  Infant vocal babbling strongly relies on jaw oscillations, especially at the
stage of canonical babbling, which underlies the syllabic structure of world
languages. In this paper, we propose, model and analyze an hypothesis to
explain this predominance of the jaw in early babbling. This hypothesis states
that general stochastic optimization principles, when applied to learning
sensorimotor control, automatically generate ordered babbling stages with a
predominant exploration of jaw movements in early stages. The reason is that
those movements impact the auditory effects more than other articulators. In
previous computational models, such general principles were shown to
selectively freeze and free degrees of freedom in a model reproducing the
proximo-distal development observed in infant arm reaching. The contribution of
this paper is to show how, using the same methods, we are able to explain such
patterns in vocal development. We present three experiments. The two first ones
show that the recruitment order of articulators emerging from stochastic
optimization depends on the target sound to be achieved but that on average the
jaw is largely chosen as the first recruited articulator. The third experiment
analyses in more detail how the emerging recruitment order is shaped by the
dynamics of the optimization process.
