replays that behavior repeatedly until
it is internalized. They also report on
how the hippocampus tracks our
brain’s decision-making centers to
remember our past choices.
Previous research has shown that
the rodent hippocampus replays or
revisits past experiences during sleep
or periods of rest. While a rat navi-
gates a maze, for example, so-called
place cells are activated and help the
animal track its position. Following
their journey through the maze, those
same cells are reactivated in the
exact same pattern. What previously
happened is mentally replayed again.
The authors of the new study were
curious if this phenomenon only
applies to previous encounters with
a particular location or if perhaps this
hippocampal replay also applies to
memory more generally, including
mental and nonspatial memories.
It turns out it does.
In the study, 33 participants were
presented with a series of images
containing both a face and a house.
They had to judge the age of either
one or the other. If during the second
trial, the age of the selected option
remained the same, the judged
category also did not change in the
subsequent trial. If the ages differed,
the judged category flipped to the
other option in the next round.
While engaged in these tasks, the
subjects underwent functional
magnetic resonance imaging (fMRI),
which allows researchers to monitor
brain metabolic activity related to
areas of the brain that are active at a
given time. In this case, the imaging
allowed the authors to view the
patterns of activity in different brain
regions before, during and after the
decision task.
Following 40 minutes of this
exercise, participants underwent
brain scanning at rest for five
minutes. The fMRI patterns recorded
in the hippocampus at rest seemed
to re-create snippets of activity that
occurred during the decision-making
task. And they did so again and
again. It is as if the brain keeps
rewinding a movie scene until it can
recite it by heart.
Further, the new research found
that people who had more replay in
their hippocampus exhibited activity
that was more similar to the task in a
brain region called the orbitofrontal
cortex, an area involved in deci-
sion-making. “We found the fact that
[activity] in the orbitofrontal cortex
relates to replay in the hippocampusreally amazing,” says Nicolas Schuck,
lead author of the paper and a
neuroscientist at the Max Planck
Institute for Human Development in
Berlin. He feels the findings suggest
that replay of task and decision
sequences in the resting hippocam-
pus helps train the cortex to better
solve similar tasks in the future.
According to the paper’s senior
author, Princeton University neuro-
scientist Yael Niv, the new results
suggest that replay in the hippocam-
pus is critical not only to forming
memories but also to learning which
of our behaviors and decisions are
most effective at accomplishing a
goal and so should be repeated.Thackery Brown, a professor in the
school of psychology at the Georgia
Institute of Technology, who was not
involved in the study, finds the new
work critical in demonstrating that
the hippocampus supports replay of
not just navigation and spatial
experiences but also decision-mak-
ing. “Although spatial navigation is
essential for daily life in both humans
and animals,” he explains, “we often
remember sequences of experiences
that take place over time as well as
space. The memory of our child’s
birthday party, for example, may
involve many events unfolding over
time at one location.” Once we are
back home on the couch, the
hippocampus, it seems, may binge-
watch the party on repeat, commit-
ting it to memory.
The long-term fate of memories
depends on connections between
hippocampal neurons and those in
the high-level cognitive-processing
regions of the cortex, as the new
work shows. Brown believes hippo-
campal activity, in particular the
replay described by Schuck and Niv,
helps strengthen those bonds so
that they provide long-lasting traces
we can access, using a variety of
memory cues. “This type of ‘neuralNEWS
“We found the
fa c t that [a c t iv it y]
in the
orbitofrontal
cortex relates to
replay in the
hippocampus
really amazing.”
—Nicolas Schuck