Reaching and grasping

Rosenbaum Ch 7

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 pastexperience.

Grasp height varies depending on height of platform where object is moved to

Pre-programmed component also depends of the goal of the movement

Role of Visual Feedback

Question: why does error increase with speed?Note: 50 cm/sec = 5cm/100msec

Mass-Spring Model

Muscles are like springs: a spring has a resting length and stiffness

length and stiffness of muscles can be neurally programmed

Spring Equation

Muscles can be modeled as springs

F = -k(x1 - x0)

Difference between current position and eq. point

k = spring constant

Evidence for Mass-Spring Model

Accurate (??) reaching following deflection even when vision and proprioception were absent (dorsal roots severed).

But is this consistent with the evidence from patients with large-fibre sensoryNeuropathy?

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

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

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 hand

Locate target Plan movement

vector

Planning reaching movements

The variability in direction errors (off-axis) are smaller than that of extent errors.

Mean endpoint

On-axis error

Off-axis error

The patterns of errors reflect limitation of CNS, provide clues to control strategy.

Are movements planned in joint space or hand space?

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.

Straight hand paths

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

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

Reaching vs. Grasping

hand position

hand velocity

grip size

aperture velocity

Peak deceleration time correlated with time to peak aperture.Also, when arm moves faster, hand opens wider (finger separation increases).

Visuo-Motor Relationships:Plasticity and Development

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.

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.

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.

Increased use of visual feedback between 5 and 11 months

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.

Development of reaching

Between 5 and 9 months see many changes to kinematics:

1) Straightening of the hand path

Development of reaching

Between 5 and 9 months see many changes to kinematics:

2) Reduced number of “submovements”

3) Reduced movement time

Development of reaching

Joint kinematics changes as well: coordination among joints

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.

Development of grasping

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

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).

Reach and grasp development

BirthIncreased myelination of corticospinal tracts

Continued refinement

Direct hand to object

reach onset fine tune

reach Coordinated torque patterns/ joint patterns

Integrate sensory-motor signals Pincer grasp

months years

Calibrating visual information to form grip

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).

Held & Hein

Role of Experience in Development of Visuo-motor coordination

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.

1

2

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.

Adaptation to different relation between vision and movement.

George Stratton– Wore inverting lens for 8 days

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).

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.

FEF

Reach

Grasp

V1

V5(MT/MST)

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

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.

Eg: pursuit, reaching, grasping

Eg: saccade, throwing

Eye velocity=image velocity

sensory

retinal velocity

Motor command

delay

wind

Learnt motor command

ballistic

guided

Neural control of Grasping

Neurons in the vPM also fires when perceiving, as well, as producing grasping movements!