DiagnosticsPractice Makes Perfect - Motor Memory Possible For Neuroprosthetic Control
"Practice makes perfect" is the maxim drummed into students struggling to
learn a new motor skill - be it riding a bike or developing a killer
backhand in tennis. In order to become proficient in any motor task, all
that practice must eventually modify the performer"s nervous system so
that
stable motor memories of the physical actions are formed. In this week"s
issue of PLoS Biology, new research reveals that macaque monkeys can
achieve a kind of consolidation of motor memory when using a
neuroprosthetic device to complete a motor action. The finding could have
a role in
increasing the ease with which physically disabled people can master the
control of artificial limbs and other disembodied devices.
Scientists at the University of California, Berkeley, taught macaque
monkeys to use their own brain signals to move a computer cursor to
various
targets using a prosthetic device, and as the monkeys learned to
proficiently use the robotic device, the researchers monitored their brain
signals.
The scientists learned that the brain develops a mental map of the
solution to the task, and that the neural patterns persisted without
deviation,
much like a driver sticks to a given route commuting to work.
"When your own body performs motor tasks repeatedly, the movements become
almost automatic," said study principal investigator Jose Carmena. "The
profound part of our study is that this is all happening with something
that is not part of one"s own body. We have demonstrated that the brain is
able to form a motor memory to control a disembodied device in a way that
mirrors how it controls its own body. That has never been shown before."
Researchers have previously demonstrated that rodents, non-human primates
and humans are able to control robotic devices or computer cursors in real
time using only brain signals. But what had not been clear before was
whether such a skill had been consolidated as a motor memory. The new
study
suggests that the brain is capable of creating a stable, mental
representation of a disembodied device so that it can be controlled with
little
effort.
To demonstrate this, Carmena and Karunesh Ganguly, a post-doctoral fellow
in Carmena"s laboratory, used a mathematical model, or "decoder." The
decoder, analogous to a simplified spinal cord, translated the signals
from the brain"s motor cortex into movement of the cursor.
It took about four to five days of practice for the monkeys to master
precise control of the cursor. Once they did, they completed the task
easily and
quickly for the next two weeks.
As the tasks were being completed, the researchers were able to monitor
the changes in activity of individual neurons involved in controlling the
cursor. They could tell which cells were active when the cursor moved in
specific directions. The researchers noticed that when the animals became
proficient at the task, the neural patterns involved in the "solution"
stabilized.
There are three major features scientists associate with motor memory;
stability - once a motor memory is consolidated it is difficult to change,
rapid recall upon demand, and resistance to interference when new skills
are learned. All three elements were demonstrated by the macaques in the
UC
Berkeley study.
To test resistance to interference, the researchers presented a new
decoder - marked by a different colored cursor - two weeks after the
monkeys
showed mastery of the first decoder.
As the monkeys were mastering the second decoder, the researchers would
suddenly switch back to the original decoder and saw that the monkeys
could
immediately perform the task without missing a beat. The ability to switch
back and forth between the two decoders shows a level of neural plasticity
never before associated with the control of a prosthetic device.
"This is a study that says that maybe one day, we can really think of the
ultimate neuroprosthetic device that humans can use to perform many
different tasks in a more natural way," said Carmena.
"Beyond its clinical applications this line of research sheds light on how
the brain assembles and organizes neurons, and how it forms a motor memory
to control the prosthetic device," Carmena said. "These are important,
fundamental questions about how the brain learns in general."
Funding:
This work was supported by the
American Heart Association and the American Stroke Association
to KG and the Christopher and Dana
Reeve Foundation to JMC. The funders had
no
role in study design, data collection and analysis, decision to publish,
or preparation of the manuscript.
Competing interests statement:
The authors declare that no competing interests exist.
Citation:
"Emergence of a Stable Cortical Map for Neuroprosthetic Control."
Ganguly K, Carmena JM (2009)
PLoS Biol 7(7):e1000153. doi:10.1371/journal.pbio.1000153
PLoS Biology