Researchers' Zone:
Why do some memories stick while others fade?
Research reveals how emotions are the ‘glue’ of our memory. Learn how emotionally charged events affect brain cells and preserve significant moments.
Can you remember when…?
You’ve likely heard this question before, and as we know, some things stick in our minds while others do not. Right? Momentous events, like the birth of a child or the loss of a loved one, remain vivid for years. In contrast, memories of what we had for dinner or conversations we’ve had often disappear within days.
Research shows that emotionally impactful events strengthen the neural connections in the brain that store our memories. These important events act as a kind of "glue," anchoring otherwise trivial experiences in our memory. For example, a dinner or conversation that might otherwise be forgotten can remain clear if it happened alongside a life-changing event, such as the birth of a child or the loss of a loved one.
Historically, this mechanism has been vital for survival, enabling our ancestors to recall essential details about food sources or life-threatening dangers. However, not all memories hold the same weight—some last a lifetime, while others fade quickly. Why? And, perhaps more importantly, how?
Scientists reveal how memory is formed
Motivated by this question, my colleagues and I conducted a study using mouse models, whose brain functions are remarkably similar to humans. Our findings offer new insights into what happens in the brain during memory formation and help explain why some things are remembered while others are not.
As a neuroscientist, I’ve always been fascinated by how memory and learning work. Memory shapes our personality and equips us with the ability to understand the present and anticipate the future. It influences our decisions and actions, but it can also fail us—we forget birthdays or the names of acquaintances, while events from decades ago remain crystal clear.
Through my research, I explore how memory functions at the cellular level, studying how brain cells communicate and change during memory formation.
Curious to know how your memories are created and preserved?
Let’s dive deeper.
Synapses – where our memories are formed
Before delving into the research, let me explain some key terms often used in neuroscience.The cells in our brain are called neurons. Neurons connect through structures known as synapses. Synapses act as communication channels, allowing neurons to send and receive information. A single neuron can have up to hundreds of thousands of synapses.
Scientists agree that memories are formed (and lost) by changes in the strength of these synapses—a process known as synaptic plasticity. Synaptic plasticity can be increased or decreased in two ways: by altering the number of synapses or by making existing synapses larger or smaller.
You can think of this as transportation between two countries. It can be improved by adding more border crossings or by widening the existing crossings so that more vehicles can pass through at once.
How scientists study memory in mouse models
Back to the research: We designed two types of experiences for mice – one emotionally significant and one trivial.
The first experience, the weaker one, was based on a method called Pavlovian conditioning, developed by the Russian researcher Ivan Pavlov. This method explains how we can learn to associate normally unrelated things by repeatedly experiencing them together.
Pavlov’s classic experiment demonstrated this by giving dogs food while ringing a bell. Initially, the dogs only reacted to the food by salivating, but after repeated trials, they began to salivate just at the sound of the bell – even when no food was present. This is an example of how we learn – and thus remember – through repetition.
In our experiment, we exposed the mice to a neutral cue followed by a physical stimulus in the form of a mild electric shock. We made the shock so weak that the experience became less unpleasant. As a result, the memory of this experience was weak, and the mice could not recall it the next day.
The second experience, which was the stronger one, was an artificially created experience where we strongly activated the neurons encoding the weak memory. For scientific reasons, we used artificial activation of the neurons as an emotionally significant experience. Remarkably, in this case, the mice were able to recall the aversive memory even the following day.
At the same time, we observed that the synapses active during the event were strengthened – either by forming more channels or by expanding existing ones. In contrast, mice that only experienced the weak aversive event without artificial activation were unable to recall it the next day, and the strength of the associated synapses remained unchanged.
The big question: How do synapses interact?
To measure the strength of synapses, we used a technique called in vivo electrophysiology. This method allows us to measure the electrical activity of neurons in freely moving animals. When a synapse becomes stronger, the neuron’s electrical activity increases proportionally.
We wanted to investigate whether a trivial experience could become memorable by artificially strengthening synapses unrelated to the event.Imagine a trivial experience, such as what you ate for dinner, and an exciting event, like learning you won the lottery. These two events are usually unrelated, but if they happen simultaneously, the exciting event might make the dinner memorable.
In our experiment, we artificially strengthened certain synapses, even though they weren’t directly linked to the trivial experience. The result? The mice could recall the "trivial" event the next day. Even more interestingly, the synapses encoding the trivial experience also became stronger.
However, if we manipulated the synapses the day after the experience, the mice could not recall the event. This is similar to the lottery example: you might remember what you ate for dinner when you heard the lottery news, but likely not what you ate the following day.
The intriguing question is: How can synapses encoding an important event (like winning the lottery) strengthen neighbouring synapses encoding a trivial experience (like dinner), making the latter memorable?
We don’t yet have a definitive answer, but our best hypothesis is that certain molecules—likely proteins—move from the strong synapses to neighbouring synapses encoding the trivial experience. When these neighbouring synapses absorb the molecules, they become stronger, stabilizing the memory they encode.
Evolutionary importance and modern relevance
You might wonder why this matters.
Historically, memory was a survival tool, enabling our ancestors to recall critical details about food sources or avoid dangerous areas.Imagine being thirsty and hungry for days, only to discover a place abundant with food and water. How could you forget it? Even small, trivial details about the location might stick because the joy of finding sustenance made a strong impression.
The same applies to danger. If you encountered a tiger, wolf, or snake in a seemingly ordinary bushy area, the fear associated with the predator would make you remember the location for a long time.
While modern life is less dangerous, memory remains essential for navigating social and professional situations. We tend to remember birthdays and details about loved ones better than those about people we don’t have strong feelings for. Similarly, remembering details from a challenging situation can help us make better decisions in the future.
Even in our predictable world, memory helps us link significant events to small details, enabling us to learn, adapt, and thrive. In short, memory remains indispensable—but what and why we remember has evolved.
