BRAIN'S COMPASS RELIES ON GEOMETRIC RELATIONSHIPS
The brain has a
complex system for keeping track of which direction you are facing as you move
about; remembering how to get from one place to another would otherwise be
impossible. Researchers from the University of Pennsylvania have now shown how
the brain anchors this mental compass.
Their findings
provide a neurological basis for something that psychologists have long
observed about navigational behavior: people use geometrical relationships to
orient themselves.
The research, which
is related to the work that won this year's Nobel Prize in Physiology or
Medicine, adds new dimensions to our understanding of spatial memory and how it
helps us to build memories of events.
The study was led by
Russell Epstein, a professor of psychology in Penn's School of Arts &
Sciences, and Steven Marchette, a postdoctoral fellow in Epstein's lab. Also
contributing to the study were lab members Lindsay Vass, a graduate student,
and Jack Ryan, a research specialist.
It was published in Nature
Neuroscience.
"Imagine coming
out of a subway stop," said Marchette. "You know exactly where you
are in the world, but you still have the experience of looking around to figure
out which way you are facing. You might think, 'I'm looking at city hall, so I
must be facing east.' It takes a second before it clicks.
"We're
interested in how people are able to reset their sense of direction in the
world and what cues they rely upon in the environment to do that."
To test how the
brain makes these inferences, the researchers designed an experiment in which
they introduced participants to a virtual environment, a set of four museums in
a park, and had the participants memorize the location of the everyday objects
on display in those museums. They then scanned their brains while asking them
to recall the spatial relationships between those objects, such as whether the
bicycle was to the left or the right of the cake.
In the scans, using
a technique that measures blood flow to different regions of the brain known as
fMRI, the researchers focused on a region known as the retrosplenial complex.
People who have severe injuries to this region are able to recognize landmarks
in their environments but are unable to recall how to get from one to another,
suggesting that it plays a specific role in the type of memory used in
navigation and orientation.
"The
retrosplenial complex is very much underexplored," Epstein said.
"While we don't have the ability to go in and look at individual neurons,
like O'Keefe and the Mosers did in their Nobel Prize-winning work, one of the
nice things about fMRI is that we didn't have to decide beforehand which areas
of the brain to record from."
There are three ways
the retrosplenial complex could conceivably encode this type of information and
serve as part of a mental compass.
One way would be a
"global" system, in which the brain tracks the absolute direction one
is facing regardless of visual cues in the environment. In fact, there is good
evidence that such a system exists in the brain, but the Penn team doubted that
the retrosplenial complex was the central component of it.
An
"idiosyncratic" system, in which the brain keeps tracks of direction
for each environment independently, was another possibility. In such a system,
remembering that your desk is on the north wall of your office would involve
recalling the room itself and picking out the relevant features.
Finally, they
considered a "geometric" system that is based on more generalized
relationships between features in an environment. There, remembering that your
desk is on the north wall of your office would involve recalling the
relationship between the desk and the door -- say, the desk is on the left when
I enter the room -- without having to specifically recall the room itself.
The architecture of
the team's virtual park was critical for being able to distinguish which of
these three types of systems participants were using to orient themselves in
regards to the objects.
The park's four
museums were laid out in a cloverleaf pattern around a central plaza, which
itself could only be approached from the south. Each museum had a single door,
all of which faced the center of the plaza. Each museum was visually distinct
but all were identical in layout: a single room containing eight unique
objects, two on each wall. The objects were contained in niches, such that
participants could only see them from straight ahead.
"We designed it
this way so that it was clear to the participants that each museum's back wall
pointed in one of the cardinal directions," said Marchette. "And by
placing the objects in the niches, we ensured that they could only see them
when they were looking due north, south, east or west."
After being allowed
to freely roam around the virtual environment, participants were tested about
the locations of the objects. They were asked to return to the lab a day or two
later, where they were given the opportunity to refresh their memories about
the layout of the objects before entering the MRI scanner. There, they were
shown words representing a pair of objects that were found in one of the
museums and asked whether the second object was to the left or the right of the
first.
The researchers used
half of a participant's responses to calibrate their measurement of that
participant's retrosplenial complex and then compared the activation patterns
they saw there to responses in that participant's other half.
"If the
retrosplenial complex supported a global system," Marchette said,
"then it shouldn't matter whether people are imagining facing the back
wall or the left wall; if you're looking north in one museum and north in the
other, the activation patterns should be similar. As we expected, that doesn't
happen.
"Likewise, for
an idiosyncratic system, we would expect that remembering the back walls of two
different museums would produce dissimilar patterns, since you would be
remembering the room itself. That doesn't happen either."
Instead, the
patterns look similar when participants imagined looking at objects that have
the same geometric relationship to the surrounding room, regardless of the
"true" direction the participant was facing. For example, remembering
objects on the back walls of two different museums produced similar activation
patterns, even though the back wall is north in one museum and east in the
other.
"We can even
reconstruct the location the participant is remembering based on those
similarities," Epstein said. "Once we know what we are looking for
based on the first half of a participant's responses, we can estimate the
location of a given view entirely from the fMRI data, and they are reasonably
close to where the views actually are. That's a pretty cool result. It's as if
we can read out a 'floor plan' of the museums from each person's brain. And
because the museums are geometrically identical, the retrosplenial cortex uses
the same 'floor plan' for all of them"
The team's research
provides a more complete picture of what is happening in the brain when people
navigate from one place to another.
"Psychologists
have long surmised that geometry is important for this kind of memory,"
Epstein said, "but this is beginning to show the neurological basis for
it. We hope this opens the door for a deeper look at this region of the
brain."
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