Low dimensionality of supraspinally induced force fields

Recent experiments using electrical and N methyl-D-aspartate microstimulation of the spinal cord gray matter and cutaneous stimulation of the hindlimb of spinal ized frogs have provided evidence for a modular organization of the frog's spinal cord circuitry. A ``module'' is a functional unit inthespinalcordcircuitrythatgeneratesaspecificmotor outputbyimposingaspecificpatternofmuscleactivation.The output of a module can be characterized as a force field: the collection of the isometric forces generated at the ankle over different locations in the leg's workspace. Different modules can be combined independently so that their force fields linearly sum. The goal of this study was to ascertain whether the force fields generated by the activation of supraspinal structures could result from combinations of a small number of modules. We recorded a set of force fields generated by the electrical stimulation of the vestibular nerve in seven frogs, andweperformedaprincipalcomponentanalysistostudythe dimensionality of this set. We found that 94% of the total variation of the data is explained by the first five principal components, a result that indicates that the dimensionality of the set of fields evoked by vestibular stimulation is low. This result is compatible with the hypothesis that vestibular fields are generated by combinations of a small number of spinal modules.