1. Reaching and grasping
Rosenbaum Ch 7

2. Factors in control of reach:
Learnt motor program for ballistic phase (feedforward)
Feedback during reach from vision or proprioception.
Ballistic component might rely on a simple property of muscles,
that is, muscles are like springs.
Planned component depends on current sensory evidence plus past
experience.

3. Pre-programmed component also depends of the goal of the movement
Grasp height varies depending on height of platform where object is moved to

4. Role of Visual Feedback
Question: why does error increase with speed?
Note: 50 cm/sec = 5cm/100msec

5. Muscles are like springs: a spring has a resting length and stiffness
Mass-Spring Model
Muscles are like springs: a spring has a resting length and stiffness
length and stiffness of muscles can be neurally programmed

6. Spring Equation Muscles can be modeled as springs F = -k(x1 - x0)
Difference between current position and eq. point
k = spring constant

7. Evidence for Mass-Spring Model
Accurate (??) reaching following deflection even when vision and
proprioception were absent (dorsal roots severed).

8. But is this consistent with the evidence from patients with large-fibre sensory
Neuropathy?

9. Velocity profiles for small and large movements: note difference throughout movement
Even at beginning. Implications?

10. Fitt’s Law. Movement time increases with accuracy requirements.

11. Planning reaching movements
Reaching movements are initially planned and represented in the brain in a simplified abstract form as vectors in which extent (amplitude) and direction is specified (perhaps planned independently).
Locate target
Plan movement vector
Locate hand

12. Planning reaching movements
The variability in direction errors (off-axis) are smaller than that of extent errors.
On-axis error
Off-axis error
The patterns of errors reflect limitation of CNS, provide clues to control strategy.
Mean endpoint

13. Are movements planned in joint space or hand space?

14. Straight hand paths
People move their hand in a straight path even when they can’t see their hand motion, and so can only rely on proprioception.

15. Straight hand paths
People move their arm so that the paths “look” straight even if it involves a curved path.

16. Grasping
Reaching and grasping actions require close coordination, but would seem to depend on different kinds of visual information...
Reaching: egocentric (where is the object relative to me)
Grasping: object-centred

17. Reaching vs. Grasping hand position hand velocity grip size
Peak deceleration time correlated with time to peak aperture.
Also, when arm moves faster, hand opens wider (finger separation increases).
hand position
hand velocity
grip size
aperture velocity

18. Visuo-Motor Relationships: Plasticity and Development

19. Problem of sensory-motor coordination:
How do we relate the visual and motor worlds? For reaching, a
visual signal about location must be transformed into a command to the
arm and hand muscles.
This is not innate, but must be learnt during development, and
maintained through adulthood.

20. Development of reaching
Within first 2 weeks, babies already directing arm towards objects. Some crude control of reach direction.
Improves by the 5th month; consistently touch targets.
Won’t reach for targets beyond arm’s length.
Catching and anticipating target motion at 6 months.
Distance accuracy develops more slowly, improving by 7 months.

21. Development of reaching
Within first 2 weeks, already directing arm towards objects.
Improves by the 5th month; consistently touch targets.
Catching and anticipating target motion at 6 months.
Distance accuracy develops more slowly, improving by 7 months.
Visual information used early on to aid in sensory-motor integration.

22. Increased use of visual feedback between 5 and 11 months

23. Early reach movements
Initially use the trunk & shoulder (proximal joints) to reach for objects; use elbow less frequently.
When babies do make large movements, can’t control inter-segmental dynamics. So hand oscillates.

24. Development of reaching
Between 5 and 9 months see many changes to kinematics:
1) Straightening of the hand path

25. Development of reaching
Between 5 and 9 months see many changes to kinematics:
2) Reduced number of “submovements”
3) Reduced movement time

26. Development of reaching
Joint kinematics changes as well: coordination among joints

27. Development of grasping
Newborns have grasp reflex (clasp object brought against the palm) – disappears by 6 months.
Use palmar grasp until about 12 months – then use fingers to grab.
Corresponds to rapid increase in the rate of myelination of corticospinal tracts at 12 months – responsible for distal musculature.

28. Development of grasping
At 5 months, babies orient hand, but only AFTER making contact with the object. Predictive orienting starts at 9 months.

29. Development of grasping
Tailoring of grasp to object size only after 9 months (grip aperture wider for larger objects).
Still adjusting grip force by 7-8 years (grip force larger for larger objects).

30. Reach and grasp development
Calibrating visual information to form grip
Integrate sensory-motor signals
Pincer grasp
Birth
Continued refinement
Increased myelination of
corticospinal tracts
months
years
reach onset
fine tune reach
Coordinated torque patterns/ joint patterns
Direct hand to object

31. More evidence that visuo-motor coordination must be learnt
during development.
Evidence: Kittens given visual experience without opportunity for movement,
and motor experience without vision, don’t learn how to control their movements
using vision. Correlating the two is necessary (Held & Hein study).

32. 1 2 Role of Experience in Development of Visuo-motor coordination
Held & Hein 1 2
Both kittens get visual experience and motor experience
K1. Visual experience correlated with motor commands/proprioceptive feedback/vision of limbs
K2. Gets both, but uncorrelated. Kitten 2 -abnormal visuo-motor coordination.

33. Adaptation to different relation between vision and movement.
George Stratton
Wore inverting lens for 8 days
If he saw an object on the right he would reach with his right hand and discover he should have reached with his left. He could not feed himself very well, could not tie his shoelaces, and found himself severely disoriented. His image of his own body became severely distorted. At times he felt his head had sunk down between his shoulders,and when he moved his eyes and head the world slid dizzyingly around.
As time went by Stratton achieved more effective control of his body. If he saw an object on the right he would reach with his left hand. He could accomplish normal tasks like eating and dressing himself. His body image became almost normal and when he moved his eyes and head the world did not move around so much. He began to feel as though his left hand was on the right, and his right hand on the left. If this new location of his body was vivid, the world appeared right side up, but sometimes he felt his body was upside down in a visually right-side-up world.
After removing the prisms, he initially made incorrect reaching movements. However, he soon regained normal control of his body.

34. Adaptation to different relation between vision and movement.
George Stratton
Wore inverting lens for 8 days
Believed that we learn visual directions by associating visual experiences with other forms of sensory feedback (e.g. proprioceptive).
Alternatively…
Adaptation results from learning correlation betweeen vision and actively generated motor commands (Held, 1965).

35. Why do we need to retain plasticity for new visuo-motor relationships?
1. Need to adjust to changes in body size during development.
2. Need to adjust to damage/aging.
3. Need to adjust to environmental changes eg ice, loads etc.
4. Need to learn arbitrary mappings for tool use etc.
5. Need to acquire new motor skills.
6. Visuo-motor coordination is a computationally difficult problem for the brain. Need flexibility to correct errors.

36. Reach
Grasp
FEF
V5(MT/MST) V1

37. Neural control of Grasping
Both vPM and AIP neurons fire for specific hand actions/objects.
For example, this neuron prefers a precision grip.
Precision grip
Power grip

38. Neural control of Grasping
vPM neurons fire for grasping movements made in the light and dark.
In contrast, AIP neurons fire far less when moving in the dark, and in general, AIP has more visual neurons than vPM.

39. Eg: pursuit, reaching, grasping
Eye velocity=image velocity
Motor command
sensory
retinal velocity
delay
Eg: saccade, throwing
wind
ballistic
Learnt motor command
guided

40. Neural control of Grasping
Neurons in the vPM also fires when perceiving, as well, as producing grasping movements!