Why can you remember the name of your best friend from childhood, whom you have not seen in years, and easily forget the name of a person you just met? In other words, why are some memories stable for decades while others fade in minutes?
Using mouse models, Caltech researchers have now discovered that strong, stable memories are encoded by "teams" of neurons, all firing in sync and providing redundancy that allows these memories to persist over time. Research has implications for understanding how memory can be affected after brain damage such as stroke or Alzheimer's.
The work was carried out in the laboratory of Carlos Lois, Research Professor of Biology, and is described in a paper that appears in the journal Science on August 23. Lois is also a member of the affiliated faculty of the Tianqiao and Chrissy Chen Institute of Neuroscience in Caltech.
Under the guidance of postdoctoral researcher Walter Gonzalez, the team developed a test to study the neuronal activity of mice as they learn and remember about a new place. In the test, a mouse was placed in a straight case about 5 feet long with white walls. Unique icons marked various places on the walls ̵
When an animal was originally put in the lane, it was not sure what to do and it wandered left and right until it hit the sugar water. In these cases, individual neurons were activated when the mouse noticed a symbol on the wall. However, through several experiences with the trail, the mouse became familiar with it and remembered the locations of the sugar. With increasing awareness of the mouse, more and more neurons were activated synchronously, making each symbol visible on the wall. In essence, the mouse recognized where it was with respect to each symbol.
To study how memories fade over time, researchers kept the mice off track for up to 20 days. On returning to the trail after this break, 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 lane was clearly recognizable in the analysis of the activity of large groups of neurons. In other words, the use of groups of neurons allows the brain to have redundancy and still remember memories, even if some of the original neurons are dumb or damaged.
Gonzalez explains: "Imagine, you have a long and complicated story to tell. To keep the story, you can tell them to five friends and then occasionally sit down with everyone to tell the story again and help each other fill in the gaps that a person has forgotten. In addition, every time you tell the story again, you can bring new friends to learn, and thus help preserve them and strengthen memory. Analogously, one's own neurons help each other in the coding of memories that persist over a longer period of time.
Memory is so basic to human behavior that any impairment of memory can significantly affect our daily lives. Memory loss that occurs as part of normal aging can be a major obstacle for seniors. In addition, the loss of memory due to various illnesses, especially Alzheimer's disease, has devastating consequences that can affect the most basic routines, including identifying relatives or remembering the way home. This work suggests that memories may fade faster with increasing age because memory is encoded by fewer neurons, and if one of these neurons fails, the memory is lost. The study suggests that one day, the development of treatments that could promote the recruitment of a higher number of neurons to encode a memory could help prevent memory loss.
Maybe you'll remember that later, "says Lois. "We now believe that this is probable, because the more you practice an action, the more neurons code the action. The conventionalories of memory storage postulate that stabilizing a memory requires strengthening the connections to a single neuron. Our results suggest that increasing the number of neurons encoding the same memory makes the memory last longer.
The article is titled "Persistence of Neuronal Representations by Time and Damage in the Hippocampus". Co-authors include Gonzalez and Lois, as well as student Hanwen Zhang and former laboratory technician Anna Harutyunyan. Funding was provided by the American Heart Association, the Della Martin Foundation, the Burroughs Wellcome Fund, and a grant from the BRAIN Initiative from the National Institute of Neurological Disorders and Stroke.