Models of motor control (wk 2)

Question 1

Describe reaching and the look-up table:

Answer 1

-Reaching -> Most models focused on reaching. Most common and inherent human movement. One of the first sensorimotor skills. Critical skill for survival.
-Look-up table -> Actions are written in a spreadsheet – with the spreadsheet being your brain. In order for actions to be produced, the spreadsheet is copied and read.

Question 2

What is the stretch reflex?

Answer 2

-> Alpha and Gamma motor neuron in muscle and muscle spindle. Stretch reflex keeps the muscles to be in a desired length – muscle as a spring. Alpha motor neurons causes muscle contraction. Gamma motor neurons adjust the sensitivity of the reflex and allows muscles to contract until they reach desired lengths.

Question 3

What is Sherrington’s reflex hypothesis?

Answer 3

-Sherrington’s reflex hypothesis (1910) -> Movements are obtained by combining stretch reflexes, i.e. by simply changing parameters of muscle springs. Brain sets the desired length of the muscle springs and then stretch reflexes are triggered, generating complex movements without central supervision. Further developed to Merton’s Servo Hypothesis (1953) – Movements from posture (movements are generated by altering the set-point of the posture control through gamma motor neuron drive)

Question 4

What are the 2 predictions from reflex hypothesis and the counter evidence?

Answer 4

1. Movement is generated by stretch reflexes, which is a sensory afferent
2. Gamma motor neuron drive precedes the movement (alpha motor neuron drive)
   -> Counter-evidence – Monkeys with deafferented fore limbs can reach to visual targets accurately, without view of their limbs (Taub and Berman 1968)
   -> Counter-evidence – Muscle spindle discharges can be recorded using microneurography

Question 5

What is the equilibrium point hypothesis?

Answer 5

-> Assumes that the brain controls a virtual equilibrium point (not physical reflexes) to control the reaching movement. No specific internal model is needed, except the ones on static poses.

Question 6

What is the EPH study (Bizzi)?

Answer 6

-> EPH – (Bizzi) Deafferented monkeys could reach the target. The reaching movements… When the arm was passively moved toward the target, it moved back toward the initial configuration -and reached the target again.

Question 7

Describe Lackner and DiZio (1994) - Reaching under velcoity-dependent force

Answer 7

Lackner and DiZio (1994) tested whether subjects reaching movement is affected by the Coriolis force. The Coriolis force depends only on velocity, that is, the force is applied only during movement, not when stopping. The brain according to EPH, should only care about driving the arm to the equilibrium point, regardless of the trajectory it takes.

Question 8

Describe the 2 predictions from EPH

Answer 8

1. Subjects cannot (or do not bother to) make straight movements
2. However, the accuracy of the final reaching position

Question 9

Describe the predictions of reaching under velocity-dependent force:

Answer 9

1. Subjects cannot (or do not bother to) make straight movements – Trajectory error will be high
2. However, the accuracy of the final reaching position should be maintained near zero – Endpoint error will be close to zero
   -> But the result was:
3. Reduced Trajectory error
4. Sustained Endpoint error -> Both results are opposite to the prediction of EPH