International Business Department           Liu Bojia           December 10, 2024

  How are memories formed? You can try to recall some of the scenarios that you have experienced in your daily life, such as how after memorising a text over and over again, most people are able to remember the article or poem all the time, or how a certain tool doesn't work well at first, but after using it many times you know how to use it with less effort. This all suggests that the brain's memory of particular events moves from repeated reinforcement of short-term memory to long-term memory.

  Neuroscientists have also argued in the past that memory is a linear process: memories are stored as short-term memories first, and those that are not useful are erased, such as a one-time mobile phone verification code that you write down and then never think about again. And then there are parts of the memory that correspond to discharge signals and representations that are preserved by specific neurons, which can be reproduced over time, and therefore can be recalled over a long period of time.

  But there are actually other memories that may be difficult to interpret in this way, such as when you walk past a grocery shop you haven't been to in a long time, and are likely to recall the last time you saw the shopkeeper petting a kitten, or suddenly remember a line from a film you saw only once a few years ago. In a recent paper published in Nature Neuroscience from the Max Planck Florida Institute for Neuroscience, they found that memory formation, unlike past cognition, is not just a single progression from short-term to long-term memory, but that there are two separate pathways responsible for each of the two types of memory formation. As a result, the pathway dedicated to forming long-term memories is not disturbed by short-term memories.

  In the experiment, the mice were placed in a space that could be explored, and the space would be separated into light and dark areas. Due to habit, the mice preferred to crawl into the dark environment, however when the mice entered the dark zone, the door separating the two spaces would be closed and the dark zone would emit a constant current stimulus, which would cause the mice to develop a fear memory.

  In parallel with this experiment, the authors used optogenetics to modulate the expression of CaMK II enzyme in some mouse amygdala neurons, which is critical for short-term memory formation.

  The study then looked at how the mice behaved when they were re-entered into the same environment after a short period of time. For those mice that did not manipulate the CaMK II enzyme, they resisted re-entry into their favourite dark environment. However, once the CaMK II enzyme was disrupted, the mice still chose to go to the dark area when they returned an hour after the initial experiment, completely forgetting that they had just been electrically stimulated there, suggesting that short-term memory formation had been impaired.

  Following the linear logic of past memory formation, without the support of short-term memory, these mice would no longer be fearful of dark environments. But the researchers were surprised to find that when the mice with impaired short-term memory returned to the same environment a week and a month later, they did not choose to go to the dark area as they had done before, and the memory of the shock stimulation was reawakened. This means that long-term memories can still be constructed effectively even when short-term memories are missing.

  ‘This is a different way of forming memories from what was thought in the past, and a parallel path exists for long-term memory that bypasses short-term memory.’ Dr Myung Eun Shin, one of the study's authors, said he and his colleagues believe this will probably change the brain's model of memory formation, but exactly how this new long-term memory formation pathway is constructed needs to be explored by more research. And unravelling the secrets is also expected to bring fresh insights into memory dysfunction diseases such as Alzheimer's disease.