Can We Control Memories at the Neuronal Level?

Memories, both good and bad, are important to everyone. Bad memories, in particular, serve as valuable experiences that help us learn and avoid unpleasant situations in the future—what people often call “not stepping on the same rake twice.” However, everyone knows how distressing bad memories can be. They weigh us down, ruin our mood, and interfere with daily life. If these memories are especially vivid and intense, they can even lead to post-traumatic stress disorder (PTSD). Fortunately, the brain tends to favor good memories over bad ones, meaning we remember positive moments better and for longer—a fact that has been known for some time.

In a recent study, scientists discovered how the brain stores “good” and “bad” information at the level of specific brain regions. Until now, it was unclear how the brain processes good and bad memories at the neuronal level. Researchers at Boston University have now answered this question—and have even learned to control memories themselves.

How Human Memory Works

According to scientists, human memory is not a video recording of the past, but rather a reconstruction. This is beneficial, as the brain can update and reshape memories. Thanks to this ability, if something frightening or traumatic happened in the past, the brain helps us forget or dull the emotional impact of those events. This suggests that there are mechanisms in the brain responsible for both good and bad memories.

But how do memories work in our brains, and why do we sometimes recall events from long ago? Memories are managed by so-called engrams, which are traces left by stimuli. Signals such as smells, sounds, or certain environments trigger specific physical and biochemical changes in these engrams.

Later, if a new signal matches the trace, the engram activates all the cells it’s connected to, reproducing the memory in full. For example, if someone hears familiar music, they are likely to remember life events associated with it. Sometimes, the brain replays memories without any external signal—this often happens during sleep.

Memories are controlled by specific engram neurons. Photo.

Engram Neurons: The Controllers of Memory

Simply put, engrams control the “playback” of our memories. At the neuronal level, they are specific neural networks. To activate, they need a “push” from a key signal. This means that engram neurons must be able to remember information, specifically the key signal that activates them.

How the Brain Handles Good and Bad Memories

Researchers at Boston University focus on memory networks located in the hippocampus, a key memory center that stores emotionally charged information.

In one study, scientists modified neurons in the hippocampus of mice so they could be artificially controlled—turned on or off at will. During the experiment, the mice were exposed to pleasant situations (like being given treats or placed with a companion) and unpleasant ones (such as mild electric shocks to the feet).

Scientists identified which engram neurons trigger bad memories. Photo.

The researchers then alternately activated different sets of engram neurons. As reported in the journal Communications Biology, the mice’s behavior changed accordingly. When certain neurons were activated, the mice froze and, under stress, anticipated another shock. When other neurons were activated, the mice became more lively and behaved as if expecting another treat.

This showed that emotional memories in the hippocampus correspond to distinct neurons, which differ from each other. The cells responsible for good and bad memories are different in both their location and their molecular profile. Differences were also found in regulatory epigenetic markers on DNA, which affect gene activity. As a result, their connections with other brain regions were also configured differently.

This research may help find treatments for PTSD and depression. Photo.

Why Good Memories Are “Stronger” Than Bad Ones

In a recent study published in Nature Communications, researchers used laser pulses to activate nerve cells responsible for unpleasant memories, which understandably caused stress in the mice. However, when scientists simultaneously stimulated neurons associated with good or even neutral memories, the negative emotions weakened or disappeared entirely.

Afterward, when the researchers activated the neurons that triggered bad emotions on their own, the mice no longer experienced the same level of stress as before. This suggests that the negative emotions linked to the electric shock were erased. The mice didn’t forget the unpleasant sensation itself, but the memories were no longer as vivid.

This research could potentially help treat depression and PTSD in humans. While such experiments can’t be conducted on people, the results align with a well-known psychological tip: if you’re troubled by bad thoughts, try to recall something good. This suggests that humans have similar memory mechanisms as mice.