Tutorial 3 - Motor Control

Question 1

What is the problem of DELAYED SENSORIMOTOR FEEDBACK for the motor system?

Answer 1

Slow sensory feedback can DESTABILIZE MOVEMENTS
–> Essentially, it results in continual over-adjustments,
(the absence of feedback leads to additional movements, and then when the feedback arrives it shows the movement has GONE TOO FAR - so a big counter-adjustment is made)

Question 2

How could a forward model solve this problem?

Answer 2

A forward model would GENERATE A PREDICTION of the position and velocity of a limb based on prior motor commands
–> this would enable feedback control to happen in real-time and avoid the instability associated with feedback delays

Question 3

What additional advantages do forward models have?

Answer 3

FMs can account for DEVIATIONS affecting the
- directly deviated muscle
- knock-on effects on other, connected muscles

Essentially: forward models can take into account the more complex physical dynamics of the limb

Question 4

What evidence is there to suggest that the CEREBELLUM is able to generate forward models?

Answer 4

TMS to the cerebellum disrupts the ability to make accurate movements to a target that has disappeared
–> Suggests that the cerebellum computes the likely position of the arm in the future (eg. when the movement is initiated) and compensates by adjusting the trajectory

Question 5

Predicting sensory consequences of motor commands on perception: What is the role of the parietal cortex?

Answer 5

After making a movement, the brain is able to combine two sources of information
- the prediction of the consequences of movement (provided by the forward model)
- the sensory feedback after movement completion

Neurons in the PARIETAL CORTEX have been shown to remap their receptive fields before an eye movement occurs, based on a prediction of the sensory consequences of movement
–> essentially: the parietal neurons provide a MODEL of what the world will look like after the eye movement

Question 6

What would happen if we couldn’t predict the sensory consequences of making eye movements?

Answer 6

When we move our eyes across a scene the brain is normally able to PREDICT that the stationary image will move across the retina (so perception is adjusted accordingly)

–> WITHOUT this ability: the world would appear to move every time we move our eyes (image would BLUR)