Modular circuitry for combining and controlling degrees of freedom in the limb

Work in the frog spinal cord, and more recently in mammals, suggests that the degrees of freedom problem in motor planning may be simplified by building motor actions from combinations of motor primitives. How motor primitives arise from spinal circuits, their properties and their plasticity are important issues for this framework. We have tested properties of primitives using various physiological perturbations in simple behaviors, and used various decomposition techniques (independent components analysis, matching pursuit cosine packet analysis, wavelet methods) in both simple and complex behaviors to examine the muscles that are controlled as groups or units, and to investigate the pattern and time scale of action of primitives in behaviors. Our work suggests that, for primitives organized in the spinal cord of the frog, modifications of time scale (e.g. temporal duration of primitives) in both simple spinal (e.g. wipe/scratch) and complex behaviors (e.g. struggling) are imposed or enabled by mechanisms residing above spinal cord in the medulla, suggesting a heterarchical control of the primitives in the spinal cord. In the absence of medulla, the spinal cord operates with a relatively fixed timing of the spinal primitives