The effects of physical object coupling on interlimb coordination

Recent research (Hughes, Haddad, Franz, Zelaznik, & Ryu, under review) has demonstrated that when the hands manipulate two objects that are physically coupled they exhibit greater spatiotemporal independence. On the basis of these results it was proposed that if the actions of the individual hands can be conceptualized as subparts of a larger action (or task) then the tendency toward spatiotemporal synchronization could be reduced. Thus, the present experiments were designed to explore interlimb coordination during the manipulation of physically coupled objects. In the first paper, participants were assigned to either a control or transfer group. Participants in the control group performed the task when the objects were separate (no-spring group) or when the objects were connected by a spring (spring group). Participants in the transfer group either performed the task when the objects were separate, and then when the objects were connected by a spring (no-spring-first group), or in the reverse order (spring-first-group). The task required participants to grasp and place two objects on a target board to either identical or different object end-orientations. Regardless of the order in which the tasks were performed, the hands were less coupled when manipulating objects that were connected compared to objects that were not connected. Thus, the spatiotemporal trajectories of the individual hands appeared to be less constrained when the objects were physically connected. This result supported the interpretation that the manner in which the manipulated objects were perceived can modulate the constraints that underlie bimanual coordination. This perceptual interpretation was directly tested in the second paper by manipulating the manner in which the objects were coupled. Participant’s manipulated objects that were not connected, connected with a telescopic rod, or connected with a band or spring in which stiffness constants varied based on the type of physical connection. There were three main findings: (1) interlimb coupling was similar between the no-stiffness and rod groups (2) interlimb coupling changed as a function of the stiffness of the connecting material, and (3) although the right hand moved toward the target in a straighter fashion when the objects were physically independent, this was reversed when the objects were connected. In this context, the non-dominant left hand performed the task in a straighter fashion than the dominant right hand. That is, the left-hand adopted a stabilizing role while the dominant right hand adopted a more manipulative role. Thus, there was no support for the perceptual interpretation forwarded in the first paper. Instead, it is proposed that the tactics employed by the central nervous system (CNS) changes in response to different behavioral contexts as well as physical demands