Combined effects of planning and execution constraints on bimanual task performance.

In this study we investigated the relative impact of planning and execution constraints on discrete bimanual task performance. In particular, in a bimanual CD-placement task, we compared people's preference to end movements comfortably with their preference to move symmetrically. In "Experiment 1" we examined the degree of interlimb coupling as participants repositioned two CDs in a CD rack by simultaneously moving their arms mirror-symmetrically or asymmetrically into comfortable or uncomfortable end postures. Interlimb coupling was stronger when the arms moved symmetrically towards uncomfortable end postures. In "Experiment 2" participants were asked to realize specific end orientations of the CDs but they were free to choose an initial grip type and subsequent direction of forearm rotation. Surprisingly, the participants did not move their arms symmetrically but preferred to end in a comfortable posture with their right hand but not with their left hand. We conclude that in discrete bimanual task performance the tendency to end movements in a comfortable posture dominates over the tendency to synchronously activate homologous muscle pairs. The lateralized end-state comfort effect suggests a hemispheric specialization for motor planning.