Control Of Reaching Flashcards
What 3 problems does the brain have to solve about the control of reaching??
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
What area of the brain is concerned with localisation and planning??
The posterior parietal cortex (in monkeys brain)
- firing of neurons in the PPC are related to the movement planning
What does the lateral intraparietal area do??
Needed for saccadic eye movements
What does the parietal reach region (PRR) and the anterior intraparietal area (AIP) do?
PRR - for reaching
AIP - for grasping
How does the PPC plan movement?
By closely coordinating the eye and hand
After localisation, the object is internally represented based in a specific coordinate system. What are the two options of coordinate systems?
Hand - centered coordinate
Eye - centered coordinate
What did Bastia et al (1999) find when looking at reach plans in eye-centered coordinates?
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
How can a monkey elicit voluntary, motor behaviours?
Microstimulation of neurons in the monkeys primary motor cortex
In monkeys, the motor cortex is concerned with what movements? (Graziano, 2016)
Climbing/leaping, hand in lower space, chewing/licking, hand to mouth, defence, reach to grasp, manipulation in central space
What did Graziano et al (2003) find when looking at purposeful reaching movements?
They are generated by stimulating reach-related neurons. Neurons tend to code the final reach position, rather than the starting position
How are the purposeful reaching movements controlled?
Neurons in the primary motor cortex have their own preferred directions in what is known as directional tuning
Why is a look up table not suitable as being the mechanism for generating muscle force patterns to reaching movements??
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)
What is the basic premise of Sherringtons reflex hypothesis?
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
What is Merton’s servo hypothesis (1953)???
Movement from posture - movements are generated by altering the set point of the posture control servo through the gamma motor neuron drive
What are the 2 predictions from the reflex hypothesis?
1) movement is generated by stretch reflexes , which is a sensory afferent
2) gamma motor neuron drive precedes the movement (alpha motor neuron drive)
What are the counter evidences of the reflex hypothesis?
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
What is the problem of delay in the reflex hypothesis?
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
Why was EPH adopted?
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)
What does EPH assume?
Assumes that the brain controls a virtual equilibrium point (not physical reflexes) to control the reaching movement
What did Polit and Bizzi (1979) find when looking at EPH?
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.
What happened in Polit and Bizzi (1979) experiment when arm was moved passively to the target ?
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.
What are the main assumptions/predictions of EPH??
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)
Lackner and Dizio (1994) tested whether subjects reaching movement is affected by the coriolis force, what would the EPH suggest?
1) subject cannot make straight hand movement
2) accuracy of reaching the target should not be changed
What do the results of Lackner and Dizio rotating room experiment (1994) actually tell us?
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
After lackner and Dizio (1994) study, what new model was introduced ?
The direct cortical control model, which suggests that the brain does not only specify the EP, but also may plan the ‘trajectory’.
How are trajectories planned? (2 of infinitely many choices)
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
What did Morasso (1981) state about trajectories?
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
What is the optimality principle?
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
Discuss smoothness as a property to optimise
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
What did Uno et al (1989) find when looking at properties of optimisation?
The brain also minimises the change of torque of the trajectory
Harris and Wolpert, 1998 suggested what about control of reaching?
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
Discuss property to optimise - signal dependent noise?
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
What is the optimal feedback control model?
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
What are some issues about the cortical control mechanism?
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
