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Princeton University researchers have used a novel virtual reality and brain imaging system to
detect a form of neural activity underlying how the brain forms short-term
memories that are used in making decisions.
By following the brain activity of mice as they navigated a virtual reality
maze, the researchers found that populations of neurons fire in distinctive
sequences when the brain is holding a memory. Previous research centered on the
idea that populations of neurons fire together with similar patterns to each other
during the memory period.
The study was performed in the laboratory of David Tank, who is Princeton’s Henry L. Hillman Professor in Molecular
Biology and co-director of the Princeton Neuroscience Institute. Both Tank and
Christopher Harvey, who was first author on the paper and a postdoctoral
researcher at the time of the experiments, said they were surprised to discover
the sequential firing of neurons. The study was published online in Nature.
The findings give insight into what happens in the brain during
“working memory,” which is used when the mind stores information for
short periods of time prior to acting on it or integrating it with other
information. Working memory is a central component of reasoning, comprehension
and learning. Certain brain disorders such as schizophrenia are thought to
involve deficits in working memory.
“Studies such as this one are aimed at understanding the basic
principles of neural activity during working memory in the normal brain.
However, the work may in the future assist researchers in understanding how
activity might be altered in brain disorders that involve deficits in working
memory,” said Tank.
In the study, the patterns of sequential neuronal firing corresponded to
whether the mouse would turn left or right as it navigated a maze in search of
a reward. Different patterns corresponded to different decisions made by the
mice, the Princeton researchers found.
The sequential neuronal firing patterns spanned the roughly 10-sec period
that it took for the mouse to form a memory, store it and make a decision about
which way to turn. Over this period, distinct subsets of neurons were observed
to fire in sequence.
The finding contrasts with many existing models of how the brain stores
memories and makes decisions, which are based on the idea that firing activity
in a group of neurons remain elevated or reduced during the entire process of
observing a signal, storing it in memory and making a decision. In that
scenario, memory and decision-making is determined by whole populations of
neurons either firing or not firing in the region of the brain involved in navigation
and decision-making.
The uniqueness of the left-turn and right-turn sequences meant that the
brain imaging experiments essentially allowed the researchers to perform a
simple form of “mind reading.” By imaging and examining the brain
activity early in the mouse’s run down the maze, the researchers could identify
the neural activity sequence being produced and could reliably predict which
way the mouse was going to turn several seconds before the turn actually began.
The sequences of neural activity discovered in the new study take place in
a part of the brain called the posterior parietal cortex. Previous studies in
monkeys and humans indicate that the posterior parietal cortex is a part of the
brain that is important for movement planning, spatial attention and
decision-making. The new study is the first to analyze it in the mouse.
“We hope that by using the mouse as our model system we will be able to
utilize powerful genetic approaches to understand the mechanisms of complex
cognitive processes,” said Harvey.
Navigating the maze
Princeton researchers studied these neurons
firing in the posterior parietal cortex of mice while they navigated a maze in
search of a reward. The simple maze, generated using a virtual reality system,
consisted of a single long corridor that ended in a T-intersection, requiring
the mouse to choose to turn left or right.
As the mouse ran down the long corridor, it saw visual patterns and object
signals on the right or left side of the corridor, like a motorist driving down
a highway might see a sign indicating which way to turn at the T-intersection.
If the mouse turned in the direction indicated by the signal, it found the
reward of a drink of water.
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The experimental setup required that the mouse notice the signal and
remember which side of the corridor the signal was on so that it could make the
correct decision when it reached the T-intersection. If it turned the wrong
way, the mouse would not find the reward. After several training runs, the mice
made the right decision more than 90% of the time.
In cases where the mice made errors, the neuronal firing started out with
one distinct pattern of sequential firing and then switched over to another
pattern. If the mouse saw a signal indicating that it should turn right but
made a mistake and turned left, its brain started off with the sequence
indicating the visual cues for a future right turn but then switched over to
the sequence indicative of a future left turn. “In these cases, we can
observe the mouse changing its memory of past events or plans for future
actions,” said Tank.
The mouse training and imaging experiments were conducted by Harvey, who is
now an assistant professor of neurobiology at Harvard Medical
School. Harvey was assisted in some experiments by
Philip Coen, a graduate student in the Princeton Neuroscience Institute.
Constructing a virtual reality
In place of a physical maze, the researchers created on using virtual reality,
an approach that has been under development in the Tank lab for the last
several years. The mice walked and ran on the surface of a spherical treadmill
while their head remained stationary in space, which is ideal for brain
imaging. Computer-generated views of virtual environments were projected onto a
wide-angle screen surrounding the treadmill. Motion of the ball produced by the
mouse walking and turning was detected by optical sensors on the ball’s equator
and used to change the visual display to simulate motion through a virtual
environment.
To image the brain, the researchers employed an optical microscope that
used infrared laser light to look deep below the surface in order to visualize
a population of neurons and record their firing.
The neurons imaged in these mice contained a “molecular sensor”
that glows green when the neurons fire. The sensor, developed in the lab of
Loren Looger, group leader at the Howard Hughes Medical Institute’s Janelia
Farm Research Campus, consisted of a green fluorescent protein engineered to
glow in response to calcium ions, which flood into the neuron when it fires.
The green fluorescent protein (GFP) from which the sensor was developed is
widely used in biological research and was discovered at Princeton in 1961 by
former Princeton researcher Osamu Shimomura, who earned a Nobel Prize in chemistry
in 2008 for the discovery.
The virtual reality system, combined with the imaging system and calcium
sensor, allowed the researchers to see populations of individual neurons firing
in the working brain. “It is like we are opening up a computer and looking
inside at all of the signals to figure out how it works,” said Tank.
These studies of populations of individual neurons, termed
cellular-resolution measurements, are challenging because the brain contains
billions of neurons packed tightly together. The instrumentation developed by
the Tank lab is one of the few that can record the firing of groups of
individual neurons in the brain when a subject is awake. Most studies of brain
function in humans involve studying activity in entire regions of the brain
using a tool such as magnetic resonance imaging (MRI) that average together the
activity of many thousands of neurons.
“The data reveal quite clearly that at least some form of
short-term memory is based on a sequence of neurons passing the
information from one to the other, a sort of ‘neuronal bucket brigade,'”
said Christof Koch, a neuroscientist who was not involved in the study. Koch is
the chief scientific officer for the Allen Institute for Brain Science in
Seattle and the Lois and Victor Troendle Professor of Cognitive and Behavioral
Biology at the California Institute of Technology in Pasadena.
The development and application of new technologies for measuring and
modeling neural circuit dynamics in the brain is the focus of Princeton’s new Bezos Center
for Neural Circuit Dynamics. Created with a gift of $15 million from Princeton alumnus Jeff Bezos, the founder and chief
executive officer of Amazon.com, and alumna MacKenzie Bezos, the center
supports the study of how neural dynamics represent and process information
that determines behavior.
