Cooperative bimanual action

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Abstract
We present an experiment on cooperative bimanual action. Right-handed subjects manipulated a pair of physical objects, a tool and a target object, so that the tool would touch a target on the object ( fig. 1). For this task, there is a marked specialization of the hands. Performance is best when the left hand orients the target object and the right hand manipulates the tool, but is significantly reduced when these roles are reversed. This suggests that the right hand operates relative to the frame-of-reference of the left hand. Furthermore, when physical constraints guide the tool place- ment, this fundamentally changes the type of motor control required. The task is tremendously simplified for both hands, and reversing roles of the hands is no longer an important factor. Thus, specialization of the roles of the hands is significant only for skilled manipulation. Informally, we observed that the operation of the interface was greatly simplified when both hands were involved in the task. But in the early design stages, we were faced with many possible ways that the two hands might cooperate. An early prototype allowed users to use both hands, but was still difficult to use. The nonpreferred hand oriented the doll's head, and the preferred hand oriented the cross-sectioning plane, yet the software did not pay any attention to the rela- tive placement between the left and the right hands. Users felt like they were trying to perform two separate tasks which were not necessarily related. We changed the interface so that relative placement mat- tered. All motion was interpreted as a distance relative to the doll's head in the left hand, resulting in a far more natural interaction. It was far easier to integrate the action of the two hands to perform a cooperative task. Thus, informally we had observed that two-handed coordi- nation was most natural when the preferred hand moved rel- ative to the nonpreferred hand. The current experiment formalizes this hypothesis and presents some empirical data which suggests right-to-left reference yields quantitatively superior and qualitatively more natural performance.

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