Download Action: What`s the Problem?

Action:
What’s the Problem?
CLPS1500 Ecological Approach
to Perception and Action
Prof. William Warren
Why Action?
Information (flow)!
Laws of!
Ecological!
Optics!
Environment!
Laws of!
Control!
Agent!
action (F)!
!   The function of perception is to control action
!   To understand how perception can control action, must
understand how action is organized
! 
! 
Coordination: organization among parts of action system
Control: regulation of action system for a task goal
What’s the Problem?
!   Not obvious -- we take movement for granted. But we still can’t program a
robot to walk down the street
! 
! 
Coordination: How walk down the street and chew bubblegum at the same time?
Control: How walk fast, step on a crack, avoid an obstacle?
!   Nikolai Bernstein (1896-1966)
!   Russian physiologist
!   Translated in 1967, large influence in West since 1980s (Turvey, 1977)
!   Bernstein’s problems
1.  Degrees-of-freedom problem
2.  Context-conditioned variability problem
1. The DF Problem
The Centipede was happy, quite
Until the Toad in fun
Said, “Pray which leg goes after which?”
And worked her mind to such a pitch,
She lay distracted in the ditch
Considering how to run.
-- Mrs. Edmund Craster, c. 1870
!   How control a system of many moving parts?
!   Central executive can’t control them independently.
The Degrees-of-Freedom Problem
!   How many people did it take to run ET?
!   Marionette (13 joints)
!   Imagine trying to make it do a pirouette
!   How control a system of many df?
! df = Number of values required to specify the state of
the system (positions of all joints)
!   If each joint is controlled independently,
computational burden too great for central executive
! 
! 
ET required 10 central executives!
In biological systems, df are not controlled
independently
!   Related to redundancy problem (motor equivalence):
!   Same goal can be achieved by different movements
!   Too many df. How constrain them to yield a unique
movement?
DF of Human Body
!   75 joints à 100 mechanical df
!   792 muscles à motor df
!   >100,000 motor units à neural df
!   An NP-complete problem: computational
burden increases exponentially with
number of df
! 
interactions
(a) Joints and (b) Muscles
Hinge Joints!
•  1 df: knee, elbow
•  ≥ 2 muscles
•  flexor/extensor
•  agonist/antagonist
Ball & Socket Joints!
•  3 df: shoulder, hip
•  Multiple muscles:
prime mover,
synergists
Biceps!
Brachialis!
Triceps!
(c) Motor Units
<--Afferents
(dorsal root)!
IA!
IB!
α
Golgi tendon
organ!
muscle
spindles!
Intrafusal!
Efferents-->
!   Golgi tendon organ
!   IB afferent fires if force on tendon
!   γ-motoneuron
!   sets sensitivity of muscle spindle
(ventral root)!
Spinal
column!
!   α-motoneuron
!   contracts muscle
!   Muscle spindle
!   IA afferent fires if stretched
extrafusal!
γ
!   Motor Unit (100s per muscle)
!   α-motoneuron
!   and innervated muscle fibers
Muscle!
!   Stretch reflex: system keeps muscle
length constant with varying load
!   Change muscle length by
coactivation of α and γ mn
Bernstein’s Solution: Synergies
head
R arm
R leg
x1
!   How control a system with many df?
!   functionally couple df in low-level “automatisms”
L arm
L leg
x2
!   Synergies, coordinative structures
!   Equation of constraint (function over df)
!   Reduces the effective df that must be controlled
!   But where do the linkages (the organization)
come from?
y = f ( x1, x 2 ,...x n )
xn
control!
€
coordination!
Locomotor Automatism
(Brown, 1911; Shik & Orlovskii, 1965)
! Decerebrate (“spinal”) cat
!   Exhibits normal gait
!   Alternate firing of flexors & extensors
!   Tonic stimulation of brain stem -->
increased speed, gait transitions
!   Gait organized at the spinal level
! 
! 
Spinal “rhythm generator” (Brown)
General speed control parameter
Remote Compensation
(Gracco & Abs, 1985)
!   Hallmark of a synergy
! df coupled to achieve functional goal
!   Perturb lower lip while saying “aba”
!   See adaptive compensation in upper lip (EMG ~50 ms)
!   Preserves goal of lip closure
Where does the organization in
behavior come from?
!   Hyp 1: Central controller
! 
! 
! 
Organization assumed a priori, imposed on the musculature
Action plan, motor program, neural pattern generator
Prior knowledge!
!   Hyp 2: Self-organized, emergent behavior
! 
! 
Organization emerges a posteriori from processes of pattern formation
Walk & chew bubblegum: entrainment between low-level automatisms
2. The Problem of Contextual Variability
!   Indeterminacy
!   No fixed relation between central command and resulting movement
!   Depends on context
!   Inverse problem: How determine the motor command for a desired
movement?
!
Equifinality
! 
Achieve functional goal despite varying context
!   Sources of indeterminacy
!   Anatomical, mechanical, physiological variability
(a) Anatomical Variability
!   The role of a particular muscle in producing a given movement
depends on context
Exercise:
!   Lowering your arm
!   Deltoid
!   Lowering arm against a
resistance
!
Latissimus dorsi
(b) Mechanical Variability
!   The relation between muscle activity and the resulting movement depends
on the current state of the limbs
! 
! 
When arm is below horizontal, pectoralis major à flexor (forward)
When arm is above horizontal, pectoralis major à extensor (backward)
!   Non-muscular forces: gravity, inertia
!   If arm is stationary, contract pectoralis major to start forward movement
!   But if arm is moving backward, contract it to stop movement
!   Each movement alters the initial conditions for the next movement
(c) Physiological Variability
!   The relation between a cortical “command” and the resulting movement
depends on the state of the spinal interneuron pool
!   e.g. Tonic stimulation of brain stem
! 
! 
Effect depends on spinal organization
Speed parameter for locomotor automatism
Bernstein’s Solution: Synergies again
!   Low-level automatisms automatically adjust for varying
conditions, at the spinal level
!   Motor system takes advantage of non-muscular forces
!   Thelen: infant kicking
!   Learn to reduce active muscle contribution to movement
Locomotor Automatism:
Context Sensitivity
!   Spinal rhythm generator is modified
by sensory fb
! 
! 
swing à flex
stance à extend
Extensors tonically active
Flexors (swing) initiated by sensing
full leg extension & leg unloading
!   Phase-dependent reflex reversal
!   Touch paw during stance --> extend
!   Touch paw during swing --> flex
Job Description for a
Coordinative Structure
df: Functionally couple df to yield stable coordination patterns
! Equifinality: Converge on goal from various initial conditions
!   Motor equivalence: Family of movements all achieve goal
!   Contextual variability: Automatically adapt to varying conditions
!
!   What sort of system can do this??