Chapter 10--a hierarchical perspective on rhythm generation for locomotor control.

The control of locomotion is a complex dynamic task solved with apparent ease by our body. How this is accomplished still remains an intriguing mystery. This chapter first describes classical and recent findings relevant to understanding the complexity of the question on the verge of several fields of neurophysiology, biomechanics, and computational neuroscience. Then, control of locomotion is analyzed with numerical simulations to reveal some basic characteristics responsible for modulation of the locomotor rhythm and high-level control of steering in the whole animal. In this study, the concept of a central pattern generator (CPG) for controlling locomotor rhythm first proposed by Brown was implemented in a "simple" model with bilateral half-center oscillators consisting of reciprocally organized integrators. The parameters of the CPG were determined by the process of optimization of its phase-duration characteristic that satisfies biomechanical requirements of the overground locomotion. The general finding of this study is that the modality of the control signal that drives CPGs for each limb corresponds to the desired speed of forward progression. This supports the idea that the descending and sensory feedback inputs to the spinal CPG are combined to produce a high-level control signal that sets forward velocity. The same mechanism may be responsible for the control of steering by generating a differential input of speed commands to different limbs.