NEURAL BASIS OF MOTOR CONTROL – LECTURE NOTES
Chloë Stevens (s1049391)
CONTENTS
Lecture 1 – Introduction & Challenges .......................................................................................... 3
Motor control for (ex.) picking up a cup ............................................................................................. 3
Speed Accuracy Trade-off ................................................................................................................... 3
Coordinate systems............................................................................................................................. 4
Concepts of motor control .................................................................................................................. 5
Lecture 2 – Methods in Behavioral Neurophysiology ..................................................................... 7
Difficulties in measuring brain activity................................................................................................ 7
Neural basis of Human behaviour ...................................................................................................... 7
Neural basis of Animal behaviour ....................................................................................................... 9
Electrical recordings ............................................................................................................................ 9
Optical Recordings ............................................................................................................................ 11
Lecture 3 – Spinal Reflexes ......................................................................................................... 13
Reflexes ............................................................................................................................................. 13
The Spinal Cord ................................................................................................................................. 13
Cell Types and Sensors ...................................................................................................................... 16
Classical Reflex Pathways.................................................................................................................. 18
Beyond Hardwired Reflexes .............................................................................................................. 20
Intepreting Reflexes Further ............................................................................................................. 22
Lecture 4 – Locomotion .............................................................................................................. 24
Basic Locomotor Movements ........................................................................................................... 24
The Decerebrate Preparation ........................................................................................................... 24
Local Circuits in the Spinal Cord ........................................................................................................ 27
Midbrain / Brainstem Control ........................................................................................................... 31
Cortical Contributions ....................................................................................................................... 31
Lecture 5 – Voluntary Movement 1............................................................................................. 32
Mapping Motor Cortex ..................................................................................................................... 32
Motor Cortex Connectivity ............................................................................................................... 34
Neural Coding ................................................................................................................................... 35
Lecture 6 – Neural Decoding ....................................................................................................... 37
The Meaning of Decoding ................................................................................................................. 37
Basic Population Decoding................................................................................................................ 38
Decoding reaching directions ........................................................................................................... 40
1
, Decoding Complete Movements ...................................................................................................... 41
Lecture 7 – Voluntary Movement 2............................................................................................. 42
Motor Representations ..................................................................................................................... 42
Movement Planning .......................................................................................................................... 42
Sensorimotor Representation........................................................................................................... 44
Coordinate Systems .......................................................................................................................... 45
Context / Task Dependence .............................................................................................................. 46
Mirror Neurons ................................................................................................................................. 47
Movement Sequences ...................................................................................................................... 47
Lecture 8 – Neural Prostheses .................................................................................................... 48
Needs and challenges of Motor Prostheses ..................................................................................... 48
Acquiring the Neural Signals ............................................................................................................. 49
Computing the Control Signal ........................................................................................................... 50
Activating a natural limb and Artificial limb ..................................................................................... 51
Integrating sensory feedback............................................................................................................ 51
Lecture 9 – Posture .................................................................................................................... 53
Body center of mass and strategies for regaining a stable posture ................................................. 53
Postural response ............................................................................................................................. 54
Synergies ........................................................................................................................................... 55
Vestibular System ............................................................................................................................. 58
Lecture 10 – Cerebellum ............................................................................................................. 61
Cerebellar Disfunctions ..................................................................................................................... 61
Structure of Cerebellum.................................................................................................................... 62
Functions of the cerebellar parts ...................................................................................................... 66
Lecture 11 – Basal Ganglia .......................................................................................................... 70
Anatomy of Basal Ganglia ................................................................................................................. 70
Pathways of the Basal Ganglia .......................................................................................................... 71
Functions of the Basal Ganglia .......................................................................................................... 74
Reinforcement learning .................................................................................................................... 74
Diseases involving the Basal GanGlia ................................................................................................ 75
Lecture 12 – Models & Robots .................................................................................................... 77
Control Architecture Overview ......................................................................................................... 77
Motor control concepts in Humans and Robots ............................................................................... 78
2
,LECTURE 1 – INTRODUCTION & CHALLENGES
MOTOR CONTROL FOR (EX.) PICKING UP A CUP
A trivial action can be decoded into different phases; (1) perceptual part : detecting the object (2)
plan motor trajectory; (3) correct for unexpected events and (4) reach for it / perform action.
1. Locate hand and bottle, (perceptual) sensing localizes target, determines retinotopic coordinates
and translate these to egocentric coordinates (where the body/head are centred). The position
of the hand is sensed using visual and proprioceptive information.
2. Plan hand movement, the trajectory from the hand’s initial position to the final position (end
point trajectory).
3. Determine intrinsic plan, the process of translating the reaching trajectory of the hand to the
corresponding joint movements (: inverse kinetics).
4. Execute movement, the commands need to take into action (taking into account weight, friction,
speed) → inverse dynamics, delivering a sequence of combined muscle forces that translate into
torques (force) at the joints/limbs.
SPEED ACCURACY TRADE -OFF
Accuracy varies with the speed of movement (not duration).
- Slower movements → higher accuracy (lower s.d.)
- Faster movements → lower accuracy (higher s.d.).
Faster movements require more extreme decelerations
and have less time for fine-adjustments, therefore
reducing the accuracy of the movement.
Fitt’s Law : difficulty = log (A/W) → predicts that the time required to rapidly move to a target area
is a function of the ration between the distance (A) to the target and the width (W) of the target.
- When task difficulty increases → motor control takes longer to maintain accuracy.
- Smaller widths (W) / larger distances (A) makes task more difficult.
3
, - Subjects adapt their movement duration / reduce their movement speed to perform harder
tasks (at the same level of accuracy).
- Only approx. correct as curves are not linear but slope up.
Motor control adapts accuracy or speed dynamically depending on which of them is constrained in a
task.
COORDINATE SYSTEMS
Coordinate systems represent the same position in space can be represented in different ways. Each
system may occur naturally but needs to be translated in the brain.
A) Cartesian coordinates : regular rectangular coordinates of space (X/Y/Z).The center can be
in the head / eyes / hand depending on the task.
B) Spherical coordinates : position in 3D is described as 2 angles and a distance, relative to a
center (can depend on the task).
C) Angles of joints : position in 3D is described by the angles of a sequence of joints. The center
is given by the first of the connected joints.
You can guess in which coordinate system the brain plans the movement by inspecting the
‘complexity’ of the same movement in different coordinate system (see experiment).
The trajectories in cartesian coordinates are largely straight (cartesian/spherical), however joint
angles are not straight as they rotate around pivots and even joints change directions
- Straight trajectories point to a cognitive / higher level motor planning in easy to handle,
straight cartesian coordinates.
- Translation to joint coordinates happens in steps through lower motor system (cerebellum,
brainstem, spinal cord) towards muscle activation.
4
Chloë Stevens (s1049391)
CONTENTS
Lecture 1 – Introduction & Challenges .......................................................................................... 3
Motor control for (ex.) picking up a cup ............................................................................................. 3
Speed Accuracy Trade-off ................................................................................................................... 3
Coordinate systems............................................................................................................................. 4
Concepts of motor control .................................................................................................................. 5
Lecture 2 – Methods in Behavioral Neurophysiology ..................................................................... 7
Difficulties in measuring brain activity................................................................................................ 7
Neural basis of Human behaviour ...................................................................................................... 7
Neural basis of Animal behaviour ....................................................................................................... 9
Electrical recordings ............................................................................................................................ 9
Optical Recordings ............................................................................................................................ 11
Lecture 3 – Spinal Reflexes ......................................................................................................... 13
Reflexes ............................................................................................................................................. 13
The Spinal Cord ................................................................................................................................. 13
Cell Types and Sensors ...................................................................................................................... 16
Classical Reflex Pathways.................................................................................................................. 18
Beyond Hardwired Reflexes .............................................................................................................. 20
Intepreting Reflexes Further ............................................................................................................. 22
Lecture 4 – Locomotion .............................................................................................................. 24
Basic Locomotor Movements ........................................................................................................... 24
The Decerebrate Preparation ........................................................................................................... 24
Local Circuits in the Spinal Cord ........................................................................................................ 27
Midbrain / Brainstem Control ........................................................................................................... 31
Cortical Contributions ....................................................................................................................... 31
Lecture 5 – Voluntary Movement 1............................................................................................. 32
Mapping Motor Cortex ..................................................................................................................... 32
Motor Cortex Connectivity ............................................................................................................... 34
Neural Coding ................................................................................................................................... 35
Lecture 6 – Neural Decoding ....................................................................................................... 37
The Meaning of Decoding ................................................................................................................. 37
Basic Population Decoding................................................................................................................ 38
Decoding reaching directions ........................................................................................................... 40
1
, Decoding Complete Movements ...................................................................................................... 41
Lecture 7 – Voluntary Movement 2............................................................................................. 42
Motor Representations ..................................................................................................................... 42
Movement Planning .......................................................................................................................... 42
Sensorimotor Representation........................................................................................................... 44
Coordinate Systems .......................................................................................................................... 45
Context / Task Dependence .............................................................................................................. 46
Mirror Neurons ................................................................................................................................. 47
Movement Sequences ...................................................................................................................... 47
Lecture 8 – Neural Prostheses .................................................................................................... 48
Needs and challenges of Motor Prostheses ..................................................................................... 48
Acquiring the Neural Signals ............................................................................................................. 49
Computing the Control Signal ........................................................................................................... 50
Activating a natural limb and Artificial limb ..................................................................................... 51
Integrating sensory feedback............................................................................................................ 51
Lecture 9 – Posture .................................................................................................................... 53
Body center of mass and strategies for regaining a stable posture ................................................. 53
Postural response ............................................................................................................................. 54
Synergies ........................................................................................................................................... 55
Vestibular System ............................................................................................................................. 58
Lecture 10 – Cerebellum ............................................................................................................. 61
Cerebellar Disfunctions ..................................................................................................................... 61
Structure of Cerebellum.................................................................................................................... 62
Functions of the cerebellar parts ...................................................................................................... 66
Lecture 11 – Basal Ganglia .......................................................................................................... 70
Anatomy of Basal Ganglia ................................................................................................................. 70
Pathways of the Basal Ganglia .......................................................................................................... 71
Functions of the Basal Ganglia .......................................................................................................... 74
Reinforcement learning .................................................................................................................... 74
Diseases involving the Basal GanGlia ................................................................................................ 75
Lecture 12 – Models & Robots .................................................................................................... 77
Control Architecture Overview ......................................................................................................... 77
Motor control concepts in Humans and Robots ............................................................................... 78
2
,LECTURE 1 – INTRODUCTION & CHALLENGES
MOTOR CONTROL FOR (EX.) PICKING UP A CUP
A trivial action can be decoded into different phases; (1) perceptual part : detecting the object (2)
plan motor trajectory; (3) correct for unexpected events and (4) reach for it / perform action.
1. Locate hand and bottle, (perceptual) sensing localizes target, determines retinotopic coordinates
and translate these to egocentric coordinates (where the body/head are centred). The position
of the hand is sensed using visual and proprioceptive information.
2. Plan hand movement, the trajectory from the hand’s initial position to the final position (end
point trajectory).
3. Determine intrinsic plan, the process of translating the reaching trajectory of the hand to the
corresponding joint movements (: inverse kinetics).
4. Execute movement, the commands need to take into action (taking into account weight, friction,
speed) → inverse dynamics, delivering a sequence of combined muscle forces that translate into
torques (force) at the joints/limbs.
SPEED ACCURACY TRADE -OFF
Accuracy varies with the speed of movement (not duration).
- Slower movements → higher accuracy (lower s.d.)
- Faster movements → lower accuracy (higher s.d.).
Faster movements require more extreme decelerations
and have less time for fine-adjustments, therefore
reducing the accuracy of the movement.
Fitt’s Law : difficulty = log (A/W) → predicts that the time required to rapidly move to a target area
is a function of the ration between the distance (A) to the target and the width (W) of the target.
- When task difficulty increases → motor control takes longer to maintain accuracy.
- Smaller widths (W) / larger distances (A) makes task more difficult.
3
, - Subjects adapt their movement duration / reduce their movement speed to perform harder
tasks (at the same level of accuracy).
- Only approx. correct as curves are not linear but slope up.
Motor control adapts accuracy or speed dynamically depending on which of them is constrained in a
task.
COORDINATE SYSTEMS
Coordinate systems represent the same position in space can be represented in different ways. Each
system may occur naturally but needs to be translated in the brain.
A) Cartesian coordinates : regular rectangular coordinates of space (X/Y/Z).The center can be
in the head / eyes / hand depending on the task.
B) Spherical coordinates : position in 3D is described as 2 angles and a distance, relative to a
center (can depend on the task).
C) Angles of joints : position in 3D is described by the angles of a sequence of joints. The center
is given by the first of the connected joints.
You can guess in which coordinate system the brain plans the movement by inspecting the
‘complexity’ of the same movement in different coordinate system (see experiment).
The trajectories in cartesian coordinates are largely straight (cartesian/spherical), however joint
angles are not straight as they rotate around pivots and even joints change directions
- Straight trajectories point to a cognitive / higher level motor planning in easy to handle,
straight cartesian coordinates.
- Translation to joint coordinates happens in steps through lower motor system (cerebellum,
brainstem, spinal cord) towards muscle activation.
4
