This proves the structural changes were causing the problem, not just associated with it.
By mid-adulthood, most inaccessible memories returned naturally. But they were fuzzier. Mice no longer feared only the specific shock chamber. They showed fear in unfamiliar environments too.
Why Old Memories Return Blurred
Is this forgetting useful? Perhaps.
Remembering the broad emotional lesson might be more valuable than remembering exact details. If you encounter a danger, knowing it was bad is important. The precise location might matter less as time goes on.
This pattern resembles the “reminiscence bump.” Adults tend to recall more memories from adolescence and early adulthood than from other periods. Those memories keep strong emotional weight even when fine details fade.
The researchers don’t know if the two phenomena share the same biology. The later return of memories might come from increasing perineuronal nets with age. Or it could be repeated exposure to similar events. Or some other mechanism still unidentified.
A Window for Mental Health?
Schizophrenia and major depression often emerge in late adolescence. This overlaps perfectly with the period of heavy circuit remodeling seen in the mice.
The temporary instability might create a sensitive window. For people with genetic susceptibility, disruptions in this process could raise the risk of psychiatric illness.
It’s important to note these experiments were done in mice. We don’t know for sure if human memory circuits behave exactly the same way. But the parallel is striking. If our brains are rewriting themselves during the teenage years, the stakes are higher than we assumed.
The door is open. The lock is changing. What happens inside the mind during those years? We are just beginning to see how the lock works.
