EIGENVECTORS OF THE INERTIA TENSOR AND PERCEIVING THE ORIENTATIONS OFLIMBS AND OBJECTS

We report several experiments directed at the ability of humans to per ceive the spatial orientation of occluded objects, to position an occl uded limb relative to targets or directions in the environment, and to match the spatial orientations of occluded contralateral limbs. Resul ts suggest that each of these abilities is tied to the inertial eigenv ectors of each object or limb, which correspond to the object's or lim b's principal axes of rotational inertia. Discussion focuses on the dy namic nature of proprioception, the importance of physical invariants for perception, and the relation of invariants to hypothesized frames of reference for proprioception and motor control. It is suggested tha t the detection of invariants revealed through movement is a major mec hanism in kinesthetic perception involving intact limbs, neuropathic o r anesthetized limbs, prosthetic devices, and hand-held tools and impl ements. The inertia tensor is identified as one such invariant.