How memories form and fade

Why are some memories stable over decades, while others fade within minutes?

New US research with mice suggests lasting memories are encoded by groups of neurons firing in synchrony, providing redundancy that enables these memories to persist over time. 

“The conventional theories about memory storage postulate that making a memory more stable requires the strengthening of the connections to an individual neuron,” says Walter Gonzalez, who led the work at California Institute of Technology.

“Our results suggest that increasing the number of neurons that encode the same memory enables the memory to persist for longer.” 

To come to that conclusion, they measured the activity of specific neurons in a mouse’s brain that are known to encode for places while the mouse moved around a 1.5-metre track that had unique symbols on the walls at different locations and sugar water at each end. 

Initially a mouse roamed until chancing upon the water, at which time single neurons were activated when it took notice of a symbol on the wall. However, as it came more familiar with the enclosure over repeat visits, more neurons were activated in synchrony by seeing each symbol on the wall. 

Essentially, the researchers say, the mouse was recognising where it was with respect to each unique symbol.

A mouse was then kept away from the track for 20 days. Upon returning, those 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 analysing the activity of large groups of neurons.

In other words, the researchers say, 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.

This, they add, suggests memories might fade more rapidly as we age because a memory is encoded by fewer neurons, and if any of these neurons fail, the memory is lost. 

Designing treatments that boost the recruitment of a higher number of neurons to encode a memory could, therefore, potentially help prevent memory loss.

“For years, people have known that the more you practice an action, the better chance that you will remember it later,” says co-author Carlos Lois. 

“We now think that this is likely, because the more you practice an action, the higher the number of neurons that are encoding the action.”

The study is published in the journal Science.