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, yet at the same time we often notice that one hand influences the other. We investigated bi-manual pointing movement towards a common target and hypothesised that if (parts of) bi-manual movements are planned as one functional unit, then there should be significant correlations between the movement parameters of each hand. Surprisingly, we found only consistently high correlation coefficients with regard to the reaction time. Our results demonstrate that trajectory formation and adjustments of bi-manual movements are largely independent and hand-specific, contradicting the notion that the left and right hand are controlled as one functional unit. Because we find that average, 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, however, there are small but very consistent timing differences between the left and right hands. These differences are small compared to the variability in reaction times and, therefore, we conclude that they result from an intrinsic control property of the system of bi-manual movement generation. (C) 1999 Elsevier Science B.V. All rights reserved.