Control of reaching Flashcards
what is localisation?
Representation of the location of the object
define planning
Plan of reaching based on the representation
Control
Generate muscle forces to drive the arm to the desired position
How and where is localisation and planning done in the brain?
in the posterior parietal cortex
does this by closely coordinating hand and eye movements through neuron firing. It is mainly eye-centred coordination
How and where is execution and control done in the brain?
Happens in the motor cortex. Stimulation of different regions evokes different behaviours.
Neurons in primary motor cortex have their own preferred directions - directional tuning
Which is the most basic control model of reaching?
Look-up table model
What is the Look-up table Model?
have a table that the brain uses that related to muscle force to the desired reaching movement.
but the data base would become too big
Explain the stretch reflex as a control model of Reaching
Keeps the muscle to be in a desired length - muscle acts as a spring
Gamma motor neurons adjust the sensitivity of the reflex
What is Sherrington’s reflex hypothesis? and how was this then further developed into merton’s servo hypothesis?
Sherrington’s - reaching movement is obtained by combining stretch reflexes
- merton’s - movements are generated by altering the set-point of the posture control servo through gamma motor neuron drive
What are the 2 predictions from the reflex hypothesis (control model of reaching)
- movement is generated by a stretch reflex, which is a sensory afferent
- Gamma motor neuron drive precedes the movement (alpha motor drive)
Aphla and gamma motor neurons contract which parts of the muscle?
Alpha = innervate extrafusal fibres Gamma = Innervate intrafusal fibres
What are the 2 pieces of evidence that disprove the predictions of the reflex hypothesis?
- When the afferent neural pathways in C2-T3 are disconnected there is no way the signal can go from your arm to the brain, however you can still reach to a target when deafferented
- Muscle spindle discharges can be recorded using microneurography - there is no gamma lead when looking at these signals
What does the equilibrium point hypothesis assume?
It assumes that the brain controls a virtual equilibrium point (not physical reflexes) to control the reaching movement
What are the main assumptions for the Equilibrium Point Hypothesis?
- the brain does not care for where the arm is (initial point). It only cares for where it should be (final end point) - therefore this hypothesis is dependent of final position and independent of initial position.
It does not care for the forces applied
what is the evidence that the equilibrium point hypothesis does not care for the forces applied during the movement?
study conducted that the coriolis force was applied tot he arm. subjects were still able to move their limb, however the accuracy was decreased.
What are the 2 main options in which trajectories are planned?
- Straight trajectory in the hand space: curved in joint space
- Straight trajectory in the joint space: curved in the hand space.
soo the curved part is what is altering the trajectory
What are trajectory plans dependent and independent of?
Independent of the joints
Dependent of the hand space
What is the name of the underlying principle of planning the trajectory and what does it mean?
Optimality principle = the brain must be optimizing a certain property of the trajectory - also known as the optimal control model
What are the 4 properties that the brain could optimise when it comes to planning a trajectory?
Smoothness - brain will minimise the jerk of the trajectory
Energy (torque) - the brain minimises the change of torque
Uncertainty - the brain minimises the uncertainty of reaching
Signal dependent noise - Movement causes noise, whose size depends on the control signal (input to the muscle) - bigger and more abrupt movements cause bigger noise
Different variability patterns suggest that…
the brain can flexibly ‘tune’ the reflex, depending on the task.
explain optimal feedback control in terms of flexible reflexes
A task is selected and the optimal feedback control law decides on a motor command. However, there is noise. This is assessed and adjusts the feedback control so the goal is reached and the noise is taken into account.
Optimal feedback control cares more about your destination rather than hoe you are getting there
