depiction of human brain

Memories: How Do They Form and Fade?

Have you ever wondered why some of your childhood memories are still fresh in your mind even after decades, while some recent ones fade in minutes? Researchers have recently discovered the neural processes that cause some memories to fade quickly while making other memories stable over time.

Using mouse models, researchers from California Institute of Technology have determined that strong, stable memories are encoded by “teams” of neurons all working in synchrony, providing redundancy that enables these memories to stay over time. The study helps in understanding how brain damage due to strokes or Alzheimer’s disease may affect memory.

Published in the journal, Science, the study was conducted at Biology research professor, Carlos Lois’s laboratory. The professor is also an affiliated faculty member of the Tianqiao and Chrissy Chen Institute for Neuroscience at Caltech.

The team, led by Walter Gonzalez, a postdoctoral scholar developed a test to examine mice’s neural activity as they learn about and remember a new place. In the test, mice explored a 5-feet-long enclosure where unique symbols denoted different locations along its white walls. A treat (sugar water) for mice was place at both ends of the track. The activity of specific neurons in the mouse hippocampus (the region of the brain where new memories are formed) known to encode for places, was measured while the mouse walked around.

The researcher noted that when a mouse was first put in the track, it was not certain about what to do and so moved left and right until it came across the treat. In these cases, when a mouse took notice of a wall symbol, single neurons were activated. But over several experiences with the track, the mouse became familiar with it and remembered the site of the treat. As it became more familiar, more and more neurons were synchronously activated by seeing each symbol on the wall. Basically, the mouse was recognizing its own location with respect to each unique symbol.

In order to investigate how memories fade over time, the researchers then withheld mice from the enclosure for up to 20 days. Upon coming back to the track after the sabbatical, mice that had formed strong memories encoded by higher numbers of neurons remembered the task promptly. The mouse’s memory of the track was clearly identifiable when analyzing the activity of large groups of neurons, in spite of some neurons showing different activity. Alternatively, using groups of neurons enables the brain to recall memories while having redundancy, even if some of the original neurons fall silent or are damaged.

Gonzalez clarifies, “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.”

While earlier theories about memory storage suggest that making a memory more stable requires the strengthening of the connections to an individual neuron, this study proposes that increasing the number of neurons encoding the same memory enables the memory to stay for longer. The study has great implications for designing future treatment that could boost the recruitment of a higher number of neurons to encode a memory, and could help prevent memory loss.