Movement Execution and Control

Question 1

how do neurons influence movement

Answer 1

• micro-stimulation of neurons in the primary motor cortex can elicit voluntary motor behaviours
• purposeful reaching movements are generated by stimulating reach-related neurons
• neurons tend to code the final reach position rather than the starting position
• neurons in primary motor cortex have their own preferred directions
  
  • directional tuning

Question 2

what are feedforward controllers in human motor control

Answer 2

• ballistic, open-loop control is needed to generate very quick movement, when there is no time to process the sensory feedbacks
• typical sensorimotor delay is ~200msec
• fast eye movements, called saccades last only <200msec
• no time to accommodate feedback information which means saccades are controlled feedforwardly
• feedforward control is also needed for movement that requires very little latency: vestibulo-ocular reflex (VOR)
• typical latency of VOR is 8~9msec initiated way earlier than visual feedback kicks in
• unlike saccades which are pre-programmable, VOR is not programmable

Question 3

what is the problem of delay is visual feedback

Answer 3

• ~100ms to be processed in the retina and transmitted to visual cortex
• central processing (among neurons) add another ~100ms

Question 4

what is the problem of delay in muscle feedback (reflexes)

Answer 4

10-40ms before a muscle spindle signal reaches CNS

Question 5

what is an issue of the delayed feedback system

Answer 5

it is often unstable and uncontrollable

Question 6

what are strategies to compensate for sensorimotor delays

Answer 6

• intermittency: pause until sensory feedback arrives and then resume
  
  • saccades
  • manual tracking
  • balancing
• prediction compensates for sensorimotor delays
  
  • if you know what is going to happen in the future, as a consequence of your own action, you can proactively control your movement based on the prediction

Question 7

how does forward model enable anticipatory control

Answer 7

• the forward model predicts the consequence of action based on the efference copy
• this prediction can be done with a significantly less delay - you can anticipate what sensory feedback will be received before it arrives
• early errors made during reaching are corrected quickly before the sensory feedback arrives (~45ms after the movement onset)

Question 8

should feedforward or feedback be used

Answer 8

• due to apparent trade-off between feedforward and feedback, the brain incorporates both control strategies (in sequence)
• predictive (anticipatory) control needed for quick reaction
• feedback adjustment needed for flexible tuning

Question 9

what is the observer model

Answer 9

• with prediction by forward model, the sensory feedback can be divided into two types:
  
  • sensory feedback that I already expected: no surprise
  • sensory feedback that I didn’t expect surprise (also called innovation
• the mismatch between prediction and senosry feedback can arise because
  
  • your forward model is wrong
  • your sensory feedback is corrupted by noise
• observer model: how to mix what we predicted and what was sensed

Question 10

what do internal models enable

Answer 10

enable prediction, by which our brain does anticipatory control of the movement