[Textbook homunculus diagram depicts how the brain controls
individual body parts — the revamp could improve treatments for
brain injury.]
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SUNDAY SCIENCE: FAMOUS ‘HOMUNCULUS’ BRAIN MAP REDRAWN TO INCLUDE
COMPLEX MOVEMENTS
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Max Kozlov
April 19, 2023
Nature [[link removed]]
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_ Textbook homunculus diagram depicts how the brain controls
individual body parts — the revamp could improve treatments for
brain injury. _
The homunculus or ‘little man’ is a foundational concept in
neuroscience., Colin McConnell/Toronto Star via Getty
The bizarre-looking ‘homunculus’ is one of neuroscience
[[link removed]]’s most fundamental
diagrams. Found in countless textbooks, it depicts a deformed
constellation of body parts mapped onto a narrow strip of the brain,
showing the corresponding brain regions that control each part.
But a study published in _Nature_
[[link removed]]1
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19 April reveals that this brain strip, called the primary motor
cortex, is much more complex than the famous diagram suggests. It
might coordinate complex movements involving multiple muscles through
connections to brain regions responsible for critical thinking,
maintaining the body’s physiology and planning actions. The new
results could help scientists better understand and treat brain
injuries.
“This study is very interesting and very important,” says Michael
Graziano, a neuroscientist at Princeton University in New Jersey.
It’s becoming clear that the primary motor cortex isn’t “just a
simple roster of muscles down the brain that control the toes to the
tongue”, he says.
Little man in the brain
The idea of the homunculus dates to the late nineteenth century, when
researchers noticed that electrically stimulating the primary motor
cortex [[link removed]] corresponded to
specific body parts twitching. Later work found that some body parts,
such as the hands, feet and mouth, took up a disproportionate amount
of space in the primary motor cortex compared with the rest of the
body. In 1937, these findings culminated with the first publication of
the motor homunculus, which translates to ‘little man’ in Latin.
Neurosurgeon Wilder Penfield’s 1948 diagram of the motor homunculus
(left) shows the areas of the primary motor cortex that control each
body part. A new study redraws the diagram (right), adding regions
connected to brain areas responsible for coordinating complex
movements.Credit: E. Gordon et al./Nature
As tempting as the idea of a ‘little man inside the brain’ is,
Graziano says that more recent evidence suggests the homunculus
isn’t completely accurate. For example, a 2002 study
[[link removed](02)00698-0]2
[[link removed]] that
he co-authored found that stimulating the primary motor cortex of
monkeys caused the animals to carry out coordinated movements rather
than simple muscle twitches.
And Angela Sirigu, a neuroscientist at the Institute of Cognitive
Science Marc Jeannerod in Lyon, France, and her colleague proposed a
new theory in 2008 [[link removed]]3
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based on data from people whose arms had been amputated above the
elbow. They suggested that there are two systems in the primary motor
cortex: one for motor commands and another for muscle synergies that
enable coordinated movement.
Quality glimpse
In the latest study, Nico Dosenbach, a neuroscientist at Washington
University in St. Louis, Missouri, and his colleagues used functional
magnetic resonance imaging (fMRI)
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which measures the changes in blood flow that occur with brain
activity, to scan individuals while they were either resting or moving
different parts of their body. The researchers devoted hours of
scanning time to each person, which afforded a high-quality glimpse of
the primary motor cortex.
Some aspects of the original motor homunculus checked out: there were
separate regions that controlled the movements of the foot, hand and
face. But the researchers also found three areas interspersed between
these regions that were strongly connected with each other and
connected to other parts of the brain responsible for goal-driven
action planning and other tasks such as regulating blood pressure and
pain. These interspersed regions were not specific to any one body
part or movement and were activated during action planning.
Three coloured spots on each half of the brain show areas that were
strongly connected to other regions responsible for goal-driven action
planning and tasks such as regulating blood pressure and pain. The
hotter the colour, the denser the connections.Credit: Evan
Gordon/Washington University
The findings weren’t a fluke: the researchers found the same regions
lighting up in large data sets from previously scanned individuals.
And when they scanned children, they found that a newborn hadn’t
developed this brain network yet, whereas an 11-month-old and a
9-year-old had, supporting the theory that this network coordinates
complex action plans, because newborns aren’t yet able to control
their movements precisely, Dosenbach says.
The study represents a prime example of the power of fMRI, says
Michael Fox, a neurologist at Brigham and Women’s Hospital in
Boston, Massachusetts. “We had no idea this secret system of brain
regions was there hiding in plain sight until functional brain imaging
came along,” he says.
Targets for therapy
The findings could lead to changes in therapy for disorders of the
primary motor cortex caused by stroke or injury. Many of the brain
areas currently targeted for neurostimulation, a technique used to
rehabilitate people with movement disorders, came from trial and
error, Fox says. The new homunculus could help explain how and why
certain targets work, and identify new or better targets.
Understanding how people recover from damage to the primary motor
cortex is really important, says Dylan Edwards, a specialist in
neurorehabilitation at the Moss Rehabilitation Research Institute in
Philadelphia, Pennsylvania. These findings could help “tailor
treatments that might be aligned with specific patterns of
deficits”, he says.
Given how long researchers have been probing the motor cortex, “we
thought we knew everything about this region”, says Sirigu. “But
its organization is much more complex than we have traditionally
thought.”
_doi: [link removed]
References
*
Gordon, E. M. _et
al._ _Nature_ [link removed] (2023).
Article [[link removed]] Google Scholar
[[link removed].]
*
Graziano, M. S. A., Taylor, C. S. R. & Moore, T. _Neuron_ 34,
841–851 (2002).
Article [[link removed]] PubMed
[[link removed]] Google
Scholar
[[link removed].]
*
Reilly, K. T. & Sirigu, A. _Neuroscientist_ 14, 195–200 (2008).
Article [[link removed]] PubMed
[[link removed]] Google
Scholar
[[link removed].]
_MAX KOZLOV writes for Nature as a life-sciences reporter. His work
has also appeared in The Atlantic, Quanta Magazine, The Scientist, The
St. Louis Post-Dispatch, Behavioral Scientist, and The Public’s
Radio. In his free time, he loves to rock climb, book last-minute
travel, and devour just about every journalistic piece he comes
across._
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