Long-term memories rely on both the repetition of events and an intricate neurological learning process in making these memories last, shows a new study by a team of neuroscientists. Its findings provide a more detailed understanding of how these types of memories are formed as well as insights into what may disrupt their creation.
“Repetition is a well-documented trigger for memory formation—the more times something is repeated, the better it is remembered,” explains New York University’s Nikolay V. Kukushkin, the lead author of the study, which appears in the journal Proceedings of the National Academy of Sciences (PNAS). “However, the brain’s machinery is more complicated than that. Our research shows that the effects of individual repeated events interact in more nuanced ways and have distinct roles in working to form long-term memories—neurons can sense not just repetition, but also the order of repeated experiences and can use that information to discriminate between different patterns of these events in building memories.”
“For example, neurons can tell the
difference between two events in escalating order of intensity and those same
two events in the opposite order, forming a memory only if the intensity
increases over time,” he adds.
The researchers, who also included
Thomas Carew, a professor in NYU’s Center for Neural Science, and Tasnim
Tabassum, an NYU researcher, sought to better understand what lies behind a
well-documented neurological process—specifically, that repeated events induce
long-term memory where individual events fail to do so.
What has been unclear is how repeated
events interact with one another to form a memory.
To explore this question, the scientists
studied Aplysia
californica, the California sea slug. Aplysia is a model organism for this type of research
because its simple memories are well understood at the molecular and cellular
level. Neurons that control them can be isolated and studied in a Petri dish,
as the study’s authors did here, reproducing all the essential components of
memory formation. The researchers “trained” these neurons by applying repeated
chemical pulses that replicated Aplysia’s
responses to stimuli, such as mild electric shocks, typically used in
experiments. They then monitored the long-term strengthening of connections
between the neurons, thereby mimicking and then observing the formation of a
long-term memory.
“Two-trial learning is a technique in
which Aplysia, or even isolated Aplysia neurons,
can be made to form a long-term memory after two experiences,” explains
Kukushkin, a researcher at NYU’s Center for Neural Science and a clinical
assistant professor in Liberal Studies at NYU. “Single trials have no effect,
but two trials, if they are appropriately spaced in time, do.”
As part of these experiments, the
researchers specifically examined the resulting activity of the protein ERK,
which is required for memory.
Previously, scientists had thought that
ERK activation should build up during the learning process. But in the PNAS study, the researchers found a more complicated
dynamic: a “tug of war” between molecules that activate ERK (and therefore
favor memory) and those that deactivate it (and therefore oppose memory).
After only a single trial, they note,
the deactivating side of the “tug of war” prevailed and ERK activity was
arrested, preventing memory formation. By contrast, a second trial was required
to prevent the decrease in ERK activity, thereby allowing memories to take
hold.
The scientists used different variations
of the training procedure—alterations that differentially affected memory
depending on the pattern of stimulation. They modified the “intensity” of
individual training events by varying the concentration of chemicals used to
mimic electric shocks. When the two-trial training included events of different
intensity, only the “weak-strong” pattern of training produced long-term
memory, whereas the inverse, “strong-weak” sequence, failed to do so. In other
words, the same combination of trials only had an effect if it increased in intensity,
but not if it decreased in intensity, over time. This might represent an
evolutionary adaptation to prioritize memory of escalating stimuli, the
scientists suggest—as they point out, events that escalate in intensity have
more predictive power than those that decrease in intensity.
“Long-term memory formation, then,
depends on which of the two competing sides of the ERK tug of war wins over
time,” explains Kukushkin. “But perhaps more significantly, the work
demonstrates that effects of repeated events do not simply accumulate. In fact,
they have distinct roles, such as to initiate and confirm the commitment of
information to long-term memory. Neurons can sense not just repetition, but the
order of stimuli, and they use that information to discriminate between
different patterns of experience.”
Journal article: https://www.pnas.org/doi/full/10.1073/pnas.2210478119
Source: Long-term
memories a matter of order–not just repetition – Scents of Science
(myfusimotors.com)
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