Linear and nonlinear measures predict swimming in the leech

Stimulation of a trigger interneuron of an isolated nerve cord preparation of the medicinal leech, Hirudo medicinalis, sometimes leads to swimming; so metimes it does not. We investigate signals transmitted in the ventral cord of the leech after stimulation and seek quantitative measures that would m ake it possible to distinguish signals that predict swimming from those tha t do not. We find that a number of linear as well as nonlinear measures pro vide statistically significant distinctions between the two kinds of signal s. The linear measures are the time dependence of (i) the standard deviatio n and (ii) the autocorrelation function at a small time delay. The nonlinea r measures are (i) a measure of nonlinear predictability and (ii) the time dependence of a measure of the size of the embedded signal trajectory. Calc ulations using surrogate data suggest that the differences between the two classes of signals are dynamical as well as statistical.