Most people know phrases like “photographic memory” or “elephant’s memory”, used for those who recall details
exceptionally well. But the brain doesn’t store everything permanently.
Some memories fade within days or weeks, while others last years or a lifetime. What determines whether the brain
forgets quickly or preserves a moment forever?
To find the answers to these questions, scientists from Rockefeller University in New York made mice perform a virtual
reality activity, wherein the rodents were subjected to different environments.
The mice formed strong, long-lasting memories for the environments that were shown to them several times, and quickly
forgot the ones they were introduced to only a few times.
Published in the journal Nature, the study compares what happens in the brain when a memory lasts for a long time with
what causes the organ to forget an event.
It says certain molecules in the brain regulate how memories are promoted or demoted, determining whether one remembers
them forever or forgets them.
The scientists studied a brain circuit that connects the thalamus, deep in the centre of the organ, and parts of the
cortex or the outer layer.
The study answers an important question: which molecular mechanisms promote important memories to the cortex and demote
less important ones to be forgotten?
It found that there is no single on-off switch that controls memories, but several timed stages or molecular timers that
turn on like a cascade over several weeks or days. In other words, it means long-term memory is a gradually evolving
process, wherein brain regions reorganise the memories into long-lasting forms and make them more durable.
Scientists have identified three key molecules that decide how long a memory lasts. Two of them, CAMTA1 and TCF4, work
in the thalamus, the brain region that helps with short-term memory and quick decisions. The third, ASH1L, works in the
cortex, where long-term memories are stored.
CAMTA1 keeps a new memory alive for the first few days; without it, the memory fades quickly. TCF4 then takes over,
strengthening links between brain regions, so the memory becomes steady over weeks. Finally, ASH1L makes the memory
long-lasting. In simple terms, the thalamus chooses important memories and sends them to the cortex for lifelong
Using correlation (variables change together), the scientists discovered which molecules are correlated with memory
To understand whether these molecules actually cause memories to last, rather than simply appearing alongside them, the
researchers turned to CRISPR gene editing to remove each molecule one at a time in mice.
When a molecule was knocked out, the mice forgot the repeated memory exactly at the stage when that molecule was
normally supposed to act. When TCF4 was removed, memories faded because they could no longer be transferred from the
thalamus to the cortex. A key discovery was that none of the three molecules are needed to form a memory, only to
stabilise and maintain it.
The study shows that long-term memory is a step-by-step process, where important memories are passed along a timed chain
of molecular “gates,” while unimportant ones drop out early.
These insights could guide Alzheimer’s research, especially because TCF4 and ASH1L appear crucial for long-term
consolidation, potentially helping scientists design alternate pathways for memory transfer.
However, the study still cannot explain how these molecular timers are switched on or how the brain decides which
memories matter, questions the researchers hope to explore next.