Steady-state and perturbed rhythmical movements: a dynamical analysis.

This study examined rhythmic finger movements in the steady state and when momentarily perturbed in order to derive their qualitative dynamical properties. Movement frequency, amplitude, and peak velocity were stable under perturbation, signaling the presence of an attractor, and the topological dimensionality of that attractor was approximately equal to one. The strength of the attractor was constant with increasing movement frequency, and the Fourier spectra of the steady-state trials showed an alternating harmonic pattern. These results are consistent with a previously derived nonlinear oscillator model. However, the oscillation was phase advanced by perturbation overall, and a consistent phase-dependent, phase-shift pattern occurred, which is inconsistent with the model. The overall phase advance also shows that any central pattern generator responsible for generating the rhythm must be nontrivially modulated by the limb being controlled.