Scientists have long known the brain's hippocampus is crucial for long-term
memory. Now a new Northwestern Medicine study has found the hippocampus also
plays a role in short-term memory and helps guide decision-making.
The findings shed light on how the hippocampus contributes to memory and
exploration, potentially leading to therapies that restore hippocampal
function, which is impacted in memory-related aging and neurodegenerative
diseases such as dementia, the study authors said.
In the study, scientists monitored participants' brain activity and tracked
their eye movements while looking at different complex pictures. The
scientists discovered that as we visually scan our environment and absorb
new information, our hippocampus becomes activated, using short-term memory
to better process new visual information to help us rapidly reevaluate
situations.
How our memory helps us scan new environments
Imagine walking down the street and noticing an awkwardly parked car on your
neighbor's lawn. Maybe you quickly dismiss it and move on. But when you see
an ambulance and fire truck approaching your location, you connect the dots
and look back to see the scene of an accident. By using short-term memory to
guide where you look, the hippocampus allows you to reexamine the car and
form a lasting memory of the accident.
"At any given moment, your brain rapidly initiates eye movements that you
are typically unaware of," said corresponding author James Kragel, a
postdoctoral research fellow at Northwestern University Feinberg School of
Medicine. "Our findings suggest the hippocampus uses memory to inform where
your eyes look, thereby priming the visual system to learn and reevaluate
our environment on the fly.
"If you didn't look back and see the crash, you might not encode that
important information, but in using short-term memory retrieval, you can tie
those clues together and remember details that cue bigger memories. It all
comes down to building connections among these disparate elements that allow
you to remember them later in a much easier way."
The study was published June 18 in the journal Science Advances.
"These findings emphasize that although hippocampal-dependent memory is
typically considered a thing of the past, in fact, it operates in the moment
to optimize our behavior and decision-making," said senior study author Joel
Voss, associate professor of medical social sciences, neurology, and
psychiatry and behavioral sciences at Feinberg. "This is key to
understanding hippocampal function and developing effective treatments for
memory disorders."
"It is as if you are using your memory to plan for what to expect, and then
when it mismatches with what is actually unfolding, your hippocampus gets
activated to reevaluate and update your current perception of what is going
on," Kragel said.
Tracking eye movements to learn more about memory
The study was conducted on patients with epilepsy who were undergoing
neurosurgical monitoring at Northwestern Memorial Hospital to localize the
source of their seizures. They had electrodes implanted in their brains to
map seizure-related brain activity. During their stay in the
epilepsy-monitoring unit, participants performed a memory task in which they
studied lists of complex scenes with multiple people and objects (e.g.
someone sitting at a park bench with food on the table, things happening in
the background) followed by a memory test.
During the test, the participants indicated whether a presented scene was
old or new. Throughout the task, the authors simultaneously recorded eye
movements and neural activity to link hippocampal activity to memory-guided
behaviors.
When studying a scene for the first time, participants often returned their
gaze to a location they had just viewed hundreds of milliseconds prior.
These "revisitation" eye movements enhanced spatiotemporal memory for scenes
(remembering where an object was located or the sequence in which something
happened). Brain recordings revealed the brain networks involved in
generating these "revisitations," as hippocampal activity shifted just
before their execution. Increases in brain activity followed revisitations,
which Kragel believes may form a lasting memory of the scene and its
elements.
"This shows that the hippocampal contribution to memory unfolds over just
hundreds of milliseconds during ongoing behavior, which is surprising given
that the timecourse of its involvement, typically seen in long-term memory
retrieval, is usually thought to be days to years," Voss said.
Reference:
Kragel JE, Schuele S, VanHaerents S, Rosenow JM, Voss JL. Rapid coordination
of effective learning by the human hippocampus. Sci Adv.
2021;7(25):eabf7144. doi:
10.1126/sciadv.abf7144