Aj. Ijspeert, A connectionist central pattern generator for the aquatic and terrestrial gaits of a simulated salamander, BIOL CYBERN, 84(5), 2001, pp. 331-348
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.