A connectionist central pattern generator for the aquatic and terrestrial gaits of a simulated salamander

This article investigates the neural mechanisms underlying salamander locom otion, and develops a biologically plausible connectionist model of a centr al pattern generator capable of producing the typical aquatic and terrestri al gaits of the salamander. It investigates, in particular, what type of ne ural circuitry can produce and modulate the two locomotor programs identifi ed within the salamander's spinal cord; namely, a traveling wave of neural activity for swimming and a standing wave for trotting. A two-dimensional b iomechanical simulation of the salamander's body is developed whose muscle contraction is determined by the locomotion controller simulated as a leaky -integrator neural network. While the connectivity of the neural circuitry underlying locomotion in the salamander has not been decoded for the moment , this article presents the design of a neural circuit that has a general o rganization corresponding to that hypothesized by neurobiologists. In parti cular: the locomotion controller is based on a body central pattern generat or (CPG) corresponding to a lamprey-like swimming controller, and is extend ed with a limb CPG for controlling the salamander's limbs. The complete con troller is developed in three stages: first the development of segmental os cillators, second the development of intersegmental coupling for the making of a lamprey-like swimming CPG, and finally the development of the limb CP G and its coupling to the body CPG. A genetic algorithm is used to determin e the parameters of the neural circuit for the different stages, given a hi gh-level description of the desired state space trajectories of the differe nt subnetworks. A controller is thus developed that can produce neural acti vities and locomotion gaits very similar to those observed in the real sala mander. By varying the tonic (i.e. non-oscillating) excitation applied to t he network, the speed, direction and type of gait can be varied.