Research Could Lead To Improvements In Prosthetic Limbs And Robots
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
University researchers have found some tantalizing clues in an unlikely
mode of motion: the undulations of tropical fish.
Their findings, published in the January 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 Johns Hopkins' Whiting School of
Engineering. In studying the fish and preparing the Neuroscience paper,
Cowan teamed up with Eric Fortune, assistant professor of psychological
and brain sciences in the Krieger School of Arts and Sciences, also at
Johns Hopkins.
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 which it uses to "see" in
the dark. According to Fortune, several characteristics, including this
electric sense, make this fish a superb subject for the study of how
the brain uses sensory information to control locomotion.
"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 systems 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
their brains and bodies via their 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 it an important project 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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