Neural Basis of Motor Control NWI-BB080C (NWIBB080C)
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Neural Basis of Motor Control: Summary of the lectures, NWI-BB080C
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Neural Basis of Motor Control NWI-BB080C (NWIBB080C)
Institution
Radboud Universiteit Nijmegen (RU)
Book
Principles of Neural Science 5e
This is a summary of lectures from the course Neural Basis of Motor control given at the Radboud University. I completed this course with a 7.5. All images are derived from the lectures.
Lectures of Neural Basis of Motor Control (NWI-BB080C)
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Radboud Universiteit Nijmegen (RU)
Bsc Medical Biology / Neurology / neurophysiology
Neural Basis of Motor Control NWI-BB080C (NWIBB080C)
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Neural basis of motor control summary
NWI-BB080C
Lecture 1: Introduction
What is needed for a trivial action:
- The perceptual part: detect the object.
- Plan motor trajectory.
- Correct for unexpected events.
- Reach for the bottle.
Motor control for picking up a cup:
- 1. First locate hand and cup.
- 2. Then plan hand movement
o Sensing and change of coordinates.
o The trajectory from the hand’s initial position to the final position is planned.
- 3. Hand trajectory is translated into joint trajectories (joint movements)
o This translation is called inverse kinetics (so inwards, not reverse).
▪ The computation of the trajectories of the joint angles.
o Requires knowledge of the body’s mechanics and the surrounding space
(obstacles) to really determine what the joint trajectories are going to be.
▪ For instance separate shoulder angles and elbow trajectories angles
are mapped out.
- 4. Then you have the execution of the command:
o Taking into account the Inverse dynamics: the speed, weight of the arm etc.
o Combined muscle forces are created when taking into account the inverse
dynamics, and these forces translate at the joints and limbs into the joint
torques (draaimomenten, draai-angles van de gewrichten).
Challenges in controlling precise movements from the human body:
- 1. The body has very complex mechanisms:
o Writing with a pen needs to be done by controlling multiple fingers that have
multiple joints Every joint has an effect on the subsequent joint.
- 2. There is the complexity of inverse model/ transform: body has to choose the right
neurons to activate and inhibit when wanting one type of movement to perform.
- 3. Different parts of the body operate in different coordinate systems. Brain has to
rapidly transform these coordinate systems to integrate sensory information with
motor actions.
o Motor actions are performed in for instance the joint- angle coordinate
system (see below).
- 4. . There is a speed- accuracy trade off. Faster actions lead to reduced accuracy.
o Brain has to choose the right speed and accuracy for the task.
- 5. Brain activity is variable: performing same movement twice, results in the activity
being different. Motor control has to be robust to this variable cause you want this
movement to be performed the same still for the second time.
1
,Speed – accuracy trade off
- The accuracy of the movement varies with the speed of the movement, not with the
duration.
o So not the acceleration, but the speed!! Acceleration probs has something to
do with it as well but is not mentioned.
o Slower movements lead to higher accuracy (so a lower standard deviation to
the point where you want the hand to stop moving (the line0
o Faster movements lead to lower accuracy (higher standard deviation).
- Fitt’s law on Speed-Accuracy Tradeoff
o When task difficulty increases, motor control takes longer to maintain
accuracy (duration becomes longer).
o How does the speed/ duration changes If the accuracy is fixed?
▪ The difficulty increases by giving the target (that needs to be hit) a
smaller width.
▪ The difficulty also increases when increasing the distance (so if the
width was fixed).
• So the duration increases when the distance increases right.
▪ And with fixed accuracy and with a fixed distance:
• With a smaller width (thus an increased difficulty), the duration
becomes longer.
Coordinate systems
There are different coordinate systems for a single point in space. For instance cartesian
coordinates, spherical coordinates and join angle coordinates.
Different coordinate systems are different ways of representing the same position in space.
This position is the target location of a movement. Each of these system occur naturally,
however they need to be translated in the brain.
- Cartesian coordinates: x,y,z
o Brain may put the centre of the coordinates in the head, eyes or hand>
depends on the task .
▪ So the centre is the 0 point in for instance x,y,z axis.
- Spherical coordinates: two angels and a distance, they describe the position of the
target location of the movement in the 3D space. They are relative to a centre
position (here the shoulder, but that depends on the task).
- Angles of joints: the position of end location of the movement in the 3D space is
described by the angels of the joint. The centre of the joints is the here the first of
the connected joints (the shoulder).
2
,For a given movement: how to determine the brain’s coordinate system?
This is determining in which of the three coordinate systems, the brain plans the movement.
This is done by inspecting the complexity of performing the same movement in different
coordinate systems. Outcome: Hand movements are compatible with non-join coordinates:
Movements are optimized in the Cartesian space! The joints have to adapt accordingly. (so
cartesian coordinates are used by the brain to plan a movement)
- This is proven by the fact that trajectories in the outside space are largely straight
this is compatible with an operation in the cartesian or spherical space, and not in
the join coordinate system. Also proved by looking at the time traces of the joint
angels, they are not straight and even change direction.
- So when looking from the joint angles coordinates, the trajectories should not be
straight, however the movement does go straight so the target location of the
movement is not planned via the joint coordinates.
Reaching errors are uncorrelated
- Knowing one error along one axis of the ellipse does not help predict the error on the
other axis.
o The precision of reaching is not related to the precision of reaching into
another direction
- The lack of correlation suggests that both direction and length of the motor
movement are independently controlled, this means: non-joint based coordinates. >
most likely is spheric coordinate system around the finger.
o If they were controlled together: there would be an error in one would lead
to an error in the other
Lecture 1, part 2: Concepts of Motor Control
Control strategies that help control a movement:
3
,Feedforward control
Feedforward control is the part of motor control that does not include sensory feedback.
Feedforward control refers to being independent of sensory information that is gathered
during the movement (it is however, of course based on sensory information gathered
before the movement) .This control starts from the desired future sate of the body, and then
uses a feedforward controller to plan the movement. The feedforward controller contains
the inverse model. The inverse model means finding the right muscle to activate to achieve
the desired movement. After these muscles are found, the feedforward controller sends the
motor command to the muscle for execution.
- Positive effects of this strategy: feedforward control of a movement is fast, the motor
command is directly executed and does not have to wait for sensory information to
be collected an processed.
- Negative effects: it is inflexible, the command is not modified by new information
once executed> example: your arm could not react to an obstacle that gets in the
way (your arm will just keep moving).
- Examples of movements that are controlled feedforward: eye movements (speed is
more important than feedback + rarely obstacles in the way of moving your eye).
o It is only accurate if nothing unexpected happens.
What is used in normal reaching movements: feedforward or feedback control?
In neural systems, both strategies are combined. First a large feedforward movement,
followed by feedback-based corrective movements (if something gets in the way?).
feedforward model is used to detailly plan reaching towards a target. Only using the
feedback strategy would take longer, there would be a sequence of smaller movements,
after each small movement, a sensory measurement would be made to indicate the distance
to the target and the further movements would be made.
Feedback control strategy
Why and when is sensory information integrated :
If there is an obstacle in the way, you need to be able to overcome it. Sensory information
during the movement can be used to adapt the behaviour during its execution (not all
properties of the object are known before beginning of the movement (will it roll or slip
away, what is the surface made of etc.)
4
,Feedback control is the case when sensory feedback is used during the movement. The
sensory information that is gathered during the movement is processed and compared with
the intended movement. In combination with the feedforward control strategy: an idea of
new movement is now being made, this is then translated again by the feedforward
controller into a corrective movement.
- Example is eye movements: visual feedback is used to perform corrective saccades
but only between individual saccades ( so, correcting the eye to a new focus point).
- Positive effect: feedback control is more flexible: a movement can be corrected in a
changing environment.
- Negative side: it is comparatively slow: the movement has to wait for the sensory
input and an updated motor command to be generated.
Combination of feedforward control and feedback information:
- Grip force is adjusted feedforward and using feedback.
- To lift an object: person has prediction of weight of the
object> based on visual appearance.
o Even before lifting the object, there is a change in grip
force that is applied.
o There are different timings between the grip, the load
and the lift.
5
, o Load force is the force needed to lift the object upwards
▪ It is kind of the sum of: mass of object +
gravity on object.
- RA2 is a rapidly adapting touch sensor.
o Senses vibration and rapid force changes.
- When presenting a heavy object (800g) after 400g, a slip
occurs. The lift does not occur (also no response from RA2
because with that force, nothing happens with the object).
This error information is used by the person to adjust the
grip force (now the RA2 response is bigger)
Forward model : using the efference copy to simulate the
movement.
The motor command is not only send to the muscles, but the command is also made
available to the brain. This is called an efference copy: an internal copy of the motor
command. This internal simulation/ prediction of the motor command and is also called the
forward model (the forward model shows the efference copy to the brain?). The forward
model is available before the actual execution of the motor command. It makes a prediction
of what should happen. The forward model is similar to the feedforward controller, but
without execution. The forward model:
- 1. It can coordinate other movements.
- 2. Or prepare the organism for consequences of the movement.
o It can coordinate balance after a shift
With the forward model, you can compare the predicted movement to the actually sensed
movement (=comparison between intended and actual movement). This makes sure that we
can make corrective movements. Corrective movements require sensory feedback.
Differences between sensory feedback model and forward model is that forward model can
be very refined (including temporal trajectory). Basic sensory feedback could be ‘touching
already or not’.
6
,So remember, the efference copy is not a copy of a already performed movement that is
captured in the brain, but it is a copy of the predicted movement and thus the motor
command.
The prediction of your own movement (aka the efferent copy) accelerates the motor control:
If the movement is external (so moved by the robot) the grip force is much bigger than the
load force (logical because the human only holds the block, it doesn’t lift it, the robot does).
The grip force lags the load force.
When the movement is created internally (so the person has to lift the object each time),
then the grip force is not delayed and matches the load force each time, this is due to the
efference copy, you know what grip force belongs to the load force because of this. With this
experiment, the precision of the internal efference copy is demonstrated.
Due to the efference copy, you cannot tickle yourself. If the tickling is delayed in time due to
delayed feedback that is not matched to the efferent copy, then the sensation of tickling
reappears. So if you delay and randomize the timing of the self – tickling than it would be
more ticklish.
Forward model creates a prediction, feedforward model creates a movement. Forward
sensory model predicts the sensory feedback.
Forward sensory model: What actually happens in a movement: Integration of Prediction
and Feedback
Every movement is composed of motor execution + control events. These events start with
the initial estimates (1). Blue circle in figure means internal uncertainty about location of
hand. The forward model is here called forward motor model as it produces the actual
command (so not just the prediction). This command drives the muscles, but this command
is also sent as an efference copy to the internal state as prediction/ simulation(2, this is the
predicted current state, so based on the efference copy, basically the efference copy is the
7
,expected state). (so the green line, from the forward model, both the motor command and
the efference copy, which stays internally, are created). The forward sensory model uses
the efference copy together with knowledge about the environment (so sensory information
acquired before the movement (not seen in this figure)), to predict the sensory feedback.
Then a difference is created between the actual sensory feedback and the predicted sensory
feedback. The difference between the predicted and the actual sensory feedback is
weighted with a gain factor and used to inform the predicted state (which is created by the
efference copy of the forward model) that there is a difference between actual and expected
relation between hand and environment . The gain factor determines how strongly this error
will adapt the model/ next movement. Now the state estimate can be updated (3) . This
new estimate becomes the new starting point for a loop of these actions, until the state
estimate is the same as the intended action.
Delay and Gain influences the Accuracy of movement.
With a small/no delay of feedback, the accuracy is highest. With a higher gain, the accuracy
is highest.
- Logical because a high gain means high correction for the difference (‘error’) between
the predicted and the sensed state, this makes it more accurate when the person
wants to follow the line (in the experiment).
Within a sensory feedback loop, the sensory information arsis with a delay. Taking the
difference between the expected state (the efference copy) and the sensed state, the gain
factor needs to be chosen (so how much is the movement going to be altered). > high gain is
making the movement more precise right? And the need for a high gain means that the
predicted state was not correct. The largest oscillations in the movement occur when there
is high gain and high delay, the system can then get unstable (so not low gain and high delay
is the worst!!). A High gain in combination with a low delay is the best.
Does your perception always align with your sensorimotor control?
The sensorimotor and perception pathway adapt in a differently.
- Where = sensorimotor. What = perception
- Can be demonstrated with size-weight illusion> same weight, different size.
o People keep saying the bigger box is heavier, even after lifting it a few times,
however the grip force has adjusted to the actual weight, meaning that the
sensorimotor system has adapted and the perception has not.
- Thus the sensorimotor pathway and perception pathway operate independently to
some degree.
Is the prediction only static or can it be dynamic?
Correction is dynamic and predictive, not just a stiffening of the joints.
- A person can adapt from perturbations and then correctly reaching all the targets in a
straight line again. When the perturbations are left out again, the reaches are
overshoot into the other directions and the person has to adapt again. This means
that the control strategy is not stiffening of the muscles, because if that was the case,
then the reaches wouldn’t overshoot when the perturbations were taken away again.
Instead, it is a dynamic predictive correction when dealing with the perturbations. So
corrective predictions are dynamic ?
8
, Lecture 3: Spinal Reflexes, 7-9-2020
Reflexes are encoded in the spinal circuits.
Which reflexes do you know
Common reflexes: Corneal reflex (blinking both eyes when one is touched), sneezing,
blinking, the reflex of the ear ossicle bones, knee jerk reflex.
Internal reflex: Baroreflex: elevated blood pressure leading to decrease in heart rate>
keeping blood pressure constant!
Infant reflexes: reflexes observed in children
o Plantar reflex> curling of toes when rubbinhte underside of the foot.
o > sensation of falling forward> then reflex is extension of the arm.
What defines a reflex
- Reflexes: Involuntary actions of your body in response to a sensory input.
Reflexes are often the little actions that save you unknowingly. They have to be fast, thus
mediated by small neural circuits (however the process of information in these circuits can
be very complex). The reflex arc is the basic circuit, this retains its speed, which is essential
to fulfil its function.
Organisation of the reflex system
The spinal cord: organisation
There ae specializations in the architecture of the spinal cord that associates with the limbs.
There are local circuits: different parts of the spinal cord have a difference in composition.
This is determined by the part of the body that the nerves from that piece of spinal cord
innervate.
Organisation of the spinal cord: Cervical nerves, thoracic nerves, lumbar nerves, sacral
nerves and coccygeal nerves.
- Cervical nerves for arms and lumbar nerves for legs.
Local anatomy of spinal cord: white and grey
matter:
You have the spinal canal, in that the spinal cord
lies, this cord lies at the back of the spine (so at
the dorsal side). Grey matter is the inside of the
spinal cord, white matter is the outside.
White matter is composed of fibre tracts. The
white matter is composed of ascending and
descending fibre tracts. The white matter is very
well organized. Subparts of the what matter tracts
are referred to as columns (e.g the dorsal
column), as they run vertically along the spinal
cord in bundles.
Grey matter contains the local circuits of neurons
(so that is the transition place of sensory neurons
to motor neurons).
9
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