Movement Primitives in the Frog Spinal Cord

We present evidence for the generation of stable convergent force field patterns and muscle synergies in the spinal cord of the frog. These synergies may form the bases of postural and trajectory adjustments. We present recent analyses which show that (1) similarly structured force fields underlie natural behaviors (2) the active fields underlying both microstimulation and natural behavioral force fields are structurally invariant but may be modulated in overall force amplitude and stiffness (3) these fields are drawn from a limited set of such fields (4) these invariant fields can be used to predict the termination position of limb endpoint trajectories in the unrestrained limb (5) in those instances tested multiple stimulations resulted in fields proportional to the vectorial summation of the fields resulting from single stimulations. This recent body of work suggests that we may view the spinal cord as possessing a small number of movement primitives: circuits that may specify invariant force fields which may be combined in a more or less flexible manner to produce adjustable behaviors.

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