Why can you remember the name of your childhood best friend who you haven't seen in years and yet easily forget the name of a person you just met? In other words, why are some memories stable for decades, while others fade within minutes?
Using mouse models, researchers have now determined that strong, stable memories are encoded by "teams" of neurons that all fire in sync, providing redundancy that allows these memories to persist over time. The research has implications for understanding how memory may be affected after brain damage, such as from stroke or Alzheimer's disease.
Led by a scientist, the team developed a test to examine the neural activity of mice as they learn and remember a new place. In the test, a mouse was placed in a straight housing, about 5 feet long with white walls. Unique symbols marked different locations along the walls – for example, a bold plus sign at the far right end and a slash near the center. Sugar water (a treat for mice) was placed at both ends of the track. As the mouse explored, the researchers measured the activity of specific neurons in the mouse's hippocampus (the area of the brain where new memories are formed) that are known to code for sites.
When an animal was initially placed in the track, it was unsure of what to do and ran left and right until it encountered the sugar water. In these cases, some neurons were activated when the mouse saw a symbol on the wall. But over multiple experiences with the track, the mouse became familiar with it and remembered the locations of the sugar. As the mouse became more famous, more and more neurons were activated in sync by seeing each symbol on the wall. Essentially, the mouse recognized where it was with respect to each unique symbol.
To study how memories fade over time, the researchers then deprived the mice of the track for up to 20 days. Upon returning to the circuit after this pause, mice that had strong memories encoded by a greater number of neurons quickly recalled the task. Although some neurons showed different activity, mouse memory of the track was clearly identifiable when analyzing the activity of large groups of neurons. In other words, using groups of neurons allows the brain to have redundancy and still recall memories even if some of the original neurons are silent or damaged.
The researcher explains:“Imagine you have a long and complicated story to tell. To preserve the story, you could tell it to five of your friends and then get together once in a while to retell the story and help each other fill in any gaps a person had forgotten. In addition, each time you retell the story, you can bring new friends to learn and therefore help preserve it and strengthen memory. In an analogous way, your own neurons help each other out to encode memories that last over time.”
Memory is so fundamental to human behavior that any impairment of memory can have serious consequences for our daily lives. This work suggests that memories fade faster as we age because a memory is encoded by fewer neurons, and if one of these neurons fails, the memory is lost. The study suggests that one day designing treatments that can stimulate the recruitment of a greater number of neurons for a memory may help prevent memory loss.
