  Animal behaviors are sometimes decomposable into discrete, stereotyped
elements. In one model, such behaviors are triggered by specific commands; in
the extreme case, the discreteness of behavior is traced to the discreteness of
action potentials in the individual command neurons. We use the crawling
behavior of the nematode C. elegans to explore the opposite extreme, in which
discreteness and stereotypy emerges from the dynamics of the entire behavior. A
simple stochastic model for the worm's continuously changing body shape during
crawling has attractors corresponding to forward and backward motion;
noise-driven transitions between these attractors correspond to abrupt
reversals. We show that, with no free parameters, this model generates
reversals at a rate within error bars of that observed experimentally, and the
relatively stereotyped trajectories in the neighborhood of the reversal also
are predicted correctly.
