Motor Synergies:A Concept in Motor Control

Marieke RohdeCCNR – Centre for Cognitive Neuroscience and

RoboticsWorkshop on the Dynamical Systems

approach to Life and CognitionUniversity of Sussex, 8 and 9 March 2005

Structure

1. Motor Synergies

2. Directional Pointing• Linear Synergies in Human Directional

Pointing• Evolutionary Robotics Example

3. Conclusions

1.) Motor Synergies

Nicholas Bernstein

• Nicolas Bernstein (1967, but really 1935)– Physiology of Activity,

Biomechanics– Degrees of Freedom

Problem

The Degrees of Freedom Problem

• The “Cartesian Puppeteer” has to control a countless number of motor units.

7

26

2600

DoF

Motor Equivalence and Context-Conditioned Variability

• Motor Equivalence – Redundancy through many

degrees of freedom

• Context-Conditioned Variability:– Anatomical (role of a muscle is

context dependent)– Mechanical (command sent to

muscles is ignorant against motion/nonmuscular forces)

– Physiological (the spinal cord is not just a relay station)

The Solution

• Systematic relationships between actuators (constraints) can reduce the degrees of freedom to form functional motor units (e.g. wheel position in a car)

Motor Synergies!

• Skill Acquisition– First freezing degrees of freedom– Then freeing them and exploiting

passive dynamics

Biological Evidence for Synergies

• Systematicities in kinetics/kinematics: – Different types of gaits– Shooting– Breathing (Overview: Tuller et. Al. 1982)– Linear relation between

shoulder and elbow torque (Gottlieb et. Al. 1999)

• Complex behaviour as composition of synergies– Frog EMG data can be

explained as linear combination of 7 linear synergies (Saltiel et. Al. 2001)

Synergy between elbowand shoulder joint in a skilled marksperson

Problems with Motor Control through Synergy Control

• The reminder of the homunculus– How does it work?

• Acquisition and maintenance of synergies:– What is a good synergy?– What mechanism controls their development?

• Combination of synergies:– Who deals with non-linearities?

• Weiss, P. and M. Jeannerod (1998): – “Motor coordination is not the goal but a means to

achieve the goal of an action”

If there’s no homunculus…

…there’s no problem.

Still, the observed phenomena require explanation.

Evolutionary Robotics

• Things we can ask ourselves:– What does it imply if we have non-redundant models?– What does it imply if we do not have context-conditioned

variability?

• Things we can investigate:– More degrees of freedom– Anatomical, mechanical, physiological context dependence – Motor synergies in the absence of a homunculus– Impact of these factors on

• Behaviour• Evolvability • The “phylogenetic learning” process

2.) Directional Pointing

Linear Synergies in Human Directional Pointing

• Gottlieb et. Al. 1997: – Directional Pointing in the sagittal

plane– Linear relation:

– Systematic variation of scaling constant with pointing direction

– Linear synergies as an outcome of learning?

• Zaal et. Al. 1999: – Linear Synergies are not learned,

they constrain learning

Direction against scaling constant

Hand trajectories for pointing

Pre-reaching period

Experiments (Work in Progress)• Simulated Robotic Arm• Proprioceptive (joint angle) +

Directional Inputs• Fitness: Position at endpoint• Motor control:

– “Garden CTRNNs” with two motor neurons per degree of freedom

– “Split Brain CTRNNs” with separate controllers for joints

– Linear Synergy networks with just one motor output and evolved scaling

function (RBFNs)– 2 vs. 4 degrees of freedom for all of the above– 2 goals vs. up to 6 goals (additional goal once it has a certain level)

• Most severe simplifications: – Hand of 4 degrees model is squashed between two planes– No gravity

Screenshot of the simulated arm

Results

• Performance– DoFs: 4 twice as good as 2– Linear synergy are much better than CTRNNs (even linear linear

synergies are comparable)– Split brains are not a lot worse than ordinary CTRNNs

• How do they solve the problems?– 2 DoF’s: Frequently just use one joint – 4 DoF’s: exploit the invisible planes. – Linear Synergy: use different techniques, look a bit smoother– Split brains: Independence of joints very obvious

Results

• Phylogeny– CTRNNs freeze degrees of

freedom at first and then include them.

– Networks use passive dynamics straight away.

• Synergies– No linear synergies in any

CTRNN controllers.

3.) Conclusions

Conclusions: Evolutionary Robotics

• Redundant degrees of freedom can facilitate evolving a controller, in spite of the much bigger search space and lead to a better solution

• Learning under the constraint of linear synergy reshapes the search space and can lead to a very quick and successful evolution of different strategies (Careful with bias through model selection).

Conclusions: Synergies• A question we cannot answer (yet) is: Why are there linear

synergies? • The acquisition of synergies:

– Learning is not necessarily building up linear synergies.– The fact that the constraint of linear synergy boosts evolution suggests

its suitability for developmental processes. – CTRNNs freeze and free DoFs.– Some kind of synergy gives CTRNNs an advantage over split brain

CTRNNs.• The concept of synergy:

– It is very useful to explain behaviour in abstract terms.– Particularly, if more complex behaviour is investigated.– Thinking in terms of synergies raises different questions– You just have to be clear about your relation to the Homunculus idea.

Future Research• Input models:

– Make more CTRNN friendly– Visual inputs

• Get rid of the invisible planes• Evolve constraints for lifetime development• Use synergies in a larger context (co-evolution of car

and driver)• Investigate other forms of context conditioned variability

Any questions?

References• Arbib, M. A. (1981):  Perceptual Structures and Distributed Motor Control. 

In: V. B. Brooks (ed.): Handbook of Physiology. Section 2: The Nervous System. Vol. II, Motor Control, Part 1. American Physiological Society, 1449-1480.

• Bernstein, N. (1967): The Coordination and Regulation of Movements. Oxford: Pergamon.

• Berthouze, L. and M. Lungarella (2004): Motor Skill Acquisition Under Environmental Perturbations: On the Necessity of Alternate Freezing and Freeing of Degrees of Freedom. Adaptive Behavior, 12(1).

• Gottlieb, G. L., Q. Song, G. L. Almeida, D. Hong, and D. Corcos (1997): Directional Control of Planar Human Arm Movement. Journal of Neurophysiology 78:2985-2998.

• Grossberg, S. and Paine, R.W.(2000): A Neural Model of Corticocerebellar Interactions During Attentive Imitation and Predictive Learning of Sequential Handwriting Movements. Neural Networks, 13, 999-1046.

• Morasso, P., F.A. Mussa Ivaldi and C. Ruggiero (1983): How a discontinuous mechanism can produce continuous patterns in trajectory formation and handwriting. Acta Psychologica 54. pp. 83-98.

References• Sporns, O., and G.M. Edelman (1993): Solving Bernstein's problem: A

proposal for the development of coordinated movement by selection. Child Dev. 64:960-981.

• Saltiel, P., K. Wyler-Duda, A. d'Avella, M.C.Tresch and Bizzi, E. (2001): Muscle Synergies Encoded Within the Spinal Cord: Evidence From Focal Intraspinal NMDA Iontophoresis in the Frog. J. Neurophysiol., 85: 605-619.

• Tuller, B., H. Fitch and M. Turvey (1982): The Bernstein Perspective: II. The Concept of Muscle Linkage or Coordinative Structure. in: S. Kelso (ed.): Human Motor Behavior. An Introduction. Hillsdale: Lawrence Erlbaum.

• Turvey, M., H. Fitch and B. Tuller (1982): The Bernstein Perspective: I. The Problems of Degrees of Freedom and Context-Conditioned Variability. in: S. Kelso (ed.): Human Motor Behavior. An Introduction. Hillsdale: Lawrence Erlbaum.

• Weiss, P. and M. Jeannerod (1998): Getting a Grasp on Coordination. News Physiol. Sci. 13. 70-75.

• Zaal, F., Daigle, K., Gottlieb, G.L., Thelen, E. (1999): An unlearned principle for controlling natural movements. Journal of Neurophysiology, 82:255-259.