andrew richardson - mit.edu fileandrew richardson mcgovern institute for brain research march 14,...
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Biological motor control
Andrew RichardsonMcGovern Institute for Brain ResearchMarch 14, 2006
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Why bother with biology?
Both plant (musculoskeletal system) and controller (nervous system) optimized by evolution for versatility and efficiency.
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Motor neuroscience: levels and themes
Mechanics of the neuromusculoskeletal systemSimplification of control by neural modulation of the mechanics
Motor behaviorModel-based adaptive control in changing environmentsCoordinate frames and methods for movement planning
Motor neurophysiologyNeural representations in motor cortexCortically-controlled neural prosthetics
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Mechanics of the neuromusculoskeletal system
velocityte
nsio
n
lengthening shortening
muscle length
tens
ion
Force production by a muscle is dependent on its length and velocity.
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Mechanics of the neuromusculoskeletal system
Changes in muscle activation cause changes in the viscoelastic properties of the muscle.
muscle length
tens
ion
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The nervous system receives information on muscle length, velocity, and force.
golgi tendon organ
muscle spindle
Mechanics of the neuromusculoskeletal system
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Reflexes are local feedback loops that can modify the viscoelastic behavior of the motor periphery.
Mechanics of the neuromusculoskeletal system
Reflex gains can be modulated by higher levels of the neural controller (e.g. cerebellum, motor cortex).
Nichols and Houk, 1976
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Thus, total neuromuscular viscoelasticity has both intrinsic (i.e. muscle) and reflexive contributions. The gain of each of these contributions can be modulated by central commands.
At least for some behaviors, the neural controller likely takes advantage of these mechanical properties …
1. Equilibrium-point (servo) control2. Impedance control
Mechanics of the neuromusculoskeletal system
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Equilibrium-point control
Equilibrium points dependent on neuromuscular elasticity & loads.Changing the stiffness ratio for antagonistic muscles shifts theequilibrium point (producing a “virtual trajectory”). Precludes need to compute inverse dynamics, thus simplifying neural computations.
Mechanics of the neuromusculoskeletal system
flexo
r ten
sion
extensor tension
joint angleBizzi et al, 1984
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Mechanics of the neuromusculoskeletal system
Impedance control
In addition to viscoelastic behavior, inertial behavior can be modulated due to the kinematically redundant skeleton.Modulate full mechanical impedance (force-length, force-velocity, and force-acceleration relationships) of the limb for improved stability or performance.
Burdet et al, 2001
Hogan, 1985
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Motor neuroscience: levels and themes
Mechanics of the neuromusculoskeletal systemSimplification of control by neural modulation of the mechanics
Motor behaviorModel-based adaptive control in changing environmentsCoordinate frames and methods for movement planning
Motor neurophysiologyNeural representations in motor cortexCortically-controlled neural prosthetics
![Page 12: Andrew Richardson - mit.edu fileAndrew Richardson McGovern Institute for Brain Research March 14, 2006. Why bother with biology? Both plant (musculoskeletal system) and controller](https://reader031.vdocument.in/reader031/viewer/2022041401/5e178216aa4f8b750247faae/html5/thumbnails/12.jpg)
Motor behavior
Adaptive control of reaching movements generally leads to proactive, not reactive (i.e. impedance control), compensation.
B = 0 B = 0B =
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Motor behavior
Lackner and DiZio, 1994
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Motor behavior
Control of reaching movements may depend on the type of task (e.g. nominally stable or unstable) and familiarity with the task.
Osu et al, 2003But how do we plan motions?
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Motor behavior
Motions are planned in endpoint coordinates.
Endpoint velocityis “bell-shaped”
Joint trajectories are complex
Morasso, 1981
Endpoint hand paths are straight
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Motor behavior
Optimization criteria have been proposed to explain the observed behavior, given the seemingly ill-posed problem of getting from point A to point B.
Minimum jerk (Hogan, 1984)
Minimum torque change (Uno et al, 1987)
Minimum endpoint variance (Harris and Wolpert, 1998)
Endpoint hand paths are straight
Endpoint velocity is “bell-shaped”
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Motor behavior
But production of a fixed “desired trajectory”, whether through optimization or not, does no account for some features of motor behavior.
For example, it doesn’t predict increased variability in dimensions of state-space that are not relevant to the task.
Todorov et al, 2005
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Todorov, 2004
Motor behavior
Optimal feedback control may provide a better description of motor planning, as well as motor execution.
But what is the cost function?
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Motor neuroscience: levels and themes
Mechanics of the neuromusculoskeletal systemSimplification of control by neural modulation of the mechanics
Motor behaviorModel-based adaptive control in changing environmentsCoordinate frames and methods for movement planning
Motor neurophysiologyNeural representations in motor cortexCortically-controlled neural prosthetics
![Page 20: Andrew Richardson - mit.edu fileAndrew Richardson McGovern Institute for Brain Research March 14, 2006. Why bother with biology? Both plant (musculoskeletal system) and controller](https://reader031.vdocument.in/reader031/viewer/2022041401/5e178216aa4f8b750247faae/html5/thumbnails/20.jpg)
Motor neurophysiology
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Motor neurophysiology
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Motor neurophysiology
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Motor neurophysiology
What is encoded in motor cortex – kinematics or dynamics?Neural recording experiments
Kakei et al, 1999
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What is encoded in motor cortex – kinematics or dynamics?Neural stimulation experiments
Graziano, 2002
Motor neurophysiology
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Nicolelis, 2001
Motor neurophysiology
Engineering application: cortically-controlled neural prosthetics
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Motor neurophysiology
Neural control of a robot
Wessberg et al, 2000