Abstract
Bingham (2001; 2004a,b) proposed a dynamical model of coordinated rhythmic movement that predicted the information used was the relative direction of motion, modified by relative speed. de Rugy et al (2008) tested this prediction by testing the dependence on speed. They reported that movement stability did not depend on relative speed. However, there were limitations that cast doubt on these findings. First, the only reported measure was of stability. It quantified consistency but not accuracy. Second, amplitude, manipulated to alter relative speed, was not reported. Whether required differences in speed were actually generated is unknown. Finally, the task used to test the model was not one the model was designed to represent. We ran the following studies to test Bingham's hypothesis more precisely. Participants used a joystick to coordinate the movement of two dots on a screen, controlled by computer and joystick respectively. First, we tested stability using the ‘switching’ paradigm. Participants attempted to produce 180° relative phase at frequencies increasing from 0.5Hz to 2.0Hz by 0.25Hz steps. Switching occurred at 1.25Hz. Visual coordination is much less stable than bimanual coordination. Next, we assessed movement stability at 0° and 180° by having participants move at 1.0Hz, 1.25Hz and 1.50Hz. The amplitude of the joystick dot was constant while that of the computer dot was either the same or three times larger. 0° with unequal amplitudes had the same relative speed difference as 180° with equal amplitudes; so, the stability should be comparable and less than 0° with equal amplitudes. Using a measure of both consistency and accuracy, we found that speed differences affected movement stability as predicted by the Bingham hypothesis (even though amplitudes were somewhat different than required). Bingham, Snapp-Childs and Wilson (submitted) revised the model for the new task and successfully captured these results.