How memories form and fade

Why do you remember the name of your best childhood friend that you have not seen in years but easily forget the name of someone you met a moment ago? In other words, why are some memories stable over decades, while others fade in minutes?

Using mouse models, Caltech researchers have now determined that strong, stable memories are encoded by synchronized "teams" of neurons, providing redundancy that allows them to persist over time. The research has implications for understanding how memory may be affected after a brain injury, such as a stroke or Alzheimer's disease.

The work was done in the laboratory of Professor Carlos Lois, biology professor, and is described in an article in the newspaper of 23 August. Science. Lois is also a faculty member at the Tianqiao Institute of Neuroscience and Chrissy Chen at Caltech.

Led by postdoctoral researcher Walter Gonzalez, the team has developed a test to examine the neuronal activity of mice when they discover and remember a new place. In the test, a mouse was placed in a straight enclosure, about 5 feet long with white walls. Unique symbols marked different places along the walls, for example, a bolder sign near the far right end and a slash near the center. Sweet water (a sweet treat for mice) was placed at each end of the trail. During the exploration of the mouse, researchers measured the activity of specific neurons in the mouse hippocampus (the region of the brain where new memories form) known to encode locations.

When an animal was first placed in the track, he did not know what to do and wandered from left to right until he came across sweet water. In these cases, the individual neurons were activated when the mouse noticed a symbol on the wall. But after many experiments with the track, the mouse became familiar with it and remembered the locations of the sugar. As the mouse became more familiar, more and more neurons were activated synchronously by seeing each symbol on the wall. Essentially, the mouse recognized the position of each unique symbol.

To study how memories fade over time, the researchers then selected mice from the track for up to 20 days. Back on the track after this break, the mice that had formed strong memories encoded by a higher number of neurons quickly remembered the task. Although some neurons showed a different activity, the mouse memory of the track was clearly identifiable when analyzing the activity of large groups of neurons. In other words, the use of neuronal groups allows the brain to be redundant while recalling memories, even if some of the original neurons are silent or damaged.

Gonzalez explains, "Imagine that you have a long and complicated story to tell.To preserve the story, you can tell it to five of your friends, and then meet with each of them. between them to tell it again and help each other, fill in the gaps that an individual had forgotten, and each time you tell a new story, you can bring new friends to learn and contribute to preserve it and strengthen the memory.In a similar way, your own neurons help each other, to encode memories that will persist in time. "

Memory is so fundamental in human behavior that any alteration of memory can have serious consequences on our daily lives. The memory loss associated with normal aging can be a major handicap for the elderly. In addition, memory loss caused by several diseases, including Alzheimer's disease, has devastating consequences that can interfere with the most basic routines, including recognizing relatives or remembering the way home. These works suggest that memories fade more quickly with age, because a memory is encoded by fewer neurons and, if one of these neurons goes down, the memory is lost. The study suggests that someday, designing treatments that could stimulate the recruitment of a higher number of neurons to code a memory could help prevent memory loss.

"People have known for years that the more action you take, the better your chances of remembering it later," says Lois. "We now think that this is likely because the more action you take, the higher the number of neurons coding the action.The classic theories on memory storage postulate that creating a more stable memory requires stronger connections to an individual neuron Our results suggest that increasing the number of neurons encoding the same memory allows the memory to last longer. "

The document is entitled "Persistence of neuronal representations through time and damage to the hippocampus".