Memory: A Dynamic Process

We often picture memories as static displays, like artifacts in a museum, helping us make sense of now and plan for what’s next.

However, recent findings indicate that memories are more like well-used books from a library, subtly altering each time they’re revisited.

Think of one happy memory. Really think about it. Close your eyes and visualize the details. Try to feel a hint of the joy or hope you experienced then. Take a minute or two.

If you engaged with this little exercise, your body has changed a bit in just those moments.

When you began to recall that memory, neurons that were quiet just moments ago started communicating with each other. This activated parts of your brain linked to emotions, which explains why you might have felt similar feelings as you did back then.

As this happens, signals travel through your body. If stress was hanging over you before, your heart likely calmed down as stress hormones like cortisol diminished. Alternatively, if you were relaxed, maybe your heart raced with excitement.

In both scenarios, areas of your brain associated with rewards lit up, releasing dopamine.

That memory not only affected you, but pulling it to the forefront also altered the memory itself, according to neuroscientist Steve Ramirez.

Some parts of that memory became more significant, while others faded. Your brain may have unconsciously added or discarded details. The emotions you felt while reminiscing influenced how that memory exists, as brain cells activated by your current mental state synced with those triggered by the past event.

Every time you revisit a cherished moment, it shifts slightly, both in your experience and in the physical network of your brain cells.

This interaction with memory is something humans have done for ages. Yet, over the last two decades, researchers have discovered remarkable ways to manipulate memories (at least in mice): implanting false memories, erasing real ones, restoring lost memories from brain damage, and even separating emotional responses from one memory and applying them to another.

Ramirez discusses this in his new book, “How to Change a Memory: One Neuroscientist’s Quest to Alter the Past.” He sees these advances not as potential mind control but as means to alleviate mental distress.

“It’s fascinating what we can achieve in modern neuroscience,” Ramirez commented recently. “The ultimate aim, though, is to heal and enhance well-being for an organism. Memory manipulation can be a valuable part of treatment.”

Memory is central to Ramirez’s own story.

His father faced a life-threatening situation in El Salvador when soldiers wrongly accused him of being a leftist and held him at gunpoint. It was through a twist of recognition—his captors realizing he was just a kind former classmate—that he was spared.

Ramirez’s parents moved to the U.S. before he was born, and he was raised in Boston along with his siblings. He earned his degree in neuroscience at Boston University and received his doctorate from MIT. During his graduate studies, he joined Nobel laureate Susumu Tonegawa’s lab, forming a close bond with postdoc Xu Liu.

Both were passionate about exploring memory as a potential therapeutic avenue, quickly becoming close friends and collaborators.

Their first big success came in 2012.

A team from the University of Toronto had previously identified the neurons that activated in mice exposed to a fearful sound paired with a shock. They then killed those specific neurons, resulting in the mice no longer reacting with fear to that sound.

Seeing that memories could be deleted led Ramirez and Liu to wonder if they could also create new ones.

For their experiment, they pinpointed neurons in a mouse’s hippocampus that became active upon receiving a shock. They placed the mouse in a new environment with no reminder of the previous shock and then used light to stimulate those same neurons—without delivering a shock this time.

The mouse behaved as if it had experienced the shock, even though it hadn’t happened again.

While researchers can’t ask a mouse about its memories, they infer findings from observable behavior. Here, it seemed they had successfully activated a memory.

“It was astounding,” noted Sheena Josselyn, who was involved in the earlier fear-erasure work. “When you can alter memories like that, it unveils the neural foundations of memory.”

In 2013, Ramirez and Liu let a mouse explore a box, noting the neurons that fired during that exploration.

They then transferred the mouse to a new setting, using light pulses to reactivate the neurons that connected to the first box while administering a shock. Upon returning to the original box, which had never caused harm, the mouse froze, now associating fear with a previously safe environment.

This experiment effectively created a false memory, a significant achievement in their research.

In their concluding project together, they observed how a mouse responded positively to social interaction in a larger setup, then placed it alone in a smaller space, which initially dampened its mood.

Typically, well-adjusted mice favor sugary water, but their stressed counterparts often don’t show a preference, reflecting how the isolated mouse acted.

But when the researchers stimulated neurons tied to the social memory, the mouse’s behavior shifted dramatically—it excitedly drank from the sweet water again. This recollection altered its behavior to more closely resemble that of a healthier mouse.

This research was published in 2015, in the journal Nature. Yet, unlike previous milestones, Liu passed away unexpectedly just before this one could be celebrated.

Ramirez reflects on grief as closely linked to memory: both linger a lifetime, reshaping our identities and priorities.

Now 37, Ramirez opened his lab at Boston University in 2017. Since then, memory science has progressed impressively, allowing studies on restoring memories lost due to amnesia, activating memories while calming associated emotions, and even altering the emotional tie to one memory to connect it to another.

Still, experts don’t foresee doctors using lasers to reshape human memories.

The experiments mainly happen with genetically modified mice that exhibit neural activity in response to light—altering human DNA this way is viewed as ethically and practically problematic.

Plus, it’s really not needed.

Memory scientists argue that we already distort our memories effectively without digital assistance. With just the right prompts, many people can be led to believe they experienced events they actually did not. Memory recall can occur spontaneously or through various sensory triggers, and our brains are constantly editing memories without conscious effort.

The primary objective of Ramirez’s research is to decode the biological underpinnings of memory and use that knowledge for non-invasive therapy options.

If researchers elucidate how to retrieve memories blocked due to brain damage, for instance, it could lead to treatments that protect or enhance memory in those grappling with dementia or cognitive issues.

Furthermore, understanding how memories are encoded may yield better treatments for conditions like post-traumatic stress disorder.

Still, there’s always a shadow side to this type of research. Understanding memory manipulation could be exploited for harmful purposes, raising concerns about identity erasure or thought control.

“The concept of altering memories can stir anxieties about a future where our relationships are erased or our identities replaced,” Ramirez remarks. Yet, he believes any tool has the potential to do harm or good, and he prefers striving for helpful advancements over stagnation.

“Memory manipulation might make sense if the goal is ethically grounded—aimed at healing and promoting well-being,” he suggests. “What if we could treat memories productively, making them part of a therapeutic toolkit? That’s something worthwhile.”