Solo versus joint bimanual coordination.

Understanding the differences between solo and joint action control is an important goal in psychology. The present study represented a novel approach in which participants performed a bimanual finger oscillation task, either alone or in pairs. It was hypothesized that performance of this task relies heavily on attention and utilizes two independent processes that differentially affect solo and joint performance. One process attempts to align the fingers correctly regardless of oscillation speed, and this is reflected in an alignment error evident even at slow oscillations. A second process attempts to minimize the time lag between the fingers as the oscillation speed increases, reflected in a temporal error indexed by the rate of error increase with increasing movement speed. In three experiments, alignment and temporal error in the finger oscillation task were compared in solo and joint actors. Overall, solo actors had much lower alignment error than joint actors. Solo actors also showed a reduction in temporal error when the fingers moved in a symmetrical rather than parallel fashion, consistent with previous research showing an increase in error with increasing movement speed. However, the effect of symmetry on temporal error did not occur with joint actors. Similar results were found with one hand inverted, suggesting that the pattern of results was not due to the use of homologous muscles. To test the role of visual feedback, we examined the effect of denying visual feedback to one of the actors in the joint condition. Paradoxically, under these conditions, there was lower temporal error in the symmetrical condition. These results are interpreted in terms of the organization of solo versus joint actions and the control of bimanual tasks in general.