Models of Motor Control Flashcards
1
Q
what is a look up table
A
- stores data on muscle forces to aid movement
- cannot deal with the situations
- when the arm is perturbed during movement
- when the weight of the arm changes (e.g. holding a coffee cup)
2
Q
what is the stretch reflex
A
- alpha and gamma motor neurons in muscle and muscle spindle
- stretch reflex keeps the muscle to be in a desired length - muscle as a spring
- alpha motor neurons cause muscle contraction
- gamma motor neurons adjust the sensitivity of the reflex and allows muscles to contract until they reach desired lengths
3
Q
what is Sherrington’s reflex hypothesis (1910)
A
- movements are obtained by combining stretch reflexes i.e. by simply changing parameters of muscle springs
- brain just sets the desired length of the muscle springs and then stretch reflexes are triggered, generating complex movements without central supervisions
- futher developed to Mertons servo hypothesis (1953)
- movement from posture - movements are generated by altering the set point of the posture control servo through gamma motor neuron drive
4
Q
what are the reflex hypothesis predictions
A
- movement is generated by stretch reflexes, which is a sensory afferent
- gamma motor neuron drive precedes the movement (alpha motor neuron drive)
5
Q
what are the reflex hypothesis counter evidences
A
- afferent neural pathways in C2-T3 disconnected
- Taub and Berman (1968) “deafferentation in monkeys: pointing at a target without visual feedback”
- monkeys with deafferented force limbs can reach to visual targets accurately, without view of their limbs
- muscle spindle discharges can be recorded using microneurography
- Vallbo (1970) “slowly adapting muscle receptors in man”
6
Q
what are the reflex hypothesis problems of delay
A
- the reflex hypothesis assumes a feedback control system: movement is generated by sensation - delay makes the system unstable
- note that this hypothesis was introduced way before the concepts of internal model and anticipatory control were introduced in motor control
- internal models are components of the cortical control
7
Q
what is equilibrium point hypothesis (EPH)
A
- by those counter-evidences, Sherrington’s reflex hypothesis was rejected
- however, the idea of altering set-point is still attractive and it has evolved to equilibrium point hypothesis (set point = equilibrium point)
- equilibrium point hypothesis assumes that the brain controls a virtual equilibrium point (not physical reflexes) to control the reaching movement
- no specific internal models is needed, except the ones on static poses
- Polit and Bizzi (1979)
- due to deafferentation, monkey cannot know whether its arm is perturbed or not
- deafferented monkeys could reach to the target
- the reaching movements were robust to the external perturbation
- equilibrium point seemed to move along a virtual trajectory
- when the arm was passively moved to the target, it moved back toward the initial configuration and reached the target again
- Bizzi’s experiment provided a strong support for EPH
- it is still one of the valid hypothesis of the reaching control
- however, critical counter evidences were also presented against the main predictions of EPH
- main assumptions and predictions of EPH
- brain does not really care where the arm is. It only cares where the arm should be
- so it does not care the forces applied only during the movement, as long as the final configuration can be reached (equifinality)
8
Q
what is reaching under velocity dependent force
A
- Lackner and DiZio (1994) tested whether subjects reaching movements is affected by the coriolis force
- subject seated in the centre of a dark rotating room (only the target was visbale)
- coriolis force, which only depends on the velocity, was applied to the arm
- predictions from EPH
- subject cannot make straight hand movements
- however, the accuracy of reaching the target should not be changed
- results
- subjects were able to make straight hand movement
- however, the accuracy of reaching the target changed
9
Q
what is the direct cortical control model
A
- rotating room experiment showed that EPH alone is not enough to explain the control of reaching
- it is suggested that the brain does not only specifiy the EP but may also plan the “trajectory” - direct corticol control model
10
Q
what is the optimal control models of movement
A
optimal control model assumes that the controller tries to minimise (or maximise) a certain cost (or a certain benefit) produces by the resultant action
11
Q
what is the property to optimise
A
- smoothness
- the brain minimises the jerks of the trajectory [Flash and Hogan (1984)]
- displacement -> speed -> acceleration -> jerk
- energy (torques)
- the brain minimises the change of torque of the trajectory [Unoetal (1989)]
- uncertainty
- the brain minimises the uncertainty of reaching [Harris and Wolpert, nature (1998)]
- considered as a predominant model of reaching
- key concept: signal dependent noise
- movement cause the noise, whose size depends on the control signal (input to the muscle)
- “bigger and more abrupt movement causes bigger noise”
12
Q
what is signal dependent noise
A
- 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
13
Q
what is the optimal feedback control model
A
- optimal feedback model assumes that brain stored policies, not the commands (i.e. plan)
- optimal feedback control assumes that brain keeps feedback policies for different motor skills, we already know that having policies rather than the plan brings important advantages (robust to the error and etc)
- however, the term “feedback” is not really about a sensory feedback. It is a virtual feedback like behaviour generated based on the internal models
