Control Of Reaching

Question 1

What 3 problems does the brain have to solve about the control of reaching??

Answer 1

Localisation - representation of the location of the object

Planning - plan of reaching based on representation

Control - generate muscle forces to drive the arm to the desired position/configuration

Question 2

What area of the brain is concerned with localisation and planning??

Answer 2

The posterior parietal cortex (in monkeys brain)

- firing of neurons in the PPC are related to the movement planning

Question 3

What does the lateral intraparietal area do??

Answer 3

Needed for saccadic eye movements

Question 4

What does the parietal reach region (PRR) and the anterior intraparietal area (AIP) do?

Answer 4

PRR - for reaching

AIP - for grasping

Question 5

How does the PPC plan movement?

Answer 5

By closely coordinating the eye and hand

Question 6

After localisation, the object is internally represented based in a specific coordinate system. What are the two options of coordinate systems?

Answer 6

Hand - centered coordinate

Eye - centered coordinate

Question 7

What did Bastia et al (1999) find when looking at reach plans in eye-centered coordinates?

Answer 7

Neural recordings of the PRR for different initial gaze directions and hand positions show that the firing pattern highly correlates with gaze direction, not hand position.

Neural recordings of PRR for different initial eye and hand positions showed that the firing pattern highly correlates with the eye position, but not the hand position

Question 8

How can a monkey elicit voluntary, motor behaviours?

Answer 8

Microstimulation of neurons in the monkeys primary motor cortex

Question 9

In monkeys, the motor cortex is concerned with what movements? (Graziano, 2016)

Answer 9

Climbing/leaping, hand in lower space, chewing/licking, hand to mouth, defence, reach to grasp, manipulation in central space

Question 10

What did Graziano et al (2003) find when looking at purposeful reaching movements?

Answer 10

They are generated by stimulating reach-related neurons. Neurons tend to code the final reach position, rather than the starting position

Question 11

How are the purposeful reaching movements controlled?

Answer 11

Neurons in the primary motor cortex have their own preferred directions in what is known as directional tuning

Question 12

Why is a look up table not suitable as being the mechanism for generating muscle force patterns to reaching movements??

Answer 12

Too large and also can’t deal when arm is perturbed during movement or when the weight of the arm changes.

Hence, the stretch reflex is proposed (gamma neurons adjust the sensitivity of the reflex)

Question 13

What is the basic premise of Sherringtons reflex hypothesis?

Answer 13

Movements are obtained by combining stretch reflexes (i.e., changing parameters of muscle springs)

The brain just sets the desired length of the muscle springs and then stretch reflexes are triggered, generating complex movements without central supervision

Question 14

What is Merton’s servo hypothesis (1953)???

Answer 14

Movement from posture - movements are generated by altering the set point of the posture control servo through the gamma motor neuron drive

Question 15

What are the 2 predictions from the reflex hypothesis?

Answer 15

1) movement is generated by stretch reflexes , which is a sensory afferent
2) gamma motor neuron drive precedes the movement (alpha motor neuron drive)

Question 16

What are the counter evidences of the reflex hypothesis?

Answer 16

Deafferented monkeys can reach the target , as afferent neural pathways in C2-T3 are disconnected.

Taub and Berman (1968) showed monkeys with deafferented forelimbs can reach to visual targets accurately, without view of their limbs. (Quality of pointing was worse than intact monkeys)

Muscle spindles discharges can be recorded using microneurography
Valbo (1970) - found slowly adapting muscle receptors in man, showed no gamma lead

Question 17

What is the problem of delay in the reflex hypothesis?

Answer 17

The reflex hypothesis assumes a feedback control system: movement is generated by sensation.
However, reflexes are inherently delayed :- 10-40 ms before a muscle spindles signal reaches the CNS, and another 40-60ms before the motor command reaches to muscles.

The delayed feedback system is often unstable and uncontrollable

Question 18

Why was EPH adopted?

Answer 18

All the counter evidences left to the reflex hypothesis being rejected. However the idea of an altering set point is still attractive and has evolved to the EPH (set point = equilibrium point)

Question 19

What does EPH assume?

Answer 19

Assumes that the brain controls a virtual equilibrium point (not physical reflexes) to control the reaching movement

Question 20

What did Polit and Bizzi (1979) find when looking at EPH?

Answer 20

When the arm is mechanically perturbed, due to deafferentation, the monkey cannot know whether the arm is perturbed or not.

The deafferented monkeys could reach to the target , and the reaching movements were robust to the external perturbation -> found that equilibrium point seemed to move along a virtual trajectory.

Question 21

What happened in Polit and Bizzi (1979) experiment when arm was moved passively to the target ?

Answer 21

It moved back toward the initial configuration and reached the target again.

Bizzi’ experiment provided strong support for the EPH and hence is still one of the most recognised hypothesis for control of reaching.

Question 22

What are the main assumptions/predictions of EPH??

Answer 22

The brain does not care where the arm is, only where the arm should be
So it doesn’t care that force is applied only during the movement, as long as the final configuration can be reached (equifinality)

Question 23

Lackner and Dizio (1994) tested whether subjects reaching movement is affected by the coriolis force, what would the EPH suggest?

Answer 23

1) subject cannot make straight hand movement

2) accuracy of reaching the target should not be changed

Question 24

What do the results of Lackner and Dizio rotating room experiment (1994) actually tell us?

Answer 24

The opposite of what was predicted by the EPH

• subject can make straight hand movement
• Accuracy of reaching target was changed

Hence, it shows us that EPH alone is not enough to explain the control of reaching

Question 25

After lackner and Dizio (1994) study, what new model was introduced ?

Answer 25

The direct cortical control model, which suggests that the brain does not only specify the EP, but also may plan the ‘trajectory’.

Question 26

How are trajectories planned? (2 of infinitely many choices)

Answer 26

Straight trajectory in the hand space: highly variable and curved in joint space

Straight trajectory in the joint space: highly curved and variable in the hand space

Question 27

What did Morasso (1981) state about trajectories?

Answer 27

Trajectories are planned in the hand space

Movements tend to occur along straight lines in the hand space
Hand speed adopts a well maintained bell shaped profile
High variability was observed in the joint space

Question 28

What is the optimality principle?

Answer 28

Considered the underlying principle behind planning the trajectory, the brain must be optimising a certain property of the trajectory - also called the optimal control model.
The ‘property’ can also be called cost - then optimisation becomes minimisation of the cost

Question 29

Discuss smoothness as a property to optimise

Answer 29

The brain will minimise the jerk of the trajectory (Flash and Hogan, 1984)

Displacement -> speed -> acceleration -> jerk

Predicted and measured are very similar for bell-shaped velocity profiles and speed profile of reaching with a via point

Question 30

What did Uno et al (1989) find when looking at properties of optimisation?

Answer 30

The brain also minimises the change of torque of the trajectory

Question 31

Harris and Wolpert, 1998 suggested what about control of reaching?

Answer 31

The brain also minimises the uncertainty of reaching, which is considered as a predominant model of the control of reaching .

Key concept is signal dependent noise. Movement causes the noise, whose size depends upon the control signal (input to the muscle) - a bigger more abrupt movement causes a bigger noise

Question 32

Discuss property to optimise - signal dependent noise?

Answer 32

The model can also explain the variability of the reaching movement.

Todorov and Jordan (2002) showed that variabilities of movement with different via points are well predicted. Different variability patterns suggest that the brain can flexibly ‘tune’ the reflex, depending on the task

Question 33

What is the optimal feedback control model?

Answer 33

Involved with flexible reflexes.

Depending on the task, reflexive behaviour of the hand is unaffective/affected by the perturbation of the other hand. This means that the brain has a flexible mechanism of optimally modulating the cortical reflexes - this is the optimal feedback control model

Question 34

What are some issues about the cortical control mechanism?

Answer 34

Cortical reflex has longer delays than reflex control model (+20-30ms)

The cortical control mechanism uses prediction (what will happen in the future)

Brain deals with stability problem with internal models