| When chronic, hemiparetic stroke survivors
make reaching movements while lifting the paretic arm against
gravity, their ability to generate the necessary independent
joint movements degrades dramatically due to abnormal muscle
coactivation patterns that couple shoulder abduction with
elbow flexion. The neural mechanisms behind the appearance
of abnormal coordination patterns during post-stroke recovery
are largely unknown, but they are possibly related to a loss
in cortical resolution and an increased usage of undamaged,
indirect descending motor pathways via the brainstem. In order
to investigate the underlying mechanisms, we have developed
a novel experimental setup that simultaneously records electroencephalographic
(EEG) signals while the test subject makes different reaching
movements with an ACT3D robot. This method allows us to map
brain activity during controlled reaching movements with different
levels of robot-mediated limb support for the first time.
Our results so far provide evidence for changes in cortical
activity driving realistic upper-extremity reaching movements
as independent joint control becomes compromised in stroke
survivors. Stroke survivors show deterioration of reaching
kinematics, such as movement trajectory and velocity, as the
level of robot-mediated gravitational support is decreased,
reflecting the loss of independent joint control. This produces
corresponding increases in the size of activation in secondary
motor cortices and increases in shifts of centers of activity
to the ipsilateral hemisphere in comparison with control subjects.
Analysis of activity in specific sensorimotor cortices such
as primary motor, premotor, supplementary motor, and primary
sensory cortex, also shows differences in the timecourse of
activations between stroke and control subjects.
This study is the first to explore the neural mechanisms
behind brain reorganization and the loss of independent joint
control during realistic reaching movements under different
gravitational support conditions in moderate to severely affected
chronic stroke subjects. The understanding of these mechanisms
is expected to provide a rational basis for the design of
new targeted rehabilitation protocols that will teach stroke
survivors to gradually overcome the negative effects of gravity,
thereby reducing abnormal joint coordination.
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