(Image caption: The
hippocampus is necessary for forming new memories, and researchers from
Arizona State University and Stanford University have found another role
for this brain structure in learning. The hippocampus (purple) sent
important information to brain areas responsible for learning, like the
striatum (teal). Specifically, the hippocampus kept track of
associations between features in the environment, like the fact that
poison ivy plants, with their three leaflets, are red in the fall and
spring but are green in the summer. This association between color and
leaf shape is necessary to learn to identify poison ivy and not mistake
it for plants that look similar, like boxelder (not pictured) that also
has three leaflets but is green in the spring. Image credit (top)
“Poison ivy” by Stilfehler is licensed under CC BY-SA 3.0. (bottom)
“Poison ivy foliage during autumn leaf coloration in Ewing, New Jersey”
by Famartin is licensed under CC BY-SA 4.0)

More than just memories: A new role for the hippocampus during learning

Avid hikers know to be cautious of plants with leaves made up
of three leaflets if they are red in the spring or fall. Parents
worldwide know the precarious relationship between proximity to bedtime
and roughhousing with their children.

How do hikers know to
link the color of a leaf with the season to determine if it is poison
ivy? How do parents know to link the time of day with a child’s
excitement level to determine the success of a bedtime routine? Just
like Pavlov’s dogs salivated when a bell rang, people learn to recognize
poisonous plants or prevent toddler tears of exhaustion by forming
associations among details in their surroundings and what happens.

Researchers from Arizona State University and
Stanford University analyzed patterns of brain activity in humans and
discovered a previously unknown role for the hippocampus, a brain area
important to memory, in forming associations during learning. The study
was published on March 6 in Nature Communications.

“This
study has important implications for understanding how the brain’s
memory systems contribute to learning and decision-making” said Vishnu
Murty, an assistant professor of psychology at Temple University who was
not involved with the research. “The findings could help us understand
the complexity of decision-making deficits in populations where the
focus has mostly been on memory deficits, like Alzheimer’s disease,
other psychopathology disorders and normal aging.”

When people
learn, they build associations among features — like the time of day and
mood of a toddler — to predict an outcome, like whether or not
roughhousing will end in tears.

“These associations have a big
influence on behavior,” said Ian Ballard, who recently earned his
doctorate in neuroscience from Stanford University and is first author
on the paper. “But in the real world, objects or events are defined by
more than one feature or combinations of features, and we wanted to
understand how the brain builds associations over similar configurations
of features.”

To unpack how the brain might handle the problem of
building associations over such complex, real-world information, the
research team focused on a small brain structure that looks like a snail
shell.

A tiny yet powerful brain area

The
hippocampus is approximately one-third the size of a gumball, and this
small brain structure is crucial for memory formation. Without it,
people cannot form new memories about facts or events, like what day it
is or the names of coworkers. During memory formation, the hippocampus
represents individual details of an event, like where you parked your
car, to be as different as possible from each other.

“One
challenge with memory is that it is hard to distinguish similar
experiences. So, if you use the same parking garage at work every day,
you have to remember which floor and space to go to at the end of the
day,” said Samuel McClure, associate
professor of psychology at ASU and senior author on the paper. “The
problem is that it is easy to confuse where you parked on different
days. The hippocampus is critical for remembering the combination of
where and when.”

How the hippocampus forms memories provided a
mechanism for how complex combinations of features could be represented
in the brain, but whether the small, coiled structure actually
contributed to how people learn about the world was an open question.

To
identify how the hippocampus might contribute to how people form
associations in the real world, the researchers designed a learning task
that required participants to use combinations of features to predict
whether an outcome would happen. A series of stimulus images, like a
single face or a face paired with a building, appeared sequentially on a
screen. The participants then had to predict whether a target image
would appear after the stimulus images. The goal for the task was to
respond as quickly as possible to any target image that showed up.

It
was only the combinations of two stimulus pictures, like a face paired
with a townhouse, that that could be used to predict when a target would
appear. The individual stimuli, like the face image, were not useful
for predicting by themselves.

As the participants worked through
the task, the research team used functional magnetic resonance imaging
(fMRI) to measure brain activity from the hippocampus and other brain
structures known to be involved in learning. The team then examined the
patterns of activity during the task and noticed something interesting
about the activity in the hippocampus. It was the only brain structure
that represented the stimulus images bound together, which is important
because success on the task required forming associations about
combinations to accurately respond to the target images.

“We found
the hippocampus uniquely represented bound features: faces and houses
were distinct from a face and house mingled together,” Ballard said.

A new way the hippocampus contributes to learning

When
the research team looked at how the patterns of activity in the
hippocampus were related to other brain areas, they found hippocampal
activity was tightly correlated with activity in the striatum. Nestled
beneath the cortex, the striatum consists of three separate structures —
the caudate, putamen and nucleus accumbens — and plays an important
role in learning what predicts desirable outcomes.

“The
hippocampus formed bound associations of multiple features that
supported learning in the striatum about the configuration of multiple
features in the environment,” Ballard said.

How the striatum
handles information during learning is well known, but where all the
information comes from is an open question. Ballard added that this
study starts to chip away at that question by showing the hippocampus
provided information about combinations of features to the striatum and
that information was used to learn how to succeed at the task.

Until
recently, the brain was thought to have separate learning systems, but
the findings suggest the hippocampal memory system and striatal
reinforcement learning system are interrelated.

“It is really
important to think of the brain as an interconnected structure, with
different parts that work together to produce our impressive mental
feats. Neuroscience and psychology have done a good job at understanding
how individual parts work. It is exciting to start trying to figure out
how they begin to work together,” McClure said. “We suspect that
understanding both normal and diseased mental functions requires
figuring out how all of the parts work together ­— or not.”



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