A Concept in Motor Control
CCNR – Centre for Cognitive Neuroscience and
Workshop on the Dynamical Systems
approach to Life and Cognition
University of Sussex, 8 and 9 March 2005
1. Motor Synergies
2. Directional Pointing
• Linear Synergies in Human Directional
• Evolutionary Robotics Example
• Nicolas Bernstein (1967,
but really 1935)
– Physiology of Activity,
– Degrees of Freedom
The Degrees of Freedom Problem
• The “Cartesian
Puppeteer” has to
control a countless
number of motor
Motor Equivalence and Context-
• Motor Equivalence
– Redundancy through many
degrees of freedom
– Anatomical (role of a muscle is
– Mechanical (command sent to
muscles is ignorant against
– Physiological (the spinal cord is
not just a relay station)
• Systematic relationships
between actuators (constraints)
can reduce the degrees of
freedom to form functional motor
units (e.g. wheel position in a
• Skill Acquisition
– First freezing degrees of freedom
– Then freeing them and exploiting
Biological Evidence for Synergies
• Systematicities in
– Different types of gaits
(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 elbow
and shoulder joint in a
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.
• 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
• 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
• The “phylogenetic learning” process
Linear Synergies in Human
• Gottlieb et. Al. 1997:
– Directional Pointing in the sagittal
– Linear relation:
– Systematic variation of scaling
constant with pointing direction
– Linear synergies as an outcome of
• Zaal et. Al. 1999:
– Linear Synergies are not learned,
they constrain learning
Direction against scaling constant
Hand trajectories for pointing
Experiments (Work in Progress)
• Simulated Robotic Arm
• Proprioceptive (joint angle) +
• 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
– 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
– 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
– CTRNNs freeze degrees of
freedom at first and then
– Networks use passive
dynamics straight away.
– No linear synergies in any
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).
• A question we cannot answer (yet) is: Why are there linear
• 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
• 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.
• 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
• Investigate other forms of context conditioned variability
• 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,
• Bernstein, N. (1967): The Coordination and Regulation of Movements.
• 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
• 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.
• Sporns, O., and G.M. Edelman (1993): Solving Bernstein's problem: A
proposal for the development of coordinated movement by selection. Child
• 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
• 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.
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