Scientists have long struggled to figure out how the
brain guides the complex movement of our limbs, from the
graceful leaps of ballerinas to the simple everyday act of
picking up a cup of coffee. Using tools from robotics and
neuroscience, two Johns Hopkins researchers have found some
tantalizing clues in an unlikely mode of motion: the
undulations of tropical fish.
Their findings, published in the Jan. 31 issue of
The Journal of Neuroscience, shed new light on the
communication that takes place between the brain and body.
The fish research may contribute to important medical
advances in humans, including better prosthetic limbs and
improved rehabilitative techniques for people suffering
from strokes, cerebral palsy and other debilitating
conditions.
"All animals, including humans, must continually make
adjustments as they walk, run, fly or swim through the
environment. These adjustments are based on feedback from
thousands of sense organs all over the body, providing
vision, touch, hearing and so on. Understanding how the
brain processes this overwhelming amount of information is
crucial if we want to help people overcome pathologies,"
said Noah
Cowan, an assistant professor of mechanical engineering
in the Whiting School of Engineering. In studying the fish
and preparing the Journal of Neuroscience paper, Cowan
teamed up with
Eric
Fortune, assistant professor of psychological and brain
sciences in the Krieger School of Arts and Sciences.
Cowan and Fortune focused on the movements of a small
nocturnal South American fish called the "glass knifefish"
because of its almost transparent blade-shaped body. This
type of fish does something remarkable: It emits weak
electrical signals that it uses to "see" in the dark.
According to Fortune, several characteristics, including
this electric sense, make the fish a superb subject for the
study of how the brain uses sensory information to control
locomotion.
The glass knifefish emits weak
electrical signals that help it 'see' in the
dark.
Photo by Will Kirk / HIPS
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"These fish are ideal both because we can easily
monitor the sensing signals that their brains use and
because the task we asked the fish to do — swim
forward and backward inside a small tube — is very
simple and straightforward," said Fortune, who also uses
the fish to study the neural basis and evolution of
behavior.
The fish prefer to "hide" inside these tubes, which
are immersed in larger water tanks. In their research,
Cowan and Fortune challenged the fish's ability to remain
hidden by shifting the tubes forward and backward at
varying frequencies. This required the fish to swim back
and forth more and more rapidly in order to remain inside
the tubes. But as the frequency became higher, the fish
gradually failed to keep up with the movement of the
tubes.
The team's detailed engineering analysis of the fish's
adjustments under these conditions suggested that the
animal's sensors and brains are "tuned" to consider
Newton's laws of motion, Cowan said. In other words, the
team found that the fish's nervous system measured
velocity, so the fish could accelerate or "brake" at just
the right rate to remain within the moving tube.
"The fish were able to accelerate, brake and reverse
direction based on a cascade of adjustments made through
their sensory and nervous systems, in the same way that a
driver approaching a red light knows he has to apply the
brakes ahead of time to avoid overshooting and ending up in
the middle of a busy intersection," Fortune said. "Your
brain has to do this all the time when controlling movement
because your body and limbs, like a car, have mass. This is
true for large motions that require planning, such as
driving a car, but also for unconscious control of all
movements, such as reaching for a cup of coffee. Without
this sort of predictive control, your hand would knock the
cup off the table every time."
The researchers' understanding of the complex
relationship between the glass knifefish's movements and
the cascade of information coming into its brain and body
via its senses could eventually spark developments in areas
as far reaching as medicine and robotics.
"That animals unconsciously know that they have mass
seems obvious enough, but it took a complex analysis of a
very specialized fish to demonstrate this," Fortune said.
"With this basic knowledge, we hope one day to be able to
'tune' artificial systems, such as prosthetics, so that
they don't have the jerky and rough movements that most
robots have, which is critical for medical
applications."
The team's use of both neuroscience and engineering
principles and tools also make this project important for
other reasons.
"So far, we have used a series of engineering analyses
to tease apart some important information about how the
nervous system works," Cowan said. "As we move forward, we
expect to discover other exciting aspects of brain function
that suggest new ways to design sensory control systems for
autonomous robots."
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