From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Motor_controlMotor control is the regulation of movement in organisms that possess a nervous system. Motor control includes reflexes as well as directed movement.
To control movement, the nervous system must integrate multimodal sensory information (both from the external world as well as proprioception) and elicit the necessary signals to recruit muscles to carry out a goal. This pathway spans many disciplines, including multisensory integration, signal processing, coordination, biomechanics, and cognition, and the computational challenges are often discussed under the term sensorimotor control.
Successful motor control is crucial to interacting with the world to
carry out goals as well as for posture, balance, and stability.
Some researchers (mostly neuroscientists studying movement, such as Daniel Wolpert and Randy Flanagan) argue that motor control is the reason brains exist at all.
Neural control of muscle force
All movements, e.g. touching your nose, require motor neurons to fire action potentials that results in contraction of muscles.
In humans, ~150,000 motor neurons control the contraction of ~600
muscles. To produce movements, a subset of 600 muscles must contract in a
temporally precise pattern to produce the right force at the right
time.
Motor units and force production
A single motor neuron and the muscle fibers it innervates are called a motor unit. For example, the rectus femoris
contains approximately 1 million muscle fibers, which are controlled by
around 1000 motor neurons. Activity in the motor neuron causes
contraction in all of the innervated muscle fibers so that they function
as a unit. Increasing action potential frequency (spike rate) in the
motor neuron increases the muscle fiber contraction force, up to the
maximal force. The maximal force depends on the contractile properties of the muscle
fibers. Within a motor unit, all the muscle fibers are of the same type
(e.g. type I (slow twitch) or Type II fibers (fast twitch)),
and motor units of multiple types make up a given muscle. Motor units
of a given muscle are collectively referred to as a motor pool.
The force produced in a given muscle thus depends on: 1) How many
motor neurons are active, and their spike rates; 2) the contractile
properties and number of muscle fibers innervated by the active neurons.
To generate more force, increase the spike rates of active motor
neurons and/or recruiting more and stronger motor units. In turn, how
the muscle force produces limb movement depends on the limb biomechanics,
e.g. where the tendon and muscle originate (which bone, and precise
location) and where the muscle inserts on the bone that it moves.
Recruitment order
Motor units within a motor pool are recruited in a stereotypical order,
from motor units that produce small amounts of force per spike, to
those producing the largest force per spike. The gradient of motor unit
force is correlated with a gradient in motor neuron soma size and motor
neuron electrical excitability. This relationship was described by Elwood Henneman and is known as Henneman's size principle, a fundamental discovery of neuroscience and an organizing principle of motor control.[8]
For tasks requiring small forces, such as continual adjustment of
posture, motor units with fewer muscle fibers that are
slowly-contracting, but less fatigueable, are used. As more force is
required, motor units with fast twitch, fast-fatigeable muscle fibers
are recruited.
High|
| _________________
Force required | /
| |
| |
| _____________|_________________
| __________|_______________________________
Low|__________|__________________________________________
↑ ↑ ↑ Time
Type I Recruit first Type II A Type IIB
Computational issues of motor control
The
nervous system produces movement by selecting which motor neurons are
activated, and when. The finding that a recruitment order exists within a
motor pool is thought to reflect a simplification of the problem: if a
particular muscle should produce a particular force, then activate the
motor pool along its recruitment hierarchy until that force is produced.
But then how to choose what force to produce in each muscle? The
nervous system faces the following issues in solving this problem.
- Redundancy. Infinite trajectories of movements can accomplish a
goal (e.g. touch my nose). How is a trajectory chosen? Which trajectory
is best?
- Noise. Noise is defined as small fluctuations that are unrelated to a
signal, which can occur in neurons and synaptic connections at any
point from sensation to muscle contraction.
- Delays. Motor neuron activity precedes muscle contraction, which
precedes the movement. Sensory signals also reflect events that have
already occurred. Such delays affect the choice of motor program.
- Uncertainty. Uncertainty arises because of neural noise, but also
because inferences about the state of the world may not be correct (e.g.
speed of on coming ball).
- Nonstationarity. Even as a movement is being executed, the state of
the world changes, even through such simple effects as reactive forces
on the rest of the body, causing translation of a joint while it is actuated.
- Nonlinearity. The effects of neural activity and muscle contraction
are highly non-linear, which the nervous system must account for when
predicting the consequences of a pattern of motor neuron activity.
Much ongoing research is dedicated to investigating how the nervous
system deals with these issues, both at the behavioral level, as well as
how neural circuits in the brain and spinal cord represent and deal
with these factors to produce the fluid movements we witness in animals.
"Optimal feedback control" is an influential theoretical framing of these computation issues.
Model systems for motor control
All organisms face the computational challenges above, so neural circuits for motor control have been studied in humans, monkeys, horses, cats, mice, fish lamprey, flies, locusts, and nematodes,
among many others. Mammalian model systems like mice and monkeys offer
the most straightforward comparative models for human health and
disease. They are widely used to study the role of higher brain regions
common to vertebrates, including the cerebral cortex, thalamus, basal
ganglia and deep brain medullary and reticular circuits for motor
control. The genetics and neurophysiology of motor circuits
in the spine have also been studied in mammalian model organisms, but
protective vertebrae make it difficult to study the functional role of
spinal circuits in behaving animals. Here, larval and adult fish have
been useful in discovering the functional logic of the local spinal
circuits that coordinate motor neuron activity. Invertebrate model
organisms do not have the same brain regions as vertebrates, but their
brains must solve similar computational issues and thus are thought to
have brain regions homologous to those involved in motor control in the
vertebrate nervous system, The organization of arthropod
nervous systems into ganglia that control each leg as allowed
researchers to record from neurons dedicated to moving a specific leg
during behavior.
Model systems have also demonstrated the role of central pattern generators in driving rhythmic movements.
A central pattern generator is a neural network that can generate
rhythmic activity in the absence of an external control signal, such as a
signal descending from the brain or feedback signals from sensors in
the limbs (e.g. proprioceptors). Evidence suggests that real CPGs exist in several key motor control regions, such as the stomachs of arthropods or the pre-Boetzinger complex
that control breathing in humans. Furthermore, as a theoretical
concept, CPGs have been useful to frame the possible role of sensory
feedback in motor control.
Sensorimotor feedback
Response to stimuli
The process of becoming aware of a sensory stimulus and using that information to influence an action occurs in stages. Reaction time of simple tasks can be used to reveal information about these stages. Reaction time refers to the period of time between when the stimulus is presented, and the end of the response. Movement time is the time it takes to complete the movement. Some of the first reaction time experiments were carried out by Franciscus Donders,
who used the difference in response times to a choice task to determine
the length of time needed to process the stimuli and choose the correct
response.
While this approach is ultimately flawed, it gave rise to the idea that
reaction time was made up of a stimulus identification, followed by a
response selection, and ultimately culminates in carrying out the
correct movement. Further research has provided evidence that these
stages do exist, but that the response selection period of any reaction
time increases as the number of available choices grows, a relationship
known as Hick's law.
Closed loop control
The classical definition of a closed loop system for human movement comes from Jack A. Adams (1971).
A reference of the desired output is compared to the actual output via
error detection mechanisms; using feedback, the error is corrected for.
Most movements that are carried out during day-to-day activity are
formed using a continual process of accessing sensory information and
using it to more accurately continue the motion. This type of motor
control is called feedback control,
as it relies on sensory feedback to control movements. Feedback control
is a situated form of motor control, relying on sensory information
about performance and specific sensory input from the environment in
which the movement is carried out. This sensory input, while processed,
does not necessarily cause conscious awareness of the action. Closed loop control
is a feedback based mechanism of motor control, where any act on the
environment creates some sort of change that affects future performance
through feedback. Closed loop motor control is best suited to
continuously controlled actions, but does not work quickly enough for
ballistic actions. Ballistic actions are actions that continue to the
end without thinking about it, even when they no longer are appropriate.
Because feedback control relies on sensory information, it is as slow
as sensory processing. These movements are subject to a speed-accuracy
trade-off, because sensory processing is being used to control the
movement, the faster the movement is carried out, the less accurate it
becomes.
Open loop control
The classical definition from Jack A. Adams is:
“An open loop system has no feedback or mechanisms for error
regulation. The input events for a system exert their influence, the
system effects its transformation on the input and the system has an
output...... A traffic light with fixed timing snarls traffic when the
load is heavy and impedes the flow when the traffic is light. The system
has no compensatory capability.”
Some movements, however, occur too quickly to integrate sensory information, and instead must rely on feed forward control. Open loop control
is a feed forward form of motor control, and is used to control rapid,
ballistic movements that end before any sensory information can be
processed. To best study this type of control, most research focuses on
deafferentation studies, often involving cats or monkeys whose sensory
nerves have been disconnected from their spinal cords. Monkeys who lost
all sensory information from their arms resumed normal behavior after
recovering from the deafferentation procedure. Most skills were
relearned, but fine motor control became very difficult.
It has been shown that the open loop control can be adapted to
different disease conditions and can therefore be used to extract
signatures of different motor disorders by varying the cost functional
governing the system.
Coordination
A core motor control issue is coordinating the various components of the motor system to act in unison to produce movement.
Peripheral neurons receive input from the central nervous system
and innervate the muscles. In turn, muscles generate forces which
actuate joints. Getting the pieces to work together is a challenging
problem for the motor system and how this problem is resolved is an
active area of study in motor control research.
Reflexes
In some cases the coordination of motor components is hard-wired, consisting of fixed neuromuscular pathways that are called reflexes.
Reflexes are typically characterized as automatic and fixed motor
responses, and they occur on a much faster time scale than what is
possible for reactions that depend on perceptual processing.
Reflexes play a fundamental role in stabilizing the motor system,
providing almost immediate compensation for small perturbations and
maintaining fixed execution patterns. Some reflex loops are routed
solely through the spinal cord without receiving input from the brain,
and thus do not require attention or conscious control. Others involve
lower brain areas and can be influenced by prior instructions or
intentions, but they remain independent of perceptual processing and
online control.
The simplest reflex is the monosynaptic reflex or short-loop reflex, such as the monosynaptic stretch response. In this example, Ia afferent neurons are activated by muscle spindles when they deform due to the stretching of the muscle. In the spinal cord, these afferent neurons synapse directly onto alpha motor neurons that regulate the contraction of the same muscle.
Thus, any stretching of a muscle automatically signals a reflexive
contraction of that muscle, without any central control. As the name and
the description implies, monosynaptic reflexes depend on a single
synaptic connection between an afferent sensory neuron and efferent
motor neuron. In general the actions of monosynaptic reflexes are fixed
and cannot be controlled or influenced by intention or instruction.
However, there is some evidence to suggest that the gain or magnitude of these reflexes can be adjusted by context and experience.
Polysynaptic reflexes or long-loop reflexes are reflex
arcs which involve more than a single synaptic connection in the spinal
cord. These loops may include cortical regions of the brain as well, and
are thus slower than their monosynaptic counterparts due to the greater
travel time. However, actions controlled by polysynaptic reflex loops
are still faster than actions which require perceptual processing.
While the actions of short-loop reflexes are fixed, polysynaptic
reflexes can often be regulated by instruction or prior experience. A common example of a long loop reflex is the asymmetrical tonic neck reflex observed in infants.
Synergies
A motor synergy
is a neural organization of a multi-element system that (1) organizes
sharing of a task among a set of elemental variables; and (2) ensures
co-variation among elemental variables with the purpose to stabilize
performance variables.
The components of a synergy need not be physically connected, but
instead are connected by their response to perceptual information about
the particular motor task being executed. Synergies are learned, rather
than being hardwired like reflexes, and are organized in a
task-dependent manner; a synergy is structured for a particular action
and not determined generally for the components themselves. Nikolai Bernstein
famously demonstrated synergies at work in the hammering actions of
professional blacksmiths. The muscles of the arm controlling the
movement of the hammer are informationally linked in such a way that
errors and variability in one muscle are automatically compensated for
by the actions of the other muscles. These compensatory actions are
reflex-like in that they occur faster than perceptual processing would
seem to allow, yet they are only present in expert performance, not in
novices. In the case of blacksmiths, the synergy in question is
organized specifically for hammering actions and is not a general
purpose organization of the muscles of the arm. Synergies have two
defining characteristics in addition to being task dependent; sharing
and flexibility/stability.
"Sharing" requires that the execution of a particular motor task
depends on the combined actions of all the components that make up the
synergy. Often, there are more components involved than are strictly
needed for the particular task (see "Redundancy" below),
but the control of that motor task is distributed across all components
nonetheless. A simple demonstration comes from a two-finger force
production task, where participants are required to generate a fixed
amount of force by pushing down on two force plates with two different
fingers.
In this task, participants generated a particular force output by
combining the contributions of independent fingers. While the force
produced by any single finger can vary, this variation is constrained by
the action of the other such that the desired force is always
generated.
Co-variation also provides "flexibility and stability" to motor
tasks. Considering again the force production task, if one finger did
not produce enough force, it could be compensated for by the other.
The components of a motor synergy are expected to change their action
to compensate for the errors and variability in other components that
could affect the outcome of the motor task. This provides flexibility
because it allows for multiple motor solutions to particular tasks, and
it provides motor stability by preventing errors in individual motor
components from affecting the task itself.
Synergies simplify the computational difficulty of motor control. Coordinating the numerous degrees of freedom
in the body is a challenging problem, both because of the tremendous
complexity of the motor system, as well as the different levels at which
this organization can occur (neural, muscular, kinematic, spatial,
etc.). Because the components of a synergy are functionally coupled for a
specific task, execution of motor tasks can be accomplished by
activating the relevant synergy with a single neural signal.
The need to control all of the relevant components independently is
removed because organization emerges automatically as a consequence of
the systematic covariation of components. Similar to how reflexes are
physically connected and thus do not require control of individual
components by the central nervous system, actions can be executed
through synergies with minimal executive control because they are
functionally connected. Beside motor synergies, the term of sensory
synergies has recently been introduced.
Sensory synergy are believed to play an important role in integrating
the mixture of environmental inputs to provide low-dimensional
information to the CNS thus guiding the recruitment of motor synergies.
Synergies are fundamental for controlling complex movements, such
as the ones of the hand during grasping. Their importance has been
demonstrated for both muscle control and in the kinematic domain in
several studies, lately on studies including large cohorts of subjects.
The relevance of synergies for hand grasps is also enforced by studies
on hand grasp taxonomies, showing muscular and kinematic similarities
among specific groups of grasps, leading to specific clusters of
movements.
Motor Programs
While synergies represent coordination derived from peripheral interactions of motor components, motor programs
are specific, pre-structured motor activation patterns that are
generated and executed by a central controller (in the case of a
biological organism, the brain).
They represent at top-down approach to motor coordination, rather than
the bottom-up approach offered by synergies. Motor programs are executed
in an open-loop manner, although sensory information is most likely
used to sense the current state of the organism and determine the
appropriate goals. However, once the program has been executed, it
cannot be altered online by additional sensory information.
Evidence for the existence of motor programs comes from studies
of rapid movement execution and the difficulty associated with changing
those movements once they have been initiated. For example, people who
are asked to make fast arm swings have extreme difficulty in halting
that movement when provided with a "STOP" signal after the movement has
been initiated. This reversal difficulty persists even if the stop signal is presented after the initial "GO" signal but before
the movement actually begins. This research suggests that once
selection and execution of a motor program begins, it must run to
completion before another action can be taken. This effect has been
found even when the movement that is being executed by a particular
motor program is prevented from occurring at all. People who attempt to
execute particular movements (such as pushing with the arm), but
unknowingly have the action of their body arrested before any movement
can actually take place, show the same muscle activation patterns
(including stabilizing and support activation that does not actually
generate the movement) as when they are allowed to complete their
intended action.
Although the evidence for motor programs seems persuasive, there
have been several important criticisms of the theory. The first is the
problem of storage. If each movement an organism could generate requires
its own motor program, it would seem necessary for that organism to
possess an unlimited repository of such programs and where these would
be kept is not clear. Aside from the enormous memory requirements such a
facility would take, no motor program storage area in the brain has yet
been identified. The second problem is concerned with novelty in
movement. If a specific motor program is required for any particular
movement, it is not clear how one would ever produce a novel movement.
At best, an individual would have to practice any new movement before
executing it with any success, and at worst, would be incapable of new
movements because no motor program would exist for new movements. These
difficulties have led to a more nuanced notion of motor programs known
as generalized motor programs. A generalized motor program is a program for a particular class
of action, rather than a specific movement. This program is
parameterized by the context of the environment and the current state of
the organism.
Redundancy
An important issue for coordinating the motor system is the problem of the redundancy of motor degrees of freedom. As detailed in the "Synergies"
section, many actions and movements can be executed in multiple ways
because functional synergies controlling those actions are able to
co-vary without changing the outcome of the action. This is possible
because there are more motor components involved in the production of
actions than are generally required by the physical constraints on that
action. For example, the human arm has seven joints which determine the
position of the hand in the world. However, only three spatial
dimensions are needed to specify any location the hand could be placed
in. This excess of kinematic degrees of freedom means that there are
multiple arm configurations that correspond to any particular location
of the hand.
Some of the earliest and most influential work on the study of motor redundancy came from the Russian physiologist Nikolai Bernstein.
Bernstein's research was primarily concerned with understanding how
coordination was developed for skilled actions. He observed that the
redundancy of the motor system made it possible to execute actions and
movements in a multitude of different ways while achieving equivalent
outcomes.
This equivalency in motor action means that there is no one-to-one
correspondence between the desired movements and the coordination of the
motor system needed to execute those movements. Any desired movement or
action does not have a particular coordination of neurons, muscles, and
kinematics that make it possible. This motor equivalency problem became
known as the degrees of freedom problem because it is a product of having redundant degrees of freedom available in the motor system.
Perception in motor control
Related, yet distinct from the issue of how the processing
of sensory information affects the control of movements and actions is
the question of how the perception of the world structures action. Perception
is extremely important in motor control because it carries the relevant
information about objects, environments and bodies which is used in
organizing and executing actions and movements. What is perceived and
how the subsequent information is used to organize the motor system is
an ongoing area of research.
Model based control strategies
Most
model based strategies of motor control rely on perceptual information,
but assume that this information is not always useful, veridical or
constant. Optical information is interrupted by eye blinks, motion is
obstructed by objects in the environment, distortions can change the
appearance of object shape. Model based and representational control
strategies are those that rely on accurate internal models
of the environment, constructed from a combination of perceptual
information and prior knowledge, as the primary source information for
planning and executing actions, even in the absence of perceptual
information.
Inference and indirect perception
Many models of the perceptual system assume indirect perception,
or the notion that the world that gets perceived is not identical to
the actual environment. Environmental information must go through
several stages before being perceived, and the transitions between these
stages introduce ambiguity. What actually gets perceived is the mind's
best guess about what is occurring in the environment based on previous
experience. Support for this idea comes from the Ames room
illusion, where a distorted room causes the viewer to see objects known
to be a constant size as growing or shrinking as they move around the
room. The room itself is seen as being square, or at least consisting of
right angles, as all previous rooms the perceiver has encountered have
had those properties. Another example of this ambiguity comes from the doctrine of specific nerve energies.
The doctrine presents the finding that there are distinct nerve types
for different types of sensory input, and these nerves respond in a
characteristic way regardless of the method of stimulation. That is to
say, the color red causes optical nerves to fire in a specific pattern
that is processed by the brain as experiencing the color red. However,
if that same nerve is electrically stimulated in an identical pattern,
the brain could perceive the color red when no corresponding stimuli is
present.
Forward models
Forward models
are a predictive internal model of motor control that takes the
available perceptual information, combined with a particular motor
program, and tries to predict the outcome of the planned motor movement.
Forward models structure action by determining how the forces,
velocities, and positions of motor components affect changes in the
environment and in the individual. It is proposed that forward models
help with the Neural control of limb stiffness
when individuals interact with their environment. Forward models are
thought to use motor programs as input to predict the outcome of an
action. An error signal is generated when the predictions made by a
forward model do not match the actual outcome of the movement, prompting
an update of an existing model and providing a mechanism for learning.
These models explain why it is impossible to tickle yourself. A
sensation is experienced as ticklish when it is unpredictable. However,
forward models predict the outcome of your motor movements, meaning the
motion is predictable, and therefore not ticklish.
Evidence for forward models comes from studies of motor
adaptation. When a person's goal-directed reaching movements are
perturbed by a force field, they gradually, but steadily, adapt the
movement of their arm to allow them to again reach their goal. However,
they do so in such a way that preserves some high level movement
characteristics; bell-shaped velocity profiles, straight line
translation of the hand, and smooth, continuous movements.
These movement features are recovered, despite the fact that they
require startlingly different arm dynamics (i.e. torques and forces).
This recovery provides evidence that what is motivating movement is a
particular motor plan, and the individual is using a forward model to
predict how arm dynamics change the movement of the arm to achieve
particular task level characteristics. Differences between the expected
arm movement and the observed arm movement produces an error signal
which is used as the basis for learning. Additional evidence for forward
models comes from experiments which require subjects to determine the
location of an effector following an unvisualized movement.
Inverse models
Inverse models
predict the necessary movements of motor components to achieve a
desired perceptual outcome. They can also take the outcome of a motion
and attempt to determine the sequence of motor commands that resulted in
that state. These types of models are particularly useful for open loop
control, and allow for specific types of movements, such as fixating on
a stationary object while the head is moving. Complementary to forward
models, inverse models attempt to estimate how to achieve a particular
perceptual outcome in order to generate the appropriate motor plan.
Because inverse models and forward model are so closely associated,
studies of internal models are often used as evidence for the roles of
both model types in action.
Motor adaptation studies, therefore, also make a case for inverse
models. Motor movements seem to follow predefined "plans" that preserve
certain invariant features of the movement. In the reaching task
mentioned above, the persistence of bell-shaped velocity profiles and
smooth, straight hand trajectories provides evidence for the existence
of such plans.
Movements that achieve these desired task-level outcomes are estimated
by an inverse model. Adaptation therefore proceeds as a process of
estimating the necessary movements with an inverse model, simulating
with a forward model the outcome of those movement plans, observing the
difference between the desired outcome and the actual outcome, and
updating the models for a future attempt.
Information based control
An alternative to model based control is information based control.
Informational control strategies organize movements and actions based
on perceptual information about the environment, rather than on cognitive models
or representations of the world. The actions of the motor system are
organized by information about the environment and information about the
current state of the agent.
Information based control strategies often treat the environment and
the organism as a single system, with action proceeding as a natural
consequence of the interactions of this system. A core assumption of
information based control strategies is that perceptions of the
environment are rich in information and veridical for the purposes of
producing actions. This runs counter to the assumptions of indirect
perception made by model based control strategies.
Direct perception
Direct perception in the cognitive sense is related to the philosophical notion of naïve or direct realism
in that it is predicated on the assumption that what we perceive is
what is actually in the world. James J. Gibson is credited with
recasting direct perception as ecological perception.
While the problem of indirect perception proposes that physical
information about object in our environment is not available due to the
ambiguity of sensory information, proponents of direct perception (like
Gibson) suggest that the relevant information encoded in sensory signals
is not the physical properties of objects, but rather the action
opportunities the environment affords. These affordances
are directly perceivable without ambiguity, and thus preclude the need
for internal models or representations of the world. Affordances exist
only as a byproduct of the interactions between an agent and its
environment, and thus perception is an "ecological" endeavor, depending on the whole agent/environment system rather than on the agent in isolation.
Because affordances are action possibilities, perception is
directly connected to the production of actions and movements. The role
of perception is to provide information that specifies how actions
should be organized and controlled,
and the motor system is "tuned" to respond to specific type of
information in particular ways. Through this relationship, control of
the motor system and the execution of actions is dictated by the
information of the environment. As an example, a doorway "affords"
passing through, but a wall does not. How one might pass through a
doorway is specified by the visual information received from the
environment, as well as the information perceived about one's own body.
Together, this information determines the pass-ability of a doorway, but
not a wall. In addition, the act of moving towards and passing through
the doorway generates more information and this in turn specifies
further action. The conclusion of direct perception is that actions and
perceptions are critically linked and one cannot be fully understood
without the other.
Behavioral dynamics
Building
on the assumptions of direct perception behavioral dynamics is a
behavioral control theory that treats perceptual organisms as dynamic
systems that respond to informational variables with actions, in a
functional manner.
Under this understanding of behavior, actions unfold as the natural
consequence of the interaction between the organisms and the available
information about the environment, which specified in body-relevant
variables. Much of the research in behavioral dynamics has focused on
locomotion, where visually specified information (such as optic flow,
time-to-contact, optical expansion, etc.) is used to determine how to
navigate the environment. Interaction forces between the human and the environment also affect behavioral dynamics as seen in by the Neural control of limb stiffness.
Planning in motor control
Individual movement optimization
There
are several mathematical models that describe how the central nervous
system (CNS) derives reaching movements of limbs and eyes. The minimum
jerk model states that the CNS minimizes jerk of a limb endpoint trajectory over the time of reaching, which results in a smooth trajectory.
However, this model is based solely on the kinematics of movement and
does not consider the underlying dynamics of the musculoskeletal system.
Hence, the minimum torque-change model was introduced as an
alternative, where the CNS minimizes the joint torque change over the time of reaching.
Later it was argued that there is no clear explanation about how
could the CNS actually estimate complex quantities such as jerk or
torque change and then integrate them over the duration of a trajectory.
In response, model based on signal-dependent noise was proposed
instead, which states that the CNS selects a trajectory by minimizing
the variance of the final position of the limb endpoint. Since there is a
motor noise in the neural system that is proportional to the activation
of the muscles, the faster movements induce more motor noise and are
thus less precise. This is also in line with the Fitts' Law and speed-accuracy trade-off. Optimal control
theory was used to further extend the model based on signal-dependent
noise, where the CNS optimizes an objective function that consists of a
term related to accuracy and additionally a term related to metabolic
cost of movement.
Another type of models is based on cost-benefit trade-off, where
the objective function includes metabolic cost of movement and a
subjective reward related to reaching the target accurately. In this
case the reward for a successful reach within the desired target is
discounted by the duration of reaching, since the gained reward is
perceived less valuable when spending more time on it.
However, these models were deterministic and did not account for motor
noise, which is an essential property of stochastic motor control that
results in speed-accuracy trade-off. To address that, a new model was
later proposed to incorporate the motor noise and to unify cost-benefit
and speed-accuracy trade-offs.
Multi-component movements
Some
studies observed that the CNS can split a complex movement into
sub-movements. The initial sub-movement tends to be fast and imprecise
in order to bring the limb endpoint into vicinity of the target as soon
as possible. Then, the final sub-movement tends to be slow and precise
in order to correct for accumulated error by the first initial
sub-movement and to successfully reach the target.
A later study further explored how the CNS selects a temporary target
of the initial sub-movement in different conditions. For example, when
the actual target size decreases and thus complexity increases, the
temporary target of the initial sub-movement moves away from the actual
target in order to give more space for the final corrective action.
Longer reaching distances have a similar effect, since more error is
accumulated in the initial sub-movement and thus requiring more complex
final correction. In less complex conditions, when the final actual
target is large and the movement is short, the CNS tends to use a single
movement, without splitting it into multiple competents.