Using mouse models, Caltech researchers have now determined that strong, stable memories are encoded by “teams” of neurons all firing in synchrony, providing redundancy that enables these memories to persist over time. The research has implications for understanding how memory might be affected after brain damage, such as by strokes or Alzheimer’s disease. The work was done in the laboratory of Carlos Lois, research professor of biology, and is described in a paper that appears in the August 23 of the journal Science.
Led by postdoctoral scholar Walter Gonzalez, the team developed a test to examine mice’s neural activity as they learn about 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 locations along the walls — for example, a bold plus sign near the right-most end and an angled slash near the center. Sugar water (a treat for mice) was placed at either end of the track. While the mouse explored, the researchers measured the activity of specific neurons in the mouse hippocampus (the region of the brain where new memories are formed) that are known to encode for places.
When an animal was initially placed in the track, it was unsure of what to do and wandered left and right until it came across the sugar water. In these cases, single neurons were activated when the mouse took notice of 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 familiar, more and more neurons were activated in synchrony by seeing each symbol on the wall. Essentially, the mouse was recognizing where it was with respect to each unique symbol.
To study how memories fade over time, the researchers then withheld the mice from the track for up to 20 days. Upon returning to the track after this break, mice that had formed strong memories encoded by higher numbers of neurons remembered the task quickly. Even though some neurons showed different activity, the mouse’s memory of the track was clearly identifiable when analyzing the activity of large groups of neurons. In other words, using groups of neurons enables the brain to have redundancy and still recall memories even if some of the original neurons fall silent or are damaged.