Why is that you remember the name of your childhood best friend, but you struggle to recall the name of the person you just met- even if they told you it just five minutes ago? They’re both names after all. Despite this outward similarity however, it’s clear some memories stick stronger than others. But what are the reasons behind this?
As it turns out, the popular idiom “practice makes perfect” may hold some of the answers. Looking at memory patterns in mice, researchers at the California Institute of Technology found that stable, long-lasting memories are formed by groups of neurons that fire in synchrony, as opposed to the activation of singular, highly-connected neurons as previously thought (Gonzalez: 2019). The activation of multiple neurons around one experience in this way, they found, is linked to repetition, or practice.
To come to this conclusion, they studied the neural activity of mice when remembering a new place. Put in a straight enclosure with white walls marked with symbols and sugar water (a mouse treat) at either end, while they explored the enclosure, the researchers measured the activity of specific neurons in their hippocampus, a part of the brain known for spatial recognition and the formation of new memories (Malewar: 2019).
They noticed that, when first placed on the track and unsure of what to do, single neurons were activated whenever the mice took notice of symbols on the wall. However, over time, as the mice become more familiar with the symbols and associated them to the locations of the sugar water, more and more neurons were activated in synchrony upon noticing each symbol on the wall, indicating that the mice were able to recognise their position relative to each symbol.
Then withholding the mice from the enclosure for 20 days to measure their long-term memory, the researchers found that mice who had formed memories encoded by high numbers of neurons remembered the task more quickly than those whose memories formed from fewer neurons.
With the activation of higher numbers of neurons around a task associated with stronger memories, these findings suggest that memories may fade more rapidly as we age as they may encode fewer neurons. This means that, should any of these few encoded neurons fail, the memory would be lost (California Institute of Technology: 2019).
However, it seems that increased activation of multiple neurons may not just come from repeated actions. From studying navigation in rats for example, researchers at the Flanders Institute of Biotechnology found that highly demanding and rewarding experiences also result in stronger memories as they lead to “memory replay” processes in the hippocampus (Michon: 2019).
According to Professor Fabian Kloosman, a leader of the research,”One of the ways in which our brains consolidate memories is by mentally reliving the experience...In biological terms, this boils down to the reactivation or replay of the neuronal activity patterns associated with a certain experience. This replay occurs in hippocampal-cortical brain networks during rest or sleep (Flanders Institute for Biotechnology: 2019).”
Although the researchers had not specifically measured for neuronal activity, it is likely that, just as repeating certain actions leads to higher activation of neurons associated with a given task, so does mental simulation, thus leading to stronger memory recall in the long run.
To conclude, it seems that the reason why we remember some things more than others is due to the number of activated neurons associated with what we’re trying to remember. While repeating actions and experiences tends to make certain events more memorable, research has also found that highly demanding and rewarding experiences tend to yield stronger memories thanks to our habit of mentally replaying them, and thus consolidating them, in our minds.
Gonzalez, Walter G.: Science
Malewar, Amit: Tech Explorist
California Institute of Technology: Science Daily
Michon, Federic et al.: Current Biology
Flanders Institute for Biotechnology: Science Daily