This is why some memories are clearer than others

All memories are not created equal. Some memories reside in our minds as crystal clear recollections of past events, while others are seemingly gone as quickly as they came.

Ask someone about their strongest, clearest memories and that person will probably start talking about their wedding day or the day their child was born. Others may have a vivid recollection of what they were doing the moment they heard a big piece of news (death of a celebrity, major world event).

What do all of these memories have in common? Emotion. Whether that emotion is joy, sadness, excitement, or fear, events in our lives that invoke a strong emotional response are almost certain to reside in our memories for a long time afterward. Conversely, ask a stranger what they ate for lunch three days ago and they’ll probably struggle to settle on an answer.

A new study from Columbia University investigated the neurological reasons why emotional memories are usually so much clearer than more unexciting past moments. Just like so many other elements of modern human behavior and tendencies, they discovered that this phenomenon can be traced back to humans’ early, prehistoric beginnings and struggle for survival in the wild.

“It makes sense we don’t remember everything,” says René Hen, Ph.D., professor of psychiatry and neuroscience at Columbia University Vagelos College of Physicians and Surgeons, in a release. “We have limited brainpower. We only need to remember what’s important for our future wellbeing.”

Indeed, there’s only so much space in our heads, meaning all of our memories just aren’t going to fit. So, it’s up to the mind to sift through and prioritize memories that are important and worth keeping. On that note, there’s no feeling the human brain takes more seriously than fear. To our minds, fear isn’t that fun feeling we get while watching a horror movie or true crime documentary, it is a critical learning experience that may just prove vital to survival at a later date.

Picture yourself in a prehistoric jungle. You accidentally step on a snake and the snake lunges at you. Frightened, you jump out of the way and narrowly avoid being bitten. In the aftermath of all that you would probably also be overcome by a wave of other emotions; relief & happiness you survived, the excitement that you’ll live to see another day, etc.

Your mind picks up on the fear and other emotions you are feeling recognizes that experience was nearly fatal and records the details of that event in its neurons so you won’t make the mistake of stepping on a snake again. Experiences like these among early humans are the reason why modern minds still make fearful/emotional memories a priority.

While that explains why emotional memories are always recorded in the first place, the research team at Columbia was still interested in ascertaining exactly why emotional memories are much clearer and more detailed than other recollections.

To that end, they placed a group of lab mice in a series of fear-inducing situations and observed how their hippocampal neurons reacted as they “reached out” to the mind’s fear center, the amygdala. Neuronal activity was also recorded the next day, as the mice remembered the past day’s events.

As the research team expected, the rodents’ fear-responsive neurons reacted accordingly to the new, stressful situations and sent out signals to the amygdala. However, what happened next surprised the study’s authors.

“What was surprising was that these neurons were synchronized when the mouse later recalled the memory,” Hen says.

“We saw that it’s the synchrony that is critical to establish the fear memory, and the greater the synchrony, the stronger the memory,” notes co-researcher Jessica Jimenez, an MD/Ph.D. student at Columbia. “These are the types of mechanisms that explain why you remember salient events.”

Those last observations are important, despite being somewhat shrouded in scientific lingo. In short, the rodents’ neurons acted similarly during both the frightening event itself and while they were remembering the event the next day. This suggests that emotional memories are so much clearer than others because our minds are, somewhat literally, reliving the past.

The study’s authors also say their work may prove incredibly useful in treating memory disorders like PTSD.

“In people with PTSD, many similar events remind them of the original frightening situation,” Hen concludes, “and it’s possible that synchronization of their neurons has become too strong. We’re really trying to dig into the mechanisms of how emotional memories form to find better treatments for people with PTSD and memory disorders in general.”

The full study can be found here, published in Nature Communications.