CHARACTERIZATION OF LOW-DIMENSIONAL DYNAMICS IN THE CRAYFISH CAUDAL PHOTORECEPTOR

ATTEMPTS to detect and characterize chaos in biological systems are of considerable interest, especially in medical science, where successfu l demonstrations may lead to new diagnostic tools and therapies(1). Un fortunately, conventional methods for identifying chaos often yield eq uivocal results when applied to biological data(2-8), which are usuall y heavily contaminated with noise. For such applications, a new techni que(1) based on the detection of unstable periodic orbits holds promis e, Infinite sets of unstable periodic orbits underlie chaos in dissipa tive systems(4,9); accordingly, the new method searches a time series only for rare events(8) characteristic of these unstable orbits(10), r ather than analysing the structure of the series as a whole. Here we d emonstrate the efficacy of the method when applied to the dynamics of the crayfish caudal photoreceptor (subject to stimuli representative o f the animal's natural habitat). Our findings confirm the existence of low-dimensional dynamics in the system, and strongly suggest the exis tence of deterministic chaos. More importantly, these results demonstr ate the power of methods based on the detection of unstable periodic o rbits for identifying low-dimensional dynamics-and, in particular, cha os-in biological systems.