Memory replay prioritizes high-reward memories
not others? Our brains prioritize rewarding memories over
others, and reinforce them by replaying them when we are
at rest, according to new research from the University of
California, Davis, Center for Neuroscience, published Feb. 11
in the journal Neuron .
"Rewards help you remember things, because you want
future rewards," said Professor Charan Ranganath, a UC
Davis neuroscientist and senior author on the paper. "The
brain prioritizes memories that are going to be useful for
future decisions."
It's estimated that we only retain detailed memories for a
small proportion of the events of each day, Ranganath
said. People with very detailed memories become
overwhelmed with information. So if the brain is going to
filter information and decide what to remember, it makes
sense to save those memories that might be most important
for obtaining rewards in the future.
Ranganath and postdoctoral researcher Matthias Gruber
put this to the test by scanning the brains of volunteers by
functional magnetic resonance imaging as they answered
simple yes-no questions on short series of objects -- for
example, "do these objects weigh more than a basketball?"
Each series of objects was shown on a background image
for context, and depending on the context, the volunteers
were told they would either get a large (dollars) or small
(cents) reward for giving correct answers. At the end of
a series, participants were told how much money they just
won.
Once participants completed this part of the experiment,
the volunteers were scanned during a resting period.
Afterward, outside of the scanner, there was a surprise
memory test for all objects that were shown during
scanning.
Although participants were not expecting the memory test
outside the scanner, they were better at remembering
objects that were associated with a high reward, said
Gruber, first author of the paper.
"Also, when an object was associated with high reward,
people remembered better the particular background scene
that was on the screen during scanning," Gruber said.
Memory could be biased toward high points of
experience
Even more interesting, participants' memory performance
was predicted by brain activity measured during rest. When
the researchers looked at brain scans of subjects at rest
after giving yes-no answers -- neither learning nor actively
recalling the memory -- they found the same pattern of
activity as when subjects were doing the high-reward task.
The subjects were apparently replaying the rewarding
memories, strengthening connections and helping to fix the
memory in place.
People who showed more replay of high-reward memories
showed better retention of these events during the post-
scan test, as well as increased interactions between the
hippocampus, a structure deep in the brain heavily involved
in memory, and an area called the substantia nigra/ventral
tegmental area complex, which is involved in reward
processing, suggesting that reward played a role in
stimulating the hippocampus after learning.
Although this study did not measure it directly, these
interactions were likely related to release of dopamine, a
neurotransmitter that is released in the brain when we
expect rewards. Conditions such as Parkinson's disease or
aging are linked to reduced dopamine and often involve
memory defects.
The results show how memory could be biased toward the
high points of experience, Ranganath said. "It speaks to a
memory process that is normally hidden from us,"
Ranganath said. "Are you remembering what you really need
to know? It could depend on what your brain does while
you are at rest."
Co-authors on the study were Maureen Ritchey, Shao-Fang
Wang and Manoj Doss, all at the UC Davis Center for
Neuroscience. The work was supported by multiple sources
including the Office of Naval Research, NIH, the
Guggenheim Foundation, a Parke-Davis Fellowship and the
German Research Foundation.
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