Toronto – Researchers at the Hospital for Sick Children have demonstrated that memories formed in young mice, even those seemingly forgotten, are not erased but remain encoded in brain cells, potentially offering new insights into the phenomenon of childhood amnesia in humans. The findings, published in recent years, suggest that early memories may be inaccessible rather than lost, locked away by the brain’s developmental processes.
Childhood, or infantile, amnesia – the inability of adults to recall episodic memories from early childhood – is a nearly universal experience. Even as toddlers demonstrably form memories, those recollections typically vanish by the age of three or four. Experiments with mice mirror this pattern; mice trained to navigate a maze or associate a chamber with a shock forget these experiences as they mature.
Paul Frankland, a senior scientist at the Hospital for Sick Children, and his team tagged brain cells activated during fear conditioning in young mice. Three months later, even after the mice had forgotten the initial fear, reactivating those same cells triggered a recall of the experience, demonstrating the persistence of the original memory trace. This suggests that, at least in mice, memories aren’t destroyed but turn into inaccessible. “It may be that something about how the brain grows makes those memories inaccessible, locked rooms whose keys have been lost,” researchers note.
The brain’s capacity to add new neurons after birth appears to play a role. Animals with limited neurogenesis, like guinea pigs, do not exhibit the same degree of infantile amnesia, according to Frankland’s research. Further investigation by Tomás Ryan, a neuroscientist at Trinity College Dublin, revealed that male mice with activated maternal immune systems during pregnancy were less prone to forgetting, highlighting a potential link between the immune system and memory retention. Ryan’s group also found that suppressing the activity of microglia – immune cells in the brain – during a critical developmental period prevented the typical forgetting observed in mice, suggesting the immune system actively contributes to infantile amnesia.
Extending this research to humans presents significant challenges. Scanning the brains of young children is difficult, but Nick Turk-Browne at Yale University and his colleagues have begun to map memory formation in children as young as one year old. Their research indicates that young children form episodic memories – recollections of past events – in a manner similar to adults, suggesting that humans, too, may create memories that later become unreachable. Understanding how these early memories are processed is crucial, even if they are ultimately forgotten.
The practical implications of this research are already prompting questions from parents. Turk-Browne reports being frequently asked how to preserve early memories. While strategies like frequent discussion and photo albums can reinforce experiences, he cautions that the uniquely personal, untarnished quality of those early memories – those not prompted by external cues – is inevitably lost. “The true, pure thing…that will soon be gone, for better or worse,” he says.
Researchers are also attempting to pinpoint the precise timing of memory formation and loss. Sarah Power at the Max Planck Institute for Human Development has created a specialized laboratory environment to study memory encoding in toddlers aged 18 to 24 months. By presenting children with experiences unique to the lab, Power’s team aims to track the development and retention of episodic-like memories over time. Preliminary data suggests a surprising capacity for memory retention in very young children.
Turk-Browne is also conducting a study where parents film events from their children’s perspective. These videos are then shown to the children during brain scans, allowing researchers to assess whether viewing their own experiences elicits detectable brain activity associated with remembering, and at what age that effect diminishes.
The underlying reasons for this early memory loss remain a mystery. Ryan speculates whether it’s a biological necessity or a byproduct of rapid learning. “Is this a biological switch, or is this just a product of exuberant learning?” he asks. Turk-Browne suggests that the value of early memories may lie not in their conscious recall, but in their contribution to building a foundational understanding of the world. “Most memory researchers think of the adaptive value of memory as being able to behave appropriately in new situations based on past experience,” he explains. “Even newborn infants are really solid at aggregating statistics”—building a mental model of the world that informs future decisions.
Power’s ongoing research, while still in its early stages, continues to monitor the development of these early memories, and the factors that contribute to their retention or loss. The question of why we forget remains open, and the implications for understanding human memory, learning, and development are profound.
