Strong memories are
encoded by teams of neurons working together in synchrony
California Institute of Technology --
August 23. 2019 -- Why is it that you can remember the name of your childhood
best friend that you haven't seen in years yet easily forget the name of a
person you just met a moment ago? In other words, why are some memories stable
over decades, while others fade within minutes?
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. Lois is also an
affiliated faculty member of the Tianqiao and Chrissy Chen Institute for
Neuroscience at Caltech.
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.
Gonzalez explains: "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."
Memory is so fundamental to human
behavior that any impairment to memory can severely impact our daily life.
Memory loss that occurs as part of normal aging can be a significant handicap
for senior citizens. Moreover, memory loss caused by several diseases, most
notably Alzheimer's, has devastating consequences that can interfere with the
most basic routines including recognizing relatives or remembering the way back
home. This work suggests that 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. The study suggests that one day, designing treatments that
could boost the recruitment of a higher number of neurons to encode a memory
could 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 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 conventional theories about memory storage postulate that
making a memory more stable requires the strengthening of the connections to an
individual neuron. Our results suggest that increasing the number of neurons
that encode the same memory enables the memory to persist for longer."
The paper is titled "Persistence of
neuronal representations through time and damage in the hippocampus." In
addition to Gonzalez and Lois, co-authors are undergraduate Hanwen Zhang and
former lab technician Anna Harutyunyan. Funding was provided by the American
Heart Association, the Della Martin Foundation, the Burroughs Wellcome Fund,
and a BRAIN Initiative grant from the National Institute of Neurological
Disorders and Stroke.
Story Source:
Materials provided by California
Institute of Technology. Original written by Lori Dajose. Note: Content
may be edited for style and length.
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