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CONSTANT EFFORT COMPUTATION AS A DETERMINANT OF MOTOR BEHAVIOR

CONSTANT EFFORT COMPUTATION AS A DETERMINANT OF MOTOR BEHAVIOR. Emmanuel Guigon, Pierre Baraduc, Michel Desmurget INSERM U483, UPMC, Paris, France INSERM U534, « Space and Action », Bron, France. Amplitude (cm). MOTOR BEHAVIOR: CONSTRAINED. AMPLITUDE / VELOCITY AMPLITUDE / DURATION.

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CONSTANT EFFORT COMPUTATION AS A DETERMINANT OF MOTOR BEHAVIOR

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  1. CONSTANT EFFORT COMPUTATIONAS A DETERMINANT OF MOTOR BEHAVIOR Emmanuel Guigon, Pierre Baraduc, Michel Desmurget INSERM U483, UPMC, Paris, France INSERM U534, « Space and Action », Bron, France

  2. Amplitude (cm) MOTOR BEHAVIOR: CONSTRAINED AMPLITUDE / VELOCITY AMPLITUDE / DURATION Gordon et al. (1994)

  3. Gordon et al. (1994) MOTOR BEHAVIOR: CONSTRAINED KINEMATIC INVARIANCE

  4. MOTOR BEHAVIOR: CONSTRAINED CONSTRAINTS ACROSS DIRECTIONS Gordon et al. (1994)

  5. MOTOR BEHAVIOR: CONSTRAINED SPEED VS ACCURACY Fitts (1954) Jeannerod (1988)

  6. MOTOR BEHAVIOR: FLEXIBLE INDEPENDENT CONTROL OF KINEMATICS AND ACCURACY Gribble et al. (2003)

  7. KNOWN PRINCIPLES OC: optimal control - OFC: optimal feedback control - SOFC: stochastic OFC EPT: equilibrium-point theory - SDN: signal-dependent noise - SEN: state-estimation noise

  8. CURRENT PRINCIPLES • Optimal feedback control Constraints: to reach the goal (zero-error) Objective (cost): to minimize the controls (effort) • Constant effort For given instructions, all movements are performed with the same effort • Cocontraction as an independent parameter • State-estimation noise Inaccuracy in estimation of position and velocity Increases with velocity Decreases with cocontraction (fusimotor control)

  9. OPTIMAL CONTROL PROBLEM No static forces. No viscosity. Same formulation for OFC. Solved numerically (Bryson 1999). Muscles as force generator. No force/length effects. No force/velocity effects. No stretch reflex. No biarticular muscles.

  10. KINEMATICS

  11. EMGs SHOULDER ELBOW

  12. AMPLITUDE / DURATION

  13. KINEMATIC INVARIANCE Also holds for changes in inertial load.

  14. DIRECTIONAL VARIATIONS

  15. SHOULDER ELBOW KINEMATICS & ACCURACY • Same amplitude • Same duration • Similar kinematics • Different accuracy • - OFC + SEN • - Estimation of endpoint position: linear forward model • Gaussian noise on velocity • Variability: determinant of terminal covariance matrix

  16. WHAT ARE THE CONTROLS? SHOULDER FLEXOR CONTROL Sergio&Kalaska (1998)

  17. FLEXOR EXTENSOR SHOULDER ELBOW DIRECTIONAL TUNING Sergio&Kalaska (1998)

  18. SUMMARY • Known principlesOPTIMAL FEEDBACK CONTROL STATE-ESTIMATION NOISE Trajectory EMG Speed/accuracy Central command • New principlesCONSTANT EFFORT COCONTRACTION Amplitude/duration Kinematic invariance Constraints across directions Kinematics/accuracy

  19. DISCUSSION • Kinematic invariance Without desired trajectory. • Constant effort Movements are selected not by minimizing a cost, but by choosing a cost level • Limitations / Extensions - Static forces - Limitations of force control (Ostry&Feldman 2003) - Accuracy/stability: viscoelastic properties - Adaptation to force fields and inertial loads

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