NEW
YORK, March 28, 2024 /PRNewswire/ --
Neuroscientists have established in recent decades the idea that
some of each day's experiences are converted by the brain into
permanent memories during sleep the same night. Now, a new study
proposes a mechanism that determines which memories are tagged as
important enough to linger in the brain until sleep makes them
permanent.
Led by researchers at NYU Grossman School of Medicine, the new
study revolves around brain cells called neurons that "fire" – or
bring about swings in the balance of their positive and negative
charges – to transmit electrical signals that encode memories.
Large groups of neurons in a brain region called the hippocampus
fire together in rhythmic cycles, creating sequences of signals
within milliseconds of each other that can encode complex
information.
Called "sharp wave-ripples," these "shouts" to the rest of the
brain represent the near-simultaneous firing of 15 percent of
hippocampal neurons, and are named for the shape they take when
their activity is captured by electrodes and recorded on a graph.
While past studies had linked ripples with memory formation
during sleep, the new study, published online in the journal
Science on March 28, found
that daytime events followed immediately by five to 20 sharp
wave-ripples are replayed more during sleep and so consolidated
into permanent memories. Events followed by very few or no sharp
wave-ripples failed to form lasting memories.
"Our study finds that sharp wave-ripples are the physiological
mechanism used by the brain to 'decide' what to keep and what to
discard," said senior study author György Buzsáki, MD, PhD, the
Biggs Professor of Neuroscience in the Department of Neuroscience
and Physiology at NYU Langone Health.
Walk and Pause
The new study is based on a known pattern: mammals including
humans experience the world for a few moments, then pause, then
experience a little more, then pause again. After we pay attention
to something, say the study authors, brain computation often
switches into an "idle" re-assessment mode. Such momentary pauses
occur throughout the day, but the longest idling periods occur
during sleep.
Buzsaki and colleagues had previously established that no sharp
wave-ripples occur as we actively explore sensory information or
move, but only during the idle pauses before or after. The current
study found that sharp wave-ripples represent the natural tagging
mechanism during such pauses after waking experiences, with the
tagged neuronal patterns reactivated during post-task sleep.
Importantly, sharp wave-ripples are known to be made up the
firing of hippocampal "place cells" in a specific order that
encodes every room we enter, and each arm of a maze entered by a
mouse. For memories that are remembered, those same cells fire at
high speed, as we sleep, "playing back the recorded event thousands
times per night." The process strengthens the connections between
the cells involved.
For the current study, successive maze runs by study mice were
tracked via electrodes by populations of hippocampal cells that
constantly changed over time despite recording very similar
experiences. This revealed for the first time the maze runs during
which ripples occurred during waking pauses, and then were replayed
during sleep.
Sharp wave-ripples were typically recorded when a mouse paused
to enjoy a sugary treat after each maze run. The consumption of the
reward, say the authors, prepared the brain to switch from an
exploratory to an idle pattern so that sharp wave-ripples could
occur.
Using dual-sided silicon probes, the research team was able to
record up to 500 neurons simultaneously in the hippocampus of
animals during maze runs. This in turn created a challenge because
data becomes exceedingly complex the more neurons are independently
recorded. To gain an intuitive understanding of the data, visualize
neuronal activity, and form hypotheses, the team successfully
reduced the number of dimensions in the data, in some ways like
converting a three-dimensional image into a flat one, and without
losing the data's integrity.
"We worked to take the external world out of the equation, and
looked at the mechanisms by which the mammalian brain innately and
subconsciously tags some memories to become permanent," said first
author Wannan (Winnie) Yang, PhD, a
graduate student in Buzsáki's lab. "Why such a system evolved
is still a mystery, but future research may reveal devices or
therapies that can adjust sharp wave-ripples to improve memory, or
even lessen recall of traumatic events."
Along with Drs. Buzsáki and Yang, study authors from the
Neuroscience Institute at NYU Langone Health were Roman Huszár and
Thomas Hainmueller. Kirill Kiselev
of the Center for Neural Science at New York
University was also an author, as was Chen Sun of Mila, the Quebec Artificial
Intelligence Institute, in Montréal. The work was supported by
National Institute of Health grants R01MH122391 and
U19NS107616.
Contact: Gregory Williams,
gregory.williams@nyulangone.org
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SOURCE NYU Langone Health System