Multiple frames of reference for bimanual co-ordination.

Different movement characteristics can be governed by different frames of reference. The present study serves to identify the frames of reference, which govern intermanual interactions with respect to movement directions. Previous studies had shown that intermanual interactions are adjusted to task requirements during motor preparation: for parallel movements directional coupling becomes parallel, and for symmetric movements it becomes symmetric. The timed-response procedure allows to trace these adjustments as they are reflected in the intermanual correlations between left-hand and right-hand directions. In the present study the adjustments remained unchanged when all target directions were rotated laterally, indicating a critical role of hand-centered frames of reference. The additional role of a body-centered frame of reference was indicated by the finding of overall higher intermanual correlations with the rotated target configurations. Intermanual interference at long preparation intervals was absent even when eccentricities in the body-centered frame of reference were different. These findings converge with results on the frames of reference that govern intermanual interactions with respect to movement amplitudes. They suggest a role of both body-centered and hand-centered frames of reference for the adjustments of intermanual interactions to task requirements, but of a hand-centered frame of reference only for the intermanual interference, which remains in spite of the adjustments.