Perception of limb orientation in the vertical plane depends on center of mass rather than inertial eigenvectors.

We performed two experiments to test the hypothesis that the perception of limb orientation depends on inertial eigenvectors (e(i)) against the alternative hypothesis that it depends on the center of mass vector (CM). Whereas e(i) constrains the dynamic torques involved in angular rotation, CM constrains the static torque necessary to keep the limb aloft in the gravitational field. Hence, possible effects of e(i) and CM on kinesthetic judgments must be related to the dynamic and static torques, respectively, involved in moving and positioning a limb. In the first experiment, blindfolded participants matched, with upper arms supported, the orientation of their forearms while the forearms' e(i) and CM were manipulated relative to the elbow. The manipulation of the vector CM alone induced a matching bias, as did the combined manipulation of e(i) and CM, whereas the manipulation of e(i) alone did not. In the second experiment, participants positioned their unseen and unsupported right arm at an indicated spatial configuration while e(i) and CM of the right forearm were manipulated as in Experiment 1. As in the first experiment, forearm positioning was affected by the independent manipulation of CM and the combined manipulation of e(i) and CM, but not by the independent variation of e(i). Moreover, none of the manipulations affected upper arm positioning. These results refute the claim that the perception of limb orientation (in the vertical plane) is based on e(i)and demonstrate, for the first time, the implication of a limb segment's CM in the perception of its orientation.