Combining modules for movement

Abstract

We review experiments supporting the hypothesis that the vertebrate motor system produces movements by combining a small number of units of motor output. Using a variety of approaches such as microstimulation of the spinal cord, NMDA iontophoresis, and an examination of natural behaviors in intact and deafferented animals we have provided evidence for a modular organization of the spinal cord. A module is a functional unit in the spinal cord that generates a specific motor output by imposing a specific pattern of muscle activation. Such an organization might help to simplify the production of movements by reducing the degrees of freedom that need to be specified.

Modeling discrete and rhythmic movements through motor primitives: a review

“…Strong evidence, notably through the concepts of central pattern generators and force fields (see resp. reviews by Grillner (2006) and Bizzi et al (2008)), supports the existence of such functional modules at the spinal level in vertebrate animals. For instance, Kargo and Giszter (2000) have demonstrated how a finite set of spinal motor primitives could account for the natural wiping reflex in the frog, showing that the central nervous system (CNS) could use such primitives to produce natural behaviors.…”

“…Indeed, they brought to light that movements were generated in a modular way by the spinal cord in frogs (for a comprehensive review, see Bizzi et al (2008)). More precisely, stimulating specific interneuronal areas of the spinal cord, they observed that the limb was moved in the direction of the same target posture (equilibrium point) whatever the initial position of the limb was.…”

“…To explain both adaptivity and robustness, a very promising perspective is the modular approach to movement generation: Movements result from combinations of a finite set of stable motor primitives organized at the spinal level (see Bizzi et al (2008) for a review). In this article, a motor primitive is defined as a network of spinal neurons that activates a set of muscles (that we call a synergy) in a coordinated way in order to execute a specific movement.…”

The impact of visual feedback on the motor control of the upper-limb

“…(1) VF can effectively increase the inter-limb similarity of synergies and activation coefficients. (2) In the case of synergies, the improvement is achieved by reducing the interlimb muscle-weight differences of synergies (3). In the case of activation coefficients, the improvement is achieved by approximating the activation coefficient of the left arm towards the activation coefficient of the right arm.…”

“…Urra, A. Casals and R. Jané are with the Universitat Politècnica de Catalunya -BarcelonaTech and Institute for Bioengineering of Catalonia (IBEC), Baldiri Reixac 4, Tower I, 9 th floor, 08028, Barcelona, Spain (phone: +34 (93) 4039706; fax: +34 (93) 4039702; e-mail: ourra@ibecbarcelona.eu, acasals@ibecbarcelona.eu, rjane@ ibecbarcelona.eu) R. Jané is also with CIBER de Bioingeniería, Biomateriales y Nanomedicina, Spain. Our brain achieves motor coordination by combining simultaneous activations of diverse muscle groups known as synergies [2][3]. In this framework, a control structure is defined as the set of synergies and activation coefficients needed to perform a movement.…”