Variability and correlations in bi-manual pointing movements

Bi-manual movement tasks present us with an intriguing paradox: on one hand it is clear that we are capable of controlling our hands independently, ye t at the same time we often notice that one hand influences the other. We i nvestigated bi-manual pointing movement towards a common target and hypothe sised that if (parts of) bi-manual movements are planned as one functional unit, then there should be significant correlations between the movement pa rameters of each hand. Surprisingly, we found only consistently high correl ation coefficients with regard to the reaction time. Our results demonstrat e that trajectory formation and adjustments of bi-manual movements are larg ely independent and hand-specific, contradicting the notion that the left a nd right hand are controlled as one functional unit. Because we find that a verage, rather than individual, movements are correlated, we conclude that coupling between the two hands occurs only at 'task level'. In symmetrical tasks, movements are initiated simultaneously. In asymmetrical tasks, howev er, there are small but very consistent timing differences between the left and right hands. These differences are small compared to the variability i n reaction times and, therefore, we conclude that they result from an intri nsic control property of the system of bi-manual movement generation. (C) 1 999 Elsevier Science B.V. All rights reserved.