Coordination dynamics of rhythmical and discrete prehension movements: Implications of the scanning procedure and individual differences

The aim of this study was to compare the coordination dynamics of discrete and rhythmical reaching and grasping movements from a dynamical systems per spective. Previous research from this theoretical perspective had focused o n rhythmical actions and it is unclear to what extent discrete movements ar e amenable to a similar dynamical systems analysis. Six adult subjects perf ormed prehension in two conditions: a discrete, non-continuous mode and a r hythmical, continuous mode. A 'scanning procedure' was implemented between pre- and post-tests in which the required time of final relative hand closu re (T-rfc) was systematically varied. It was shown that the error in the re aching and grasping pattern was least at an attractor region and systematic ally increased with deviation from the attractor. Results also indicated th at there were no differences between condition or trial block for the group . However, there were several within-subject effects of interest. The valid ity of the scanning procedure was found to be questionable in the discrete condition, where four subjects showed differences in T-rfc between pre- or post-test and the predicted T-rfc of the scanning procedure. Four out of si x subjects also had different preferred T-rfc values for discrete and rhyth mical movement, indicating that individual specific models might need to be constructed for future dynamical modelling of discrete movement. (C) 1998 Elsevier Science B.V. All rights reserved.