Why Some Older Memories Are Easier To Recall Than Newer Ones

Most of us have instant recall when it comes to pulling up a tale from our childhood but when it comes to remembering what we had for lunch we draw a blank. Why are some memories longer lasting while others vanish within moments?

With the help of mouse models, Caltech researchers have found that strong, withstanding memories are encrypted by “teams” of neurons all firing together, providing redundancy that allows these memories to endure over time. Neural redundancy is the theory that multiple neural units have similar or identical functions. An example of this is when a person is shot in the brain, there are still redundant neural units that can perform the functions of the damaged neural units.

This study could help comprehend how memory might be affected after brain damage that could be caused by events like stroke or Alzheimer’s disease.

Performed in the laboratory of Carlos Lois, the study was led by postdoctoral scholar Walter Gonzalez. His team devised a method to observe mice’s neural activity as they learn about and remember a new environment.

This was carried out by placing a mouse in a white-walled straight enclosure about five feet in length. The walls were marked with various symbols on different locations on the walls. These symbols could be in the form of a bold plus sign near the right most end and an angled slash near the centre. At the either end of the track were sources of sugar water, a dessert of sorts for mice. Researchers recorded the activity of specific neurons in the hippocampus while the mice wandered around. The hippocampus is part of limbic system and serves important functions in the consolidation of information from short-term memory to long-term memory, and in spatial memory that enables navigation.

When initially placed in the track, a mouse would be unclear about what action it should take and meander until it chanced upon the sugar water. In these cases, single neurons were activated when the mouse took notice of a symbol on the wall. With increased exposure to the track the mouse would become acquainted with it and know where to find the sugar water. An increased familiarity with the track would result in increased synchronous neuronal activation whenever the mouse recognised the symbols on the wall. Essentially, the mouse was using these symbols as signboards to know where it was.

Then, to investigate how memories faded over time, the mice were kept away from the track for up to 20 days. When returned to the track post-hiatus, mice that had stored strong memories using a greater number of neurons recalled the task quickly. Despite some neurons displaying different activity, the mouse’s recollection of the track was clearly noticeable when examining the activity of large groups of neurons. Basically, employing groups of neurons allows the brain to have redundancy and still remember memories even if not all of the neurons are properly functioning.

Gonzalez explained: "Imagine you have a long and complicated story to tell. In order to preserve the story, you could tell it to five of your friends and then occasionally get together with all of them to re-tell the story and help each other fill in any gaps that an individual had forgotten. Additionally, each time you re-tell the story, you could bring new friends to learn and therefore help preserve it and strengthen the memory. In an analogous way, your own neurons help each other out to encode memories that will persist over time."

Memory is such an essential aspect of human behaviour that any deterioration can dramatically affect our daily lives. Senior citizens can be drastically handicapped by memory loss that happens as part of regular aging. Additionally, memory loss that occurs as part of diseases such as Alzheimer’s has devastating outcomes that can affect even the most basic routines including recognizing family members or remembering the way back home.

This study suggests that memories fade faster as we get older because a memory is encoded by fewer neurons. If these neurons break down, there are fewer neurons to carry on their duties and thus, the memory disintegrates. It also suggests that devising treatments that could engage more neurons to encode a memory could aid in memory preservation.

"For years, people have known that the more you practice an action, the better chance that you will remember it later," says Lois. "We now think that this is likely, because the more you practice an action, the higher the number of neurons that are encoding the action. The conventional theories about memory storage postulate that making a memory more stable requires the strengthening of the connections to an individual neuron. Our results suggest that increasing the number of neurons that encode the same memory enables the memory to persist for longer."