Controlling Movements

1
Controlling Movements

• Reaching and Grasping - Prehension

2
Upper Extremity Control

• Upper extremity has a role in posture and Gait
• Major role in recovery from loss of balance and
  crawling
• Again
• Task
• Environment
• Individual
• All play a major role in control and function

3
What is involved in prehension?

• Locating target in space
• Transport of the arm reaching
• Hand preparation grasping and pre-grasping
• Manipulation once object is grapsed

4
Feedforward vs. Feedback

• Feedforward
• Anticipate the requirements of the task and
  potential perturbations to its successful
  completion
• Feedback
• Correct for under/overshoot or perturbations

5
Eye-Head-Trunk Coordination

• Since tasks could require one or all to be
  controlled there may be more than one mechanism
  for control
• E.g., Task only requires eyes to move, vs. head
  and eyes, vs. all three.

6
Eye movement control

• Eye movement to a stationary target causes image
  movement across the eye but it appears stationary
  
• Parietal cortex updates movement of the eye to
  update the visual space
• Why is this important?
• We need to have a stable visual platform to
  control and guide movement.
• Corollary discharge of eye movements to other
  brain regions to update movement progress.

7
How do the eye movements sync with arm movement?

• Ares in Pre-motor cortex that neurons for both
  saccades and arm movements
• Hand movements are smoother when accompanied by
  vision
• Eye tracking is smoother with accompanied hand
  movements
• Yours or your partners!
• Mirror Neurons???

8
Reach Grasp Point

• Prehension and pointing are very different in how
  they are controlled
• Reach Grasp change greatly as a function of the
  goal.

9
Class Interaction

• Discuss what would impact reaching and grasping
  and what would change as a function of the change
  made
• Ie., make a change to the task the environment or
  the individual and discuss what changes would
  result

10
Steps in Control of Prehension

• Decision to move
• Parietal and premotor cortex
• Action plan developed
• Association areas (frontal, parietal, temporal,
  occipital)
• Action plan is initiated and details added
• Motor cortex, cerebellum and basal ganglia
• Plan is executed
• Spinal cord and brain stem
• Feeback from sensory systems allow for updates
  time permitting

11
Conscious vs. Unconscious Control

• Separate streams of information seem to allow for
  both conscious and unconscious control
• Grasping Tau

12
Two Pathways for Reach and Grasp

• Movement of the arm is achieved while refinement
  of the hand is also achieved
• Different Grasp movements also controlled
  differently
• For all grasps 2 steps
• Hand must be adapted to object
• Hand must begin to close at appropriate movement
• Grasping Tau

13
Grasp and Lift

• 4 Phases
• Contact with fingers
• Grip force applied
• Load overcome and object begins to move
• Decrease in grip prior to putting down object
• With different needs more of less time needed to
  establish grasp
• Cerebellum plays key role in modulating grip
  forces
• Adding necessary details to commands

14
Studying the Coordination of Reach and Grasp

• Invariant features in reach and grasp
• Perturbations in one affects the other
• Kinematic coupling of the two
• At a basic level how do we go about studying
  these movements???

15
Do Single-Joint movements Exist?

• Single Joint Movements One joint, one degree of
  freedom, often constant load
• Simplest form one joint, agonist antagonist
  muscles.
• Adding to the complexity
• Degrees of freedom ()
• Number of muscles at the joint
• Number of joints the muscle crosses
• In essence we dont have single-jt. Movements
• So why study?

16
What does a simple movement look like?

• EMG
• Agonist antagonist burst
• Torque
• Flexor to start and extensor to stop
• Acceleration
• Follows torque curve
• Velocity
• increases then returns to zero

17
Basic strategies for movement

• Speed-Insensitive
• Control is achieved by the duration of the
  excitation pulse
• Movement time is not an issue
• Ss. move as fast and accurate as possible

18
Basic strategies for movement

• Speed-sensitive
• Control is achieved by the overall magnitude of
  the excitation pulse
• Involved in movements that require control over
  movement speed

19

• What differences do we see in the rate of muscle
  activation?
• Why would the Ss. show such slow recruitment
  patterns for the solid line (green graph)?
• What is the movement constraint?

20
Multi-Joint Movements
21
Unidirectional movements

• Movement being considered involves multiple
  muscles and joints moving in a linear direction
• Working Point?
• Location of working point relative to performer?
• Proximal, distal, internal, external?

22
Components of voluntary movements

• Common reaching movements will typically involve
• Trajectory of endpoint will be strait
• Velocity profile will be symmetrical (bell curve)
• Acceleration of end-point will be bimodal
• Individual joints and muscles will have varied
  acceleration, velocities, movement paths

23
Difficulties in Controlling Reaching Movements

• Complications added when controlling
  multiple-linked joints
• Coriolis and centrifugal forces
• Bi-articular muscles
• Interjoint and interlimb reflexes
• Degrees of Freedom
• Possible configurations to achieve movement goal?
• The number of DOF possible for a movement is
  greater than that needed to achieve the movement
• Knowing this - Do we choose one configuration and
  repeat it?

24
What is Controlled?

• Things not controlled
• Forces produced by individual muscles
• Activation patterns of individual muscles
• Torques in individual muscles
• Rotations in individual joints

25
What is Controlled?

• Studies have shown the working point (e.g.,
  hammer head-finger tips in B-Ball shooting) to be
  the most reproducible performance variable
• supporting the working point control notion.
• Equilibrium vector the working point moves in a
  spring like manner when perturbed
• Suggesting that the individual components behave
  as springs
• Support of this from studies demonstrating MC
  neurons responsible for stiffness and others for
  responsible for movement of the muscle.

26

• Equilibrium Point Model
• For a given efferent command a joint will assume
  a set joint angle
• If external load changes without changes in
  efferent command the joint can move to a range of
  angles
• Change in load results in changes in peripheral
  activation (reflex arcs)
• If efferent signal changes then a new range of
  joint locations are possible

27
How Does the CNS Plan Movements?

• We have the intention to move, but what does the
  CNS use to plan the movement?
• Does it Coordinate based on
• Muscles
• Joint Angles (intrinsic), or
• End-point (extrinsic) strategies

28
If Based on Muscle Commands

• We would not need to perform transformations for
  endpoint or joint angles,
• Problem arises - activation patterns depend
  highly upon initial joint position
• High degree of variability in movement with same
  muscle command

29
If Based on Joint Angles

• Advantage no longer required to transform
  endpoint locations
• CNS could determine that the joint would need to
  be in a set configuration to achieve the goal
• Problem is that CNS would still need to transform
  Jt. Angles into muscle activation patterns.

30
If Based on Endpoint Location

• If movement was planned on the end point location
• The CNS would have to transform endpoints into
  complex joint angle configurations/movements,
• Then transform the joint angles into muscle
  activation patterns

31
So where does that leave us?

• If we moved based on Muscle commands
• Adding weights or changing movement speed should
  alter movement patterns
• Such is not the case!
• If we moved based on joint movements
• we should move in a curved path
• Due to the movement about an axis
• Such is not typically the case!

32
So NOW Where does that leave us?

• Movement based on endpoint locations.
• Studies assessed hand movement when pointing.
• Hand tends to follow a straight line
• Even when asked to follow a path hand moved in
  a series of straight lines.

33
Speed-Accuracy Trade-off

• Woodsworth (1899).
• Initial adjustment phase
• propel the limb
• Current-control phase
• Home in on the target
• Fitts (1954)
• Relationship between amplitude of the movement
  and the size of the target effects movement time
• MT a bLog2 (2A/W)

34
Fitts Law cont.

• Two tasks with different A W can have the same
  index of difficulty
• W .5 .25 inches
• A 16 8 inches
• From the formula
• MT a bLog2 (2A/W)
• a intercept
• b slope
• Different effectors will have different
  relationships

35
Fitts Law cont.

• The slope represents an interaction of IDs with
  the controllability inherent to the IV
• Limb used
• Age
• Skill level
• Goal of maintaining a constant rate of
  information processing across different A W
  combinations

36
Correction Models for Speed Accuracy Trade-Off

• Crossman-Goodeve Model
• Multiple ballistic movements each with their own
  inherent error (1/7th the distance)
• Fitts Law was accounted for by the number of
  corrections needed to decrease error to zero.
• Problem time constraints on visually based
  correction.
• Also most movements were only comprised of 1 or 2
  sub-movements
• Kinematic data does not support the multiple
  corrections needed.

37
Correction Models for Speed Accuracy Trade-Off

• Optimized Submovement Model
• Initial movement is still driven by an impulse to
  enable movement
• Target may be reached or not
• Submovements, typically one or two, will correct
  the error
• MT is optimized using the principles of impulse
  variability (discussed earlier)
• On some occasions more than one correction is
  needed (again optimizing based on impulse
  variability)

38
(No Transcript)
39
Hand-space vs. Joint-space
40
Hand-space vs. Joint-space
41
Velocity and Acceleration Profiles

• Note the bell-shaped curve for velocity
• What is the small increase at the end for?
• Bimodal acceleration profile

42
Movement Occurs When All Conditions are Ideal